Gaëtan Buvat

Houssem Sellemi

Uday K Ravella

Maud Barré

Sandrine Coste

Gwenaël Corbel

Philippe Lacorre

Reduction Kinetics of La2Mo2O9 and Phase Evolution during Reduction and Reoxidation

Keywords: La 2 Mo 2 O 9, amorphous oxide, reduction kinetics, hydrogen reduction, thermogravimetric analysis, exsolution

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potentially interesting as anode material in conventional doublechamber SOFC. 10 Further reduction produces an amorphous compound hereafter called La 2 Mo 2 O 7-y . The amorphous reduced phase was successfully tested as a sulfur-tolerant anode material for SOFC. 11 Therefore, so far, the most promising applications of La 2 Mo 2 O 9 -derived materials in SOFC devices are either for oxidized forms as electrolyte in single-chamber devices 12 (with less stringent reducing conditions) or for reduced amorphous form as anode material in double-chamber devices. 11,13 Very recently, Liu et al. 14 showed that La 2 Mo 2 O 9 nanowires prepared by electrospinning exhibit a 3 orders of magnitude increase of ionic conductivity with respect to the bulk material. They attribute most of this increase to the presence of a highly conducting surface layer of strained amorphous phase with composition La 2 Mo 2 O 8.5 around the nanowires. If confirmed, such a huge ionic conductivity would bolster research and development on such materials, among which partially reduced amorphous forms of the lanthanum molybdate. From a fundamental point of view, a better understanding of the stability and existence ranges of these amorphous phases is highly desirable as well as of their basic properties.

Previous reports already described La 2 Mo 2 O 9 reduction into amorphous phase. 15,[START_REF] Vega-Castillo | Centro Atoḿico[END_REF] Vega Castillo et al. obtained the amorphous compound La 2 Mo 2 O 6.88 from a pellet after 80 h of reduction at 608 °C under 10% H 2 /Ar atmosphere with a flow rate of 6 L•h -1 . 8 However, above 900 °C, the amorphous phase undergoes a slight crystallization, and at 1000 °C under a partial oxygen pressure (pO 2 ) below 10 -10 Pa, the reduction leads to mixtures of crystallized molybdates. The same authors investigated the thermodynamics of the reduction process by studying the phase stability at different temperatures in a wide range of partial oxygen pressures 8,10 (see Figure 1). Thus, at 718 °C, amorphous La 2 Mo 2 O 7-y phase appears below pO 2 = 10 -17 Pa. At 608 °C, the slightly reduced and crystallized MIEC phase La 2 Mo 2 O 8.96 is formed in the range 10 -20 < pO 2 < 10 -18 Pa, whereas below 10 -20 Pa, the amorphous La 2 Mo 2 O 7-y form is obtained.

In the present paper, the kinetic aspect of La 2 Mo 2 O 9 reduction to the amorphous forms is studied. For that purpose, powders from different synthesis routes with different specific surface areas and treated at two temperatures are used. The phase evolution during reduction is analyzed, and the stability of La 2 Mo 2 O 7-y is discussed. Reoxidation of the amorphous reduced phase is also studied.

■ EXPERIMENTAL SECTION

Synthesis of La 2 Mo 2 O 9 Powders. Two synthesis routes were used in order to obtain powders with different sizes and shapes of particles and therefore different specific surface areas of the samples. First, La 2 Mo 2 O 9 powders were prepared by solid state reaction (SSR). La 2 O 3 and MoO 3 were weighed in stoichiometric amounts and mixed in an agate mortar with acetone. La 2 O 3 was previously dehydrated and decarbonated at 1000 °C for 12 h. The mixed oxides were annealed at 500 °C for 12 h in order to avoid Mo oxide sublimation and then heat treated twice at 900 °C for 12 h. The powders were ground between each heat treatment. La 2 Mo 2 O 9 powders with higher specific surface areas were also prepared by the polyol process using two glycol solvents, ethylene glycol (EG) and diethylene glycol (DEG), with a final rapid heat treatment of 5 min at 600 °C. [START_REF] Sellemi | [END_REF] The details regarding this synthesis are reported in previous papers by Sellemi et al. [START_REF] Sellemi | [END_REF]19 Hydrogen Reduction and Reoxidation Characterization. Reduction kinetics of La 2 Mo 2 O 9 was studied at 700 and 760 °C using a Setaram TG92 microbalance under a flow rate of 4.8 L•h -1 of 10% H 2 diluted in Ar. A quartz crucible was used as a container for the reduction. The furnace with crucible and powder in it was first purged in the hydrogen-diluted atmosphere for 1 h at room temperature before heating the sample up to high temperature with a fast rate of 30 °C• min -1 . The sample weight was corrected from the effect of Archimede pressure by subtracting, from the measurement performed on the sample, the measurement performed on the empty quartz crucible under the same conditions. Since powders prepared by solid state reaction and by the polyol process have very different specific surface areas (see Table 1 from the BET measurements), we preferred to use for each measurement the same occupied volume in the crucible rather than the same mass. Around 300, 150, and 50 mg of raw samples were used, respectively, for SSR, EG, and DEG sample for each TG measurement.

The reoxidation of La 2 Mo 2 O 7-y for 12 and 60 h was performed under flowing air in the same thermogravimetric apparatus. The measurements were carried out in a quartz crucible containing about 50 mg of raw samples.

Other thermal analyses for the study of the reoxidation were carried out in air with a TGA/DTA Q600 SDT TA Instruments apparatus under a flow rate of 6 L•h -1 with a heating/cooling rate of 10 °C•min -1 using platinum crucibles with Al 2 O 3 as a reference.

Characterization Techniques. X-ray powder diffraction (XRPD) patterns were recorded at room temperature on a PANalytical θ/θ Bragg-Brentano X'pert MPD PRO diffractometer (Cu Kα radiations) equipped with the X'Celerator detector. Step and recording speed were 0.03°and 53 s/step for regular patterns and 0.03°and 440 s/step for Rietveld quantification, respectively. N 2 adsorption measurements were performed at 77 K on a Micromeritics Tristar II system (Micromeretics, Norcross, GA). Approximately 260 mg and 1.9 g of powders were employed, respectively, for EG/DEG and SSR sample in each measurement. The specific surface area was calculated using the Brunauer-Emmet-Teller (BET) model in the range of around 0.06 < P/P 0 < 1. ■ RESULTS AND DISCUSSION Hydrogen Reduction of La 2 Mo 2 O 9 . The reduction of La 2 Mo 2 O 9 was characterized in situ by thermogravimetric analysis, the weight loss being assumed to result from a loss of oxygen atoms according to the following reaction

+ → + - z z La Mo O H La Mo O H O z 2 2 9 2(g) 2 2 9 2 (g) (1)
Reduction kinetics of the sample is determined by plotting the evolution with time of the mass and, therefore, of the oxygen content at a fixed temperature under constant flowing atmosphere. In the first part, the mechanism and reduction kinetics are compared at different temperatures, 700 and 760 °C, on the SSR sample. In the second part, a comparison is made for powder samples having different specific surface areas.

Effect of the Reduction Temperature. Two different temperatures, 700 and 760 °C, were used for the reduction of La 2 Mo 2 O 9 compounds synthesized by solid state reaction. A temperature of 760 °C was chosen with respect to the previous work by Goutenoire et al. 7 We also chose a temperature of 700 °C because it is close to the operating temperature expected for an intermediate temperature SOFC and for the different phase change during reduction compared to what happens at 760 °C, as described hereafter. The reduction kinetics is presented in Figure 2 through the average oxygen content versus time (Figure 2a) and the time dependence of the reduction rate (Figure 2b), calculated from the derivative vs time of the oxygen loss per formula unit. Phase evolution during reduction was followed by X-ray powder diffraction (XRPD) after cooling the sample down to room temperature from different reduction temperatures in diluted hydrogen. For a reduction temperature of 760 °C (Figure 3a), after 30 min, three phases can be identified as La 2 Mo 2 O 9 , La 7 Mo 7 O 30 (La 2 Mo 2 O 8.57 ), and an amorphous phase detected through the background undulation, whereas after 2 h of reduction only the amorphous phase and crystallized La 7 Mo 7 O 30 compound are present. The amount of La 7 Mo 7 O 30 decreases with time, resulting in a pure amorphous phase after a few hours. Further reduction of the amorphous La 2 Mo 2 O 6.2 phase induces the exsolution of metallic molybdenum, which started being detected after 20 h annealing. It implies a substoichiometry in molybdenum in the amorphous phase. However, for a reduction temperature of 700 °C (Figure 3b), during the first 6 h a direct progressive amorphization of the La 2 Mo 2 O 9 phase is observed, with disappearance of the peaks corresponding to crystallized domains until complete amorphization. XRD and thermogravimetric analyses show two reduction regimes (see Figure 2b). The first regime corresponds to the reduction of La 2 Mo 2 O 9 into amorphous La 2 Mo 2 O 7 and the second one to the reduction of the amorphous phase and exsolution of metallic molybdenum from it when further reduced. For the first regime, the reduction rate depends on temperature: 0.4 and 1.2 oxygen atoms loss per unit cell and per hour at 700 and 760 °C, respectively. However, in the second regime, the reduction rate does not seem to depend on temperature: after 18 h it is 0.012 both at 700 and 760 °C. Effect of the Powder Specific Surface Area. The reduction kinetics was studied at 700 °C on powders with different specific surface areas. The starting powders used were synthesized by solid state reaction and by the polyol process using ethylene glycol (EG) and diethylene glycol (DEG). The specific surface areas measured by the BET method are reported in Table 1. High values are obtained for the powder samples prepared by the polyol process, for very thin platelet particles synthesized with DEG, and porous spherical grains synthesized with EG. [START_REF] Sellemi | [END_REF]19 Reduction kinetics are presented in Figure 4a. We can notice for EG and DEG samples that reduction has already begun during the heating process. As expected, the kinetics is higher for powders prepared by the polyol process. After 18 h of reduction, oxygen stoichiometries x of La 2 Mo 2 O x samples obtained by SSR, DEG, and EG are 6.57, 6.04, and 6.11, respectively. In Figure 4b are shown the reduction rates calculated from the derivative vs time of oxygen loss. As previously shown, two reduction regimes are observed: one corresponding to the reduction of La 2 Mo 2 O 9 and the other to the reduction of amorphous phase La 2 Mo 2 O 7 . The average reduction rates of the first regime are consistent with the specific surface areas. DEG sample has the highest average reduction rate of 4.1 atoms lost per unit cell per hour, whereas average reduction rates of EG and SSR samples are 3.1 and 0.4, respectively. Reduction kinetics of the powder obtained with EG is only slightly smaller than that of the powder obtained with DEG despite a factor of about 3 between the two specific surface areas. This difference could be explained by the morphology of the particles; the DEG process produces platelet particles, while EG leads to spherical grains. Then it indicates that spherical morphology seems to favor the phase reduction.

Concerning the second regime, the reduction rate is the same for the three samples. It seems that the reduction of the amorphous phase is influenced neither by the temperature nor by the microstructure of the original particles. It might be due to the fragmentation of the platelets/grains during the last part of the first reduction regime, resulting in amorphous subparticles of similar sizes whatever the raw sample. Phase evolution characterized by XRD shows first a reduction into the amorphous phase without any formation of intermediate La 7 Mo 7 O 30 phase for the DEG and EG samples. Then, as for the reduction at 760 °C, an exsolution of metallic molybdenum from the amorphous phase is observed for severe reduction of polyol samples, resulting in an amorphous Mo-substoichiometric phase. In order to measure the weight fraction of metallic molybdenum, quantitative Rietveld analyses of the amorphous and molybdenum metallic phases were carried out by XRD using the internal standard method. For this purpose, cerium oxide was used as the internal standard, because of its microabsorption coefficient close to those of metallic molybdenum and amorphous reduced phase. An exact amount of 10% in weight was added to the powders analyzed by XRD. The internal standard method is based on the calculation of the weight fraction of each compound

= × × × ∑ × × × W S Z M V S Z M V ( ) ( ) j j j j j i i i i i (2)
where W j is the weight fraction of phase j, S j is the Rietveld scale factor of phase j, Z j is the number of formula units per cell of phase j, M j is the mass of formula unit of phase j, and V j is the volume unit cell of phase j.

Quantitative Rietveld analyses were performed on two samples of DEG reduced for 17 and 34 h, for which the scale factor was determined. Figure 5 presents Rietveld analysis on the XRD pattern of the 34 h reduced sample. The quantification of the weight fraction of metallic molybdenum and amorphous phase allowed us to determine the stoichiometry of this last one, as summarized in Mathematical Modeling of the Reduction Kinetics. On the basis TGA analysis and on the phase evolution of the compound determined by XRD, a modeling of the reduction kinetics was performed using a kinetic law developed by Bessieres et al. for the reduction of iron oxide. 20,21 The motivation for such type of model rests on the shape of the reduction rate curves, which does not pass through a maximum with time as would be the case if the reduction kinetics were nucleation limited. [START_REF] Haber | Less-Common Met[END_REF] The decrease in reduction rate with time shows that here nucleation is very fast, and in this case, a "shrinking core model" is usually used to describe kinetics. [START_REF] Sloczynski | [END_REF] The Bessieres model we used is based on a chemical reaction interface which progresses with a constant rate V c , being proportional to the concentration of the reducing gases and to the area of the reaction interface

π = -* V B C R ( ) 4 c i o 2 (3)
In this model, chemical reactions can take place either separately or simultaneously at the different interfaces. Figure 6 presents an illustration of this kinetic law model in the case of spherical grains with either one or two reactions progressing. The kinetic law is of first order and can be expressed as

- = - = -* = = R R R X B C C R mq t t tf k t 1 ( ) ( ) i i i c i i o o 1/3 o o ( 4
)
where C o is the reducing gas concentration, C* is the equilibrium concentration of the gas with the two oxides present, B is a constant depending on the temperature, R o is the initial particle radius, R i is the interface radius of reaction i, m is the mass of the oxygen atom, q is the number of oxygen atoms to be taken away per unit volume for the reaction considered, (tf c ) i is the time needed to obtain the total reduction of the interface in the pure chemical reaction, X i is R i 3 /R 0 3 , 1 -X i is the progress of the reduction, 1 -X i 1/3 is the relative penetration, and k i is the reaction rate.

Thermogravimetric data allow us to calculate the progress of the global reaction 1 -X, where X = ∑ i (α i X i ) and α i are the coefficients depending on the oxygen loss for each single reaction (≡ proportion of lost oxygen for each reaction). From the progress of the global reaction we can plot the relative penetration 1 -X 1/3 as a function of time. Assuming this type of kinetic law we can study the evolution of each interface as a function of time. The kinetic law for a single reaction can be expressed as

- = X kt 1 i i 1/3 (5)
However, when a reaction begins with a delay time it becomes

- = - X k t t 1 ( ) i i 1/3 0 ( 6
)
where t 0 is the time delay.

Then X i values of the single reaction of eqs 5 and 6 are, respectively

= - X k t ( 1
) i i 3 (7) = - - X k t t (1 (
)

) i i 0 3 (8)
The total reduction time of a single interface (tfc) i can be calculated from the kinetic constant

k i = k (tfc) 1 i i (9)
The relative deviation (RD) between experimental (1 -X 1/3 ) exp and calculated (1 -X 1/3 ) calcd curves has been calculated as follows

= ∑ | - - - | ∑ - × X X X RD(%) (1 ) (1 )
(1 ) 100

1/3 calcd 1/3 exp 1/3 calcd (10
) 12
Reaction A is the reduction of La 2 Mo 2 O 9 into the amorphous phase La 2 Mo 2 O 7-y . However, it seems that the oxygen stoichiometry for which the amorphous phase starts is not well defined. According to the literature, 8,10 y should be between 0 and 0.3. Reaction B is the reduction of the amorphous phase until the start of metallic molybdenum exsolution.

During the first period, there are two interfaces present at the same time, with reactions A and B proceeding simultaneously. The relative penetration is given by the following relation

α α - = - - - - = X k t k t g t 1 1 [ (1 ) (1 ) ] ( ) 1/3 A A 3 B B 3 1/3 (13)
The slope at the origin of g(t) is given by its derivative for t = 0

α α ′ = + g k k (0) A A B B (14)
Hereafter, when reaction A is finished, there is only one chemical reaction front (B). In this case the relative penetration is given by the following relation Experimental data of the ordinate at the origin (1 -α B 1/3 ) and the slope (α B 1/3 k B ) in the range where only reaction occurs allow us to calculate α B and the reaction constant k B . Finally, from expression 14, k A can be calculated. Experimental data and the associated fit are presented in Figure 7a, and fitted parameters of each single reaction reported in Table 3.

α α α - = - - = - + X k t k t 1 1 [ (1 ) ] 1 1/3 B B 3 1/3 B 1/3 B 1/3 B ( 15
)
The fit is poor in the 4-6 h time interval, probably due to the assumption of fast nucleation of our model, whereas the penetration curve shows a slight tendency to a sigmoidal character during the first 6 h.

Contrary to the previous SSR sample, in the case of the EG sample reduced at 700 °C, a third reaction is present which corresponds to the reduction of the amorphous phase into metallic molybdenum (reaction C). On the basis of XRD and TGA observations, reaction C seems to start around amorphous composition La 2 Mo 2 O 6.2 , i.e., this reaction starts with a time delay t 0 compared to the initial stage time t = 0. Since reduction seems to evolve continuously, we assumed that this reduction step ends when all molybdenum is reduced to the metal, lanthanum remaining under the form of elementary oxide La 2 O 3 .

During the first reduction step only reactions A and B are progressing at the same time. However, since reaction C has not begun yet, the progress of this reaction 1 -X C = 0 and then

α α α = = + + X X X X 1: C A A B B C (19)
The relative penetration is given by the following equation

α α α - = - - + - + X k t k t 1 1 [ (1 ) (1 ) ] 1/3 A A 3 B B 3 C 1/3 (20)
When reaction A ends and reaction C has not yet begun the relative penetration is

α α - = - - + X k t 1 1 [ (1 ) ] 1/3 B B 3 C 1/3 (21)
Reaction B continues until the reaction front corresponding to the exsolution of metallic molybdenum starts with a time delay t 0 (reaction C). Two interfaces are then present at the same time (reactions B and C)

α α - = - - + - - X k t k t t 1 1 [ (1 ) (1 ( )) ] 1/3 B B 3 C C 0 3 1/3 (22)
When the front of reaction B has vanished, only reaction C is present and the relative penetration is given by

α α α - = - - - = - + + X k t t k t k t 1 1 [ (1 ( )) ] 1 ( 1 ) 1/3 C C 0 3 1/3 C 1/3 C 0 C 1/3 C (23)
The ordinate at the origin 1 -α C 1/3 (1 + k C t 0 ) and the slope α C 1/3 k C allow us to calculate the constant reaction k c and therefore the time delay t o .

Fitting the relative penetration of each segment given by eqs 20, 21, 22, and 23 to the experimental data (Figure 7b) allows us to deduce the kinetic constants of each single reaction, which are reported in Table 3.

For the SSR sample reduced at 760 °C, the process is quite more complex due to the existence of the intermediate phase Successively, the same relations as previously shown can be associated as follows

α α α α ′ + ″ + - = - - + - + - + ′ ′ ″ ″ X k t k t k t Reaction A A B: 1 1 [ (1 ) (1 ) (1 ) ] 1/3 A A 3 A A 3 B B 3 C 1/3 (28) α α α ″ + - = - - + - + ″ ″ X k t k t Reaction A B: 1 1 [ (1 ) (1 ) ] 1/3 A A 3 B B 3 C 1/3 (29) α α - = - - + X k t Reaction B: 1 1 [ (1 ) ] 1/3 B B 3 C 1/3 ( 30
)
α α + - = - - + - - X k t k t t Reaction B C:1 1 [ (1 ) (1 ( )) ] 1/3 B B 3 C C 0 3 1/3 (31) α - = - - - X kt t Reaction C: 1 1 [ (1 ( )) ] 1/3 C C 0 3 1/3 ( 32
)
Experimental data of the relative penetration and previous relationships enabled us to calculate the reaction constants of each reaction k i (see Figure 7c and Table 3).

Discussion. A complex reduction behavior of La 2 Mo 2 O 9 is observed. Depending on the temperature and on the specific surface area of particles, different reactions are occurring with different kinetics. According to the fitted models, the kinetic constant of the amorphous phase reduction (reaction B) seems to vary with sample, which is not clearly visible in the derivative of the thermogravimetric data. The kinetic law used in this work is based on the hypothesis that (i) the particles are spherical and (ii) the influence of diffusion and nucleation are negligible because the flow rate used is supposed to be large enough to not interfere with the reduction mechanism. Since it might not be strictly the case for all samples, one has to keep in mind that some approximation or interpretation errors can occur in our analyses. Nevertheless, as expected, reduction kinetics is influenced by the microstructure of the particles. Higher specific surface areas induce a larger solid/gas interface which improves diffusion of hydrogen and of oxide ions at the grains surface. The same observation was reported in a study by Goel et al. on dense and porous pellets, in which higher reduction was obtained for higher porosity fraction. 24 Moreover, since H 2 O is formed during reduction, the diffusion of oxide ions on the grain surface could be influenced by the presence of water vapor. In fact, it was found that the oxygen surface exchange process in La 2 Mo 2 O 9 is improved for a wet exchange (H 2 [START_REF] Sellemi | [END_REF] O/ 16 O). 25,26 In this case, H 2 O formed during reduction could play a role in the reduction mechanism. Using another reductive gas, Jacquens et al. reported that LAMOX samples are stable when annealed in dry propane:air mixture, while they get reduced in wet propane:air mixture. 27 One can therefore suppose that the water vapor formed could accelerate reduction, despite the fact that pO 2 increases with pH 2 O. In addition, for particles with higher specific surface area, the higher reduction rate could be due to the higher surface exchange of oxygen, hence more important formation of H 2 O.

No matter the cause, in all cases, temperature or specific surface area, the reduction of La 2 Mo 2 O 9 leads to the amorphous phase La 2 Mo 2 O 7-y and for severe reduction to exsolution of metallic molybdenum. In the literature, the exsolution of metallic nanoparticles such as Ni dissolved as an oxide by cationic substitution into the compound and precipitated under reductive atmosphere after heating has been reported in nonstoichiometric perovskites, 28 and it was shown to increase the electrocatalytic properties. 29,30 In the same way, the presence of metallic molybdenum could enhance the electronic and catalytic properties of the materials. However, the decomposition into metallic molybdenum suggests that amorphous phase La 2 Mo 2 O 7-y is not thermodynamically stable under reductive atmosphere, as already mentioned by Vega-Castillo et al. 10 Indeed, the reduction kinetics under diluted H 2 is very slow (Figures 2 and4) but does not show any stabilization of the oxygen content of the amorphous phase: according to the model, the kinetic constant of Reaction C is very weak, and a correlatively very long reduction time would lead to full decomposition into metallic Mo. Definitely no thermodynamic equilibrium is reached for the amorphous phase.

The slow evolution of the metallic Mo weight fraction is also noticed in the quantitative analysis of the amorphous phase and metallic Mo through XRD (Table 2). Figure 8 shows XRD data recorded for powders synthesized with DEG and reduced for 34 and 60 h.

The average oxygen content of the latter is 5.09, and no quantitative Rietveld analysis was possible because of the small quantity of powder. However, the integration of the molybdenum peak at 2θ = 41°in comparison with the other samples suggests an extrapolation to 1.5 wt % of metallic molybdenum. Nevertheless, the XRD pattern recorded on this sample reduced for 60 h shows at 2θ = 22°an abnormally large hump of the amorphous phase (see arrow in Figure 8). Comparatively, the corresponding hump in the XRD pattern of the sample reduced for 34 h is narrower. It suggests either that amorphous La 2 Mo 2 O 7-y can accept a certain substoichiometry in molybdenum or that a second amorphous phase with higher lanthanum content is formed when Mo exsolution becomes important. In the La 2 O 3 -MoO 3 phase diagram, the closest composition in the molybdenum-deficient side is La 2 MoO 6 , 31 which is often observed when the molybdenum of La 2 Mo 2 O 9 reacts with an alkaline element. 32 This compound can get reduced to La 2 MoO 5 , and its main diffraction peak lies close to the extra hump observed in the XRD pattern. However, if this extra hump was due to an amorphized reduced phase of this type, its amount would be no more than a few percent, hence hardly visible in the diffraction pattern. We therefore consider that the most likely is an extension of the amorphous range to La:Mo > 1 stoichiometry. In any case, these results seem to confirm the instability of the amorphous phase La 2 Mo 2 O 7-y in a reductive atmosphere. However, when amorphous La 2 Mo 2 O 7-y was tested as anode in a SOFC device, negligible losses in electrical properties were observed for over 4 days 11 and even for over 30 days. 13 This suggests that the amorphous phase was stable under the reducing atmosphere of a SOFC in operating conditions. Determinant effects as anodic overpotential, oxide ion migration from the electrolyte, or water vapor emission during fuel cell operation might lead to stabilization of the amorphous phase.

Reoxidation of the Reduced La 2 Mo 2 O 7-y Amorphous Phase. The reoxidation of a LAMOX material was already used in order to estimate the oxygen loss after reduction. 6 It is faster than the reduction. However, the phase evolution during reoxidation of the amorphous reduced phase La 2 Mo 2 O 7-y had never been described. The thermogravimetric and DTA measurements were performed in air on two reduced samples (SSR and DEG) and are presented in Figure 9.

Beforehand, both initial La 2 Mo 2 O 9 powders were reduced to the amorphous phase in TG apparatus at 700 °C under 10% H 2 / Ar and a flow rate of 4.8 L•h -1 , resulting in compounds with an approximate oxygen content of 6.7 (corresponding to a mass loss of 6.1%). The thermogravimetric curves of reoxidation, recorded upon heating up to 700 °C at 10 °C•min -1 in flowing air (6 L• h -1 ) and then cooling down, show a reoxidation depending on the specific surface area of the initial compounds: the reduced DEG sample starts to reoxidize around 200 °C, whereas the reduced SSR sample starts to reoxidize around 350 °C (Figure 9a). After a complete cycle the mass gains suggest complete reoxidation. In the same way, DTA measurements present a comparable thermal history with an exothermic transfer hump corresponding to reoxidation of the material, in agreement with the mass gain (Figure 9b). At 475 °C for the reduced SSR sample and 525 °C for the reduced DEG, the samples show a second exothermic peak. XRD patterns recorded below this exothermic peak's temperature show a purely amorphous phase, whereas above this temperature crystalline La 2 Mo 2 O 9 appears. It indicates that these exothermic signals correspond to crystallization peaks. The thermal behaviors around 560 °C (see inset) match up with the structural α/β phase transition of La 2 Mo 2 O 9 on the DTA measurements. 5 In order to observe the reoxidation kinetics of the amorphous phase, thermogravimetric measurements in air at 350 °C for 60 h were performed on the amorphous La 2 Mo 2 O 6.66 compound, obtained from SSR synthesis followed by reduction in TG apparatus at 700 °C (Figure 10). The mass gain is faster during the first hours than later on. In this condition, after 12 h of reoxidation the sample is still amorphous, and after 60 h (see inserted box on the Figure 10), a weak reflection appears at 2θ = 28°, corresponding to the most intense peak of La 2 Mo 2 O 9 . Note that after 60 h the sample is still gaining weight, indicating that thermodynamic equilibrium has not been reached yet. Regarding to the mass gains measured after cooling down to room temperature, the stoichiometries corresponding to 12 and 60 h of reoxidation are, respectively, La 2 Mo 2 O 8.81 and La 2 Mo 2 O 8.85 . The 12 h reoxidized powder presents a dark brown color, whereas after 60 h it presents a light brown/orange color. The same mathematic model as used previously for the reduction did not enable us to fit correctly the reoxidation kinetics (may be because of oxygen diffusion).

The same types of experiments were also performed in a muffle furnace. A reduced SSR sample was heated up by placing the powder directly in a preheated furnace at different constant temperatures during different annealing times (see Figure 11a, for the RT DRX patterns after cooling down). As mentioned previously, the reoxidized compounds keep their amorphous structure after 12 h and even after 4 weeks at 350 °C. For a temperature of 400 °C, after 1 h, the structure remains mostly amorphous, with a weak reflection at 2θ = 28°on the XRD pattern. However, at higher temperature it recrystallizes (only a slight undulation of the background is detected in the compound reoxidized at 450 °C). A similar behavior is observed for reoxidation of sample obtained by DEG (Figure 11b) in muffle furnace in the same conditions. However, recrystallization of this sample occurs at much lower temperature than with sample obtained by SSR, which is consistent with the specific surface areas.

After 12 h of annealing in air at 100 °C, the DEG compound remains amorphous. At higher temperature, mixtures of amorphous and crystalline phases are observed (as, for instance, after 12 h at 150 °C, see Figure 11b). At 500 °C after 1 h, the compound appears totally (or at least largely) crystallized as La 2 Mo 2 O 9 with a mean crystallite size of 53 nm compared to the initial powder of 41 nm (as estimated from XRD peaks fwhm after deduction of apparatus contribution, according to the Scherrer equation). After annealing, the powders of all these samples do not present a homogeneous color: a brown color matrix with few white dots, while white phase content increases with annealing temperature (confirming that thermodynamic equilibrium has not been reached in any of these samples). We can also notice a difference on the crystallization temperature between DTA and muffle furnace, due to a thermal dynamic regime with air flow in DTA compared to the static regime in muffle furnace.

As for the reduction, higher gas/ceramic contact surface is related to faster oxidation kinetics. Also, these results show that the molybdenum cations, into the amorphous phase, accept a large variety of oxidation states and mixed valences. It results in a complex valence continuum during reoxidation. For instance, the La 2 Mo 2 O 6.66 compound, where molybdenum has an average None of these experiments left trace amounts of crystallized La 2 Mo 2 O 7 . 33 The exsolution of metallic molybdenum is an interesting feature. It could improve the electrocatalytic properties of the materials. Anyway, it shows that the amorphous phase is not in a thermodynamically stable state. The operating conditions of a SOFC fuel cell could however be responsible for its stabilization.

Finally, we reported here for the first time the ability of the amorphous phase to get highly reoxidized in air at low temperature while keeping its amorphous structure, thus enlarging its ranges of mixed Mo oxidation states and ionicelectronic conductivity properties. This is a significant result, especially when considering the presumed role of such type of slightly reduced amorphous phase in the huge increase of conductivity recently measured in La 2 Mo 2 O 9 nanowires. 14

Figure 1 .

 1 Figure 1. Thermodynamic stability of La 2 Mo 2 O 9 and reduced forms under low oxygen pressure at 608 (blue), 718 (green), and 1000 °C (red) (adapted from refs 8 and 10).

Figure 2 .

 2 Figure 2. Thermogravimetric measurements of La 2 Mo 2 O 9 reduction at 700 and 760 °C under 10% H 2 /Ar for SSR sample: (a) Average oxygen content versus time; (b) reduction rate versus time.

  After 25 h at 760 °C, the reduction rate is 0.009. According to the XRD patterns, the mechanisms occurring during the reduction at 700 and 760 °C are different. Reduction at 700 °C does not occur through the intermediate phase La 7 Mo 7 O 30 , which is consistent with previous papers on the formation of La 7 Mo 7 O 30 : Goutenoire et al. synthesized this phase by reduction of La 2 Mo 2 O 9 under a flow of 6% H 2 and 94% N 2 at 760 °C, 7 and Vega-Castillo et al. (see Figure 1) were able to stabilize this intermediate phase in the pO 2 range of 10 -17 Pa < pO 2 < 10 -11 Pa at 718 °C, while at 608 °C no La 7 Mo 7 O 30 phase was detected above 10 -20 Pa O 2 . 10

Figure 3 .

 3 Figure 3. XRD patterns of the phase evolution of La 2 Mo 2 O 9 SSR samples during reduction at (a) 760 and (b) 700 °C.

Figure 4 .

 4 Figure 4. Thermogravimetric measurements of La 2 Mo 2 O 9 reduction at 700 °C under 10% H 2 /Ar for SSR, DEG, and EG samples: (a) Average oxygen versus time (SSR = upper curve); (b) reduction rate versus time.

Figure 5 .

 5 Figure 5. Results of the quantitative Rietveld analyses of the amorphous and molybdenum phases on the DEG sample reduced 34 h at 700 °C under 10% H 2 /Ar, with cerium oxide as internal standard (R p = 22.8%, R wp =18%, R exp = 8.07%, χ 2 = 5%).

)

  Since this kinetic law is based on spherical particles, only the reduction made on the SSR and EG samples were fitted.Let us start with the model of the SSR sample reduced at 700 °C. The following reactions are deduced from the XRD and TGA observations presented previously

Figure 6 .

 6 Figure 6. Illustration of the kinetic law model with interface progression for (a) a single reaction and (b) a double reaction.

Figure 7 .

 7 Figure 7. Relative penetration and average oxygen content in La 2 Mo 2 O 9-δ versus time of reduction under 10% H 2 /Ar at (a) 700 °C on SSR sample, (b) 700 °C on EG sample, and (c) 760 °C on SSR sample.

Figure 8 .

 8 Figure 8. XRD patterns of the reduced DEG sample after 34 and 60 h. Note the low angle distortion in the first amorphous hump of the sample reduced at 60 h (see arrow and comment in the text) compared to the other.

Figure 9 .

 9 Figure 9. Thermal behavior during reoxidation of the reduced SSR and DEG samples: (a) thermogravimetric measurements; (b) DTA measurements, (inset) α/β La 2 Mo 2 O 9 transition of the SSR sample.

Figure 10 .

 10 Figure 10. Thermogravimetric measurement during reoxidation of the La 2 Mo 2 O 6.66 SSR sample at 350 °C; (insert) XRD pattern after 60 h of reoxidation.

Figure 11 .

 11 Figure 11. XRD patterns after reoxidation in a muffle furnace for different annealing times at different temperatures of reduced SSR (a) and DEG (b) samples.

Table 1 .

 1 Specific Surface Areas, According to the BET Method, of Powders Obtained by SSR and Polyol Process

	synthesis	specific surface area
	SSR	<1 m

  2

  2

  2

Table 2 .

 2 Compositions after 17 and 34 h of reduction are La 2 Mo 1.95 O 6.05 and La 2 Mo 1.94 O 5.90 , respectively. In these compositions, molybdenum cations present average oxidation states of +3.1 and +3.0, respectively.

Table 2 .

 2 Results of the Quantitative Rietveld Analyses of the Metallic Molybdenum and Amorphous Phases Using the Internal Standard Method

	reduction	oxygen	amorphous weight	Mo weight
	time	loss	fraction	fraction	formula
	17 h	7.76%	99.2%	0.80%	La 2 Mo 1.95 O 6.05 + 0.05Mo
	34 h	8.09%	98.9%	1.10%	La 2 Mo 1.94 O 5.90 + 0.06Mo

  La 7 Mo 7 O 30 during the transformation into the amorphous phase La 2 Mo 2 O 7-y (reactions A′ and A″). As previously shown, the exsolution of metallic molybdenum has been considered until reduction into primary oxide La 2 O 3 + 2Mo.

	′ Reaction A :
	La Mo O 2 2 9	→	La Mo O (La Mo O ) 7 7 30 2 2 8.57	α A	′	=	0.43 6
											(24)
	′′ Reaction A : La Mo O 7 7 30	→	La Mo O 2 2 7	-	y	α A	″	=	+ 6 1.57	y
											(25)
	Reaction B: La Mo O 2 2 7	-	y	→		La Mo O 2 2 6.2	α B	=	0.8	-6	y
											(26)
	Reaction C: La Mo O 2 2 6.2	→	La O 2 3	+	2Mo	α C	=	3.2 6
											(27)

Table 3 .

 3 Summary of the Parameters of Each Single Reaction Obtained by Modelization of the Kinetic Reduction

		SSR sample reduction 760	°C	SSR sample reduction 700	°C	EG sample reduction 700	°C
	k A			0.090 h -1		0.80 h -1
	(tfc) A			11 h		1.3 h
	α A			0.768		0.370
	k A′	0.50 h -1
	(tfc) A′	2.0 h
	α A′	0.072
	k A″	0.23 h -1
	(tfc) A″	4.3 h
	α A″	0.298
	k B	0.037 h -1		0.0090 h -1		0.080 h -1
	(tfc) B	27 h		1.1 × 10 2 h		12.5 h
	α B	0.097		0.232		0.097
	y	0.22		0.15		0.22
	k C	0.70 × 10 -3 h -1			0.90 × 10 -3 h -1
	(tfc) C	1.4 × 10 3 h				1.1 × 10 3 h
	α C	0.533				0.533
	t 0	15.2 h				7.44 h
	RD	0.48%		2.60%		0.67%

DOI: 10.1021/acs.inorgchem.5b02876 Inorg. Chem. 2016, 55, 2522-2533

■ AUTHOR INFORMATION

Corresponding Author *E-mail: philippe.lacorre@univ-lemans.fr.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge Region Pays de la Loire for financial support (PERLE2 project, convention no. 2010 10302) and Pr. Francois Goutenoire for the helpful discussion concerning quantitative Rietveld analyses.


M Gtzeskowiak

P Descamps

J Vindevoghel

CONCEPTION OF INTEGRATED POLYMER ANTENNAS FOR CHARACTERISATION OF DIELECTRIC PROPERTIES

To design microstrip antennas (l), we have to know the relative permittivity and losses of the antenna substrate. In the case of polymers, these electrical properties are given at 1 MHz by the constructor and are not the same ones at other frequencies. We decide to take this problem back to front and realise IS-band antennas to obtain the electrical properties of the polymer (2) in the K-band (from 20 to 30 GHz).

INTRODUCTION

Compact antenna are required for millimeter-wave active applications, such as tracking or people identification. To obtain small-sized and light antemas, we have to realise them by means of photolithography processes and to use thick dielectric polymer, whose electrical characteristics are only known at 1 NHZ.

There are several measurement methods to determine the relative permittivity and the losses of a dielectric in K-band (3,4). However, to realise the measurement prototype, it is necessary to add many technological steps (as baking on hotplate for instance) to the process that allows to realise antennas, which can change the electrical properties of the dielectric. We decided to realise different antennas: firstly to determine the electrical properties of the dielectric and secondly to venfy these obtained values.

ANTENNA SUBSTRATE AND DESIGN

We report on integrated proximity-fed antennas, the configuration of which allows the optimisation of both the antenna and active substrates. The active component substrate is gallium arsenide and the substrate of the antenna is an ultrathick dielectric polymer, Amoco Ultradel 7505 (2) (a photosensitived epoxy resin). The antenna configurations are presented in figures 1 and 2. The resonators are made on a polymer substrate and the transmission lines are made on a GaAs substrate, whose thickness, relative permittivity and losses are respectivelly 400 pn, 12.8 and tan& = 0.08 in the 18-26.5 GHz band. The first antenna, whose photography can be seen in figure 3, is presented in figure 1. The resonator is electromagnetically coupled to the transmission lines through the polymer. The design of the second antenna (figure 2) is presented in figure 4. The driving patch is electromagnetically coupled to a microstrip line. The driving patch is loaded by parasitic ones and they form the array of the antenna These parasitic patches are electromagnetically influence-coupled together.

TECHNOLOGICAL PROCESS

The fabrication process (Figure 5 ) is d i s e d in clean rooms :

The resin 1400-27 is deposited by spincoat operation on a GaAs substrate (Figure 5,A). Following a pre-bake on hotplate for 3 min at 1OO"C, it is exposed to near ultraviolet light to create the desired pattern (Figure 5, B). We develop using 1400-27 developer. The exposed resin is removed in the developer (Figure 5,C). A plating plane is deposited by thermal evaporation of Time and then of Au on the resin 1400-27 (Figure 5,D). The operation of lift-off allows us to remuve the resin, where it has been exposed and to obtain transmission line fabrication (Figure 5, E), Polymer is then applied by spin-coating to achieve the wanted thickness of the dielectric. Following a pre-bake at 100°C on a hotplate, the polymer is exposed to near ultra-violet light. A pre-bake in a 175OC nitrogen oven is made, then we develop using D510 developer and finally a-post-bake on a hotplate is made during about 3 minutes. Then the resonator is deposited on the polymer by thermal evaporation of Ti and Au. Then the resin is deposited by spin-coat operation (Figure 5, G) and we follow again the processes described in B, C, D, E Figures. We realise a square element.

EXPERIMENTAL RESULTS

The antennas mounted in Kcell are shown in figures 3 and 4. A K-cell (5) allows the trmmission between microstrip line (of the microstrip antenna) and K- connections (of the measurement systems). We measure the frequency evolution of SI1 with the HP-3561A dynamic signal analyser and the gain of the antenna with HP-E4418A power meter in 18-26.5 GHz (Figures

Dielectric Materials, Measurements and Applications Conference Publication

No. 473, 0 IEE 2000 6, 7). The measufed resonant frequencies are respectively 26 GHz in the case of the single square resonator, whose dimension is taken as 1.86 mm, and 25.7 GHz in the case of the 3 element-amy, whose the spacing between the elements is 0.5 mm. The measured gain of antennas are respectively 1 ClB and 3 dE3 at the resonant fkequencies.

RETROSIMULATION

The CAD software Ensemble (6) is used for retrosimulation. The Ensemble Design simulation engine is based on a full-wave approach using the mixed-potential integral equation formulation in conjonction with the method of moments.lt takes into account all mutual effects. In the mixed-potential integral equation approach, the field everywhere in space is expressed in terms of the unknown electric and magnetic surface currents on the finite-shape conductors and apertures through the use of the appropriate scalar and vectorial Green's functions. The Green's functions for a layered medium are generally expressed in terms of a semi-infinite spectral integral known as the Sommerfield integral. The simulation engine uses both electric and magnetic aments to modelise mctures with patches on metal layers and apertures on infinite ground layers. The solution process in the simulation engine involves decomposing the integral equation into a matrix equation by applying the method of moments to solve for the unknown current (6). However no detailed technological knowledge on the analytical development of the simulation engine is reqwed to use Ensemble Design. We define the Werent substrates by thickness, relative permittivity, losses, permeability and the metallisation by conductivity and thickness. The thickness of the polymer is h2 = 40 pm. The properties of the GaAs substrate are : h, = 400 Ĩ I U , nl = 12.8 and tan81 = 0.08.

These values are well-known and constant. The resonant frequency depends on the relative permittivity when the jpin and the matching depend on the losses. The relative permittivity ( E ~z ) and the losses (tan%) of the polymer are varied in order to fit the measuTed results. We obtain a relative permittivity of Er2 =2 to have the resonant frequency at 26 GHz and losses of tan& = 0.1 to obtain a gain of 1 dB and matching of = -30 dB at the resonant frequency (Figure 6). We do simulations for this 3 elements antenna with relative permittivity and losses obtained for the single element antenna, and compare with measurement of 3 elements antenna (Figure 7). The gain and the matching at the resonant frequency show the same gain and a slight dscrepancy for the matching (-15 dB instead of -18 dT3 expected). We observe a discrepancy of the

Figure 1 :

 1 Figure 1 : Structure of proximity-fed antenna

Figure 4 :Figure 3 :

 43 Figure 4 : 3 elements antema mounted in a K-cell

Figure 2 :Figure 6 :Figure 7 :

 267 Figure 2 : Explosed vi 4 of proximity-fed 3 elements antenna

  GHz band by means of experimental measurement: frequency evolution and gain of a microstrip antenna integrated on the polymer layer. The discrepancy between the relative permittivity obtained with the tint antenna and the second one is equal to 12 % and can be amibuted to K-cell measurements, but not to the technological processes or dimensions of the antennas (2 pm.accuracy) The losses of the dielectric are well determined but appear too important for electronic applications.

	CONCLUSION
	We have obtained a relative permittivity Erz between 1.8
	and 2, and losses tan& equal to 0.1 for the polymer in
	the 18-26.5
	resonant frequency (25.7 GHz when measured instead
	of 25.2 GHz when simulated). The measured resonant
	fresuency (25.7 GHz) agtees with a relative permittivity
	of Er2 = 1.8

PERSPECTIVES

Other antennas, such as electromagnetically-coupled through a slot, have been investigated, but are limited by too important losses due to thicker dielectric

[START_REF] Grzeskowiak | ~Antennes multicouches integrcks sur Arskniure de Gallium pour applications antennes actives faible port&>>[END_REF]

. A comparative study with other electrical properties characterisations (3,4) is being investigated.


Monolithic Microwave Integrated GaAs Electromagnetically-Coupled Antenna Marjorie Grzeskowiak, Jean Vindevoghel IEMN, Villeneuve d'Ascq, France Abstract-Monolithic antenna on GaAs to be used in miniature active sensors is presented. After a first study of technological processes and the characterisation of various substrates in K-band, the feasibility of an antenna, operating in K-band, electromagnetically-coupled and integrated on GaAs by monolithical realisation, has been demonstrated. A low profile (1mm) and wide bandwidth (2.3 GHz around 25 GHz) antenna is obtained. Index Terms -Active antennas, GaAs MMIC I. INTRODUCTION In recent years, there has been growing interest in the development of communication systems for localisation identification of objects, automatic tolling and livestock management. Compared to other techniques, the use of microwaves in such systems offers a variety of advantages including contactless information and large bandwidth for high-speed data transfer.

These systems ask for compact antenna design. The use of electromagnetic-coupled antenna in the range of millimeter waves allows to obtain small-sized and light antennas. Up to now, most active multilayer antennas were realised by reporting the active components directly on the antenna substrate [1]. In order to avoid bad connections due to air gaps and provide an accurate feeding position the antennas are fully realised by means of technological photolithography processes. A microwave proximity-fed antenna [START_REF] Grzeskowiak | A wideband compact antenna for K-band applications[END_REF] is realised through the technological optical processes and a bandwidth of 400 MHz is obtained for a single resonator at 26 GHz.

This paper reports on a monolithical electromagnetically-coupled antenna on GaAs substrate. This structure avoids any interaction between the resonator and the transmission line and allows to process on the two sides of GaAs substrate: on one side the antenna and on the other side the active components. However this structure requires more technological processes (thermal constraint because of thicker dielectric substrate, two-side alignment).

II. DESCRIPTION OF THE ANTENNA A. Structure of the antenna

The structure of the antenna is presented in Fig. 1. It is a two-layer structure whose radiating part is a single resonator. This patch is deposited on a dielectric substrate with a low relative permittivity E ,giving a good radiating efficiency. To avoid any interaction with the resonator, the transmission line is built on the opposite side of the antenna on a high-permittivity substrate with a relative permittivity E r The patch is fed by electromagnetic coupling to a 50-Q microstrip feedline through a slot-aperture in the ground plane [START_REF] Pozar | Microstrip antenna aperture-coupled to a microstrip line[END_REF]. Furthermore, this design allows both a reduction of the complexity of the feeding network (which is reduced to a simple transmission line) and reduces the losses, The CAD software Ensemble [4] is used to design the antenna. The Ensemble Design simulation engine is based on a full-wave approach using the mixed- potential integral equation formulation in conjunction with the method of moments. It takes into account all mutual effects. In the mixed-potential integral equation approach the field everywhere in space is expressed in terms of the unknown electric and magnetic surface currents on the finite-shape conductors and apertures through the use of the appropriate scalar and vectorial Green's functions. The Green's functions for a layered medium are generally expressed in terms of a semi infinite spectral integral known as the Sommerfeld integral. The simulation engine uses both electric and magnetic currents to modelise structures with patches on metal layers and apertures on infinite ground layers. The solution process in the simulation engine involves decomposing the integral equation into a matrix equation by applying the method of moments to solve for the unknown current [4]. However no detailed technological knowledge of the analytical development of the simulation engine is required to use Ensemble Design. We define the different substrates by thickness, relative permittivity, losses, permeability and the metallisation by conductivity and thickness.

The properties of the GaAs substrate are : hr = 460 i m, E ,l= 12.8 and tan 6 = 0.08. The thickness of the polymer is h2= 300 t m and a relative permittivity of E, =2 is taken in first step to determine the design of the antenna.

The width of the square resonator is 3.034 mm. The size of the coupling aperture is 2x0.4 mm2. The size of the single resonator antenna is 10 mm2and the thickness of the antenna is thinner than 0.8 mm.

C. Optimised technological process

The optimised process is the following one (Fig. 2).

On one side of the GaAs substrate a transmission line is deposited by thermal evaporation of Ti (200 A) and Au (5000 A ) and on the other side (ground plane) a slot is made : the resin 1400-27 is deposited by spincoating operation on both sides of the GaAs substrate (Figure 2,A). Following a pre-bake on a hotplate for 3 min at 100°C, it is exposed to near ultraviolet light to create the desired patterns (Figure 2: B). We develop using 1400-27 developer. The exposed resin is removed in the developer (Figure 2,C). A plated plane is deposited by thermal evaporation of Titane and then of Au on the resin 1400-27 (Figure 2,D). The lift-off operation allows us to remove the resin where it has been exposed and to obtain transmission line and slot fabrication (Figure 2,E).

Resin Su-8 [5] is then applied on the ground plane by spin-coating to achieve the wanted thickness of the dielectric. Following a pre-bake at 100°C on a hotplate and a bake in a 105°C oven during 5 hours, the resin is exposed to near ultra-violet light. These different steps polymerize the resin Su-8 and have been optimised to avoid thermal constraints. Then the resin is deposited by spin-coating operation (Figure 2: G) and we follow again the processes described in figures B, C, D, E. We realise a square element.

The alignment through a thick dielectric needs to use a see-through dielectric.

D. Technological constraints

During the technological realisation of the antenna. we were confronted with thermal constraints. On the figure 3, we can see the result of cooling of Resine Su- 8 on GaAs substrate: the GaAs substrate -460 urn thick -has been broken by action of 300 urn of Resine Su-8 during the temperature decrease from 105°C to 25°C.

Figure 3 Thermal constraints

To solve this problem we used a slow decrease of temperature associated with a silicium substrate sticked under the GaAs substrate in order to rigidify it. This solution allowed to realise antenna with resine Su-8 substrate.

E. Antenna in K-cell

Figure 2: Technological process In order to characterise microstrip antennas at 25 GHz, we use K-cell [6]. A K-cell allows the connexion between the transmission line of antenna and the ports of the measurement set-up. In figure 4.a we can see how the microstrip line antenna is connected to a K connector.

The originality of this K-cell is in the insert shape (Figure 4.b)) due to the structure of antenna: the microstrip line and the resonator are each on a side of the antenna. The slot in the insert allows the radiation of the antenna. The size (Figure 4.b) a, b, c) of the slot is calculated to prevent any coupling between the resonator and the K-cell. The height d of the insert depends on the mechanical constraints due to the diameter of the screw. The antenna was built and mounted in a K-cell [6] (Fig. 5.) to allow to measure the frequency evolution of the Sl parameter (Fig. 6) and the radiation patterns (Fig. 7) at the resonating frequency.

A. Frequency evolution of the S11parameter Figure 6: S,, measured vs frequency for the single resonator antenna The resonating frequency is 24.985 GHz: moreover the measurements show a wide bandwidth (Fig. 6): 2.3 GHz for this single resonator antenna. It can be noted that the bandwidth measured for a single electromagnetically-coupled resonator is very wide (2.3 GHz) compared with a 1 element proximity-fed antenna (400 MHz) [2]. They have been measured at 24.985 GHz. We can see in figure 7 the aperture angle. It is equal to 60 'C in the B-plane and 44 'C in the H-plane. As for every 1- element microstrip antenna, we can note that the aperture angle is large.

The measured gain is 0.5 dB at 24.985 GHz: this low value can be explained by important dielectric losses.

D. Substrates of Monolithic Microwave Integrated GaAs Antenna

It must be noted that the commercial polymers. used as antenna substrates. are only characterised at 1 MHz by the maker. so that the electrical characterisation of polymer properties after the technological processes optimisation is required in K-band to obtain an accurate design of the antenna [ 7].

IV. CONCLUSION

The feasibility-study of monolithical antenna on GaAs substrate in K-band for active antenna applications has been presented. Size reduction. full integration. wide bandwidth have been obtained for this antenna. but however a too low gain due to important dielectric losses is obtained An accurate characterisation of the electrical properties of the polymer in K-band is being investigated.

3 .

 3 Figurel1 Multilayer structure of the antenna B. Dimensions of the antenna

Figure 4 :

 4 Figure 4: a) K-cell b) insert (a=5 mm; b=15 mm: c=15 mm; d=9 mm)


Keywords:

Hardware Trojans (HTs) are ultimately a dangerous threat in semiconductor industry. The serious impact of HTs in security applications and global economy brings extreme importance to their detection and prevention techniques. This paper focuses on developing a HT prevention techniques through a layout level design approach. The principle is to let no available space on silicon for an attacker to insert a HT. Experiments determine the maximum occupational rate and critical empty spaces while filling with standard cells. The proposed technique makes HT insertion nearly impossible.

Original design Added circuit in empty space Benchmark Occupation ratio (Initial)

	Attack			Output Signature
	on empty	Attacked component	Input	Before	After
	space			attack	attack
			Occupation ratio (Final)	Added FFs	Added cells	Left usable space (%)
	View publication stats View publication stats


Benjamin Moreau

Nicolas Terpolilli

Patricia Serrano-Alvarado

A Semi-Automatic Tool for Linked Data Integration

Linked Data (LD) is a set of best practices to publish data in RDF format. Transforming structured datasets into RDF datasets is possible thanks to RDF Mappings. To be able to define such mappings, it is necessary to be familiar with the LD practices and to know perfectly concerned datasets. An obstacle to the democratisation of the LD is that few people satisfy these two conditions. We believe that tools making easy the process of LD integration will foster the LD growth. In this demonstration, we present a chatbot-like tool that can semi-automatically generate RDF mappings for existing structured datasets. The challenge is to automate part of the integration process that requires getting familiar with RDF.

Introduction and Motivation

Linked Data (LD) is a set of best practices to publish data in RDF 3 format. Data published as LD are described according to ontologies that represent relationships (i.e., properties) between concepts (i.e., classes) of a domain. RDFS 4 and OWL 5 are semantic web languages to describe ontologies. Using existing and widely used ontologies increases interoperability among LD datasets.

An RDF Mapping defines the transformation of a structured dataset (columnbased, JSON, etc.) into an RDF dataset. It maps columns of a dataset to terms of an RDF graph.

Writing mappings is not easy. Consider Figure 1 that shows an excerpt of a structured dataset describing Roman Emperors and Figure 2 that represents an RDF mapping allowing to transform such dataset in RDF. Writing this mapping requires to answer several questions, for instance: (i) what concepts contain the Name and Birth city columns? In this case, Name contains entities that are Persons (emperors) and Birth city contains entites that are Places (cities). (ii) What are the relationships between these two concepts? Here, Places are birth places of Persons. (iii) Which existing ontologies are relevant to describe these concepts? In this example, DBpedia, GeoNames, etc. Answering these questions requires both, to know the dataset perfectly and to be familiar with RDF concepts such as RDFS, OWL and RDF mapping languages. Unfortunately, many data producers are not familiar with RDF and are not yet ready to invest time to integrate their data. In this work, we focus on how to simplify as much as possible the integration of existing structured datasets as Linked Data. The challenge we face is to automate part of the integration process that requires getting familiar with RDF.

RML [START_REF] Dimou | RML: A Generic Language for Integrated RDF Mappings of Heterogeneous Data[END_REF] and SPARQL-Generate [START_REF] Lefrançois | A SPARQL Extension For Generating RDF From Heterogeneous Formats[END_REF] are two RDF mapping languages. Even if there exist simplified and human-readable syntaxes of mapping languages like YARRRML [START_REF] Heyvaert | Declarative Rules for Linked Data Generation at Your Fingertips![END_REF], writing a mapping requires to be familiar with RDF. Recently, interesting tools like KARMA [START_REF] Gupta | Karma: A System for Mapping Structured Sources Into the Semantic Web[END_REF], RMLeditor [START_REF] Heyvaert | RMLEditor: a Graph-Based Mapping Editor for Linked Data Mappings[END_REF] or Juma [START_REF] Junior | An Editor that Uses a Block Metaphor for Representing Semantic Mappings in Linked Data[END_REF] have been proposed to assist users during the creation of an RDF mapping. However, these tools are not easy to use for users that are not familiar with RDF concepts.

We propose a chatbot-like tool that can generate an RDF mapping from a structured dataset by only asking simple questions to users about their dataset. Our tool can simply and quickly integrate datasets as Linked Data and can encourages new users to make their first steps into Linked Data.

A Chatbot-Like Tool for Linked Data Integration

To generate an RDF mapping from a structured dataset, our tool uses two knowledge graphs, DBpedia and YAGO, the ontologies of LOV 6 , and the semantic web languages OWL and RDFS.

Roughly speaking, from a set of instances of each column, our tool searches corresponding entities in DBpedia and YAGO. The goal is to find a class corre-sponding to each column. Then, similarly, LOV is used to find the most relevant properties that may correspond to column names, such that instances of a column correspond to the object of a property. To confirm and complete these correspondences, the tool asks simple questions to the user. User confirmed correspondences allow to generate a first RDF mapping. Finally, this mapping is saturated with entailment rules of OWL and RDFS. 7Using the Roman emperor dataset of Figure 1, in the class correspondence step the Augustus value of the Name column corresponds to the entity http: //dbpedia.org/resource/Augustus of the class dbo:Person in DBpedia. Thus, Augustus is identified as an entity of the class dbo:Person. In this example, we obtain two class correspondences suggesting that columns Name and Birth City contain respectively entities of the classes dbo:Person and dbo:Place. These correspondences are suggested to the user with simple yes or no questions: "Does the column Name in your dataset contain Persons?". In order to hide URIs, questions are built using the rdfs:label property of classes.

In the property correspondence step, our tool obtains 5 property correspondences suggesting that columns Name, Birth, Birth City, Lat and Long are respectively objects of properties dbo:name, locah:dateBirth, dbo:birthPlace, geo:lat and geo:long. Again, these correspondences are suggested to the user with simple yes or no questions: "It seems that the column Lat is the latitude of a Spatial thing. Is it true?". Question is built using the rdfs:label and the rdfs:domain of the property.

To complete confirmed correspondences, the tool asks the user to select the column of the dataset that will correspond to the subject of the property. If the user confirms the geo:lat correspondence with column Lat, the tool asks "latitude is a characteristic of a Spatial Thing. Select the column that contains Spatial Thing.". In our example, if the user answers correctly, the column Birth Province is used as the subject of the geo:lat property.

Our tool uses heuristics to reduce the number of questions. It suggests at most one class and one property for each column. Only the class that corresponds to the most instances of a column is suggested. The property that is suggested for a column is the property that has the best popularity score in the LOV answer. Our tool does not suggest a property if its LOV score is lower than a fixed lower bound. Moreover, to improve the pertinence of suggested properties, the type of a column can also be added in the text search. In our example, it searches for the property Birth date instead of Birth.

From user confirmed correspondences, our tool generates a first RDF mapping. In a final step, our tool saturates this mapping by applying RDFS and OWL entailment rules. Using the range and domain of all properties (rdfs:range and rdfs:domain), new classes are infered. This is possible because the domain of a property represents the class of the subject and the range represents the type (i.e., a class or the literal datatype) of the object. In our example, for instance, the user defined Birth Province as the subject of the latitude property. In the GeoNames ontology, the rdfs:domain of this property is geo:SpatialThing. Thus, our tool infers that the column Birth Province contains entities of type geo:SpatialThing. Our tool also takes into account owl:equivalentClass, owl:equivalentProperty, rdfs:subClassOf and rdfs:subPropertyOf properties of concerned ontologies. The RDF mapping in YARRRML of our example is available at https://git.io/fjKY6 and the result of the transformation in turtle is available at https://git.io/fjKYo.

Fig. 1 .

 1 Fig. 1. Excerpt of a structured and typed dataset describing Roman emperors.

Fig. 2 .

 2 Fig. 2. RDF mapping for Roman emperors dataset. Text highlighted in grey and beginning with $ are references to a column of the dataset.

https://lov.linkeddata.es/dataset/lov/

We only consider rules 2, 3, 5, 7, 9 and 11 from RDFS: https://www.w3.org/ TR/rdf11-mt/#rdfs-entailment and rules based on owl:equivalentClass and owl:equivalentProperty from OWL: https://www.w3.org/TR/owl-ref

https://data.opendatasoft.com

https://github.com/opendatasoft/ontology-mapping-chatbot

Roman emperors dataset is available at https://data.opendatasoft.com/explore/ dataset/roman-emperors%40public/table/ and mapping of the dataset can be generated at https://chatbot.opendatasoft.com/chatbot/roman-emperors@public

Demonstration

We implemented a chatbot-like tool that is able to generate YARRRML mappings for datasets of the OpenDataSoft's data network 8 . We chose YARRRML because, at our knowledge, it is the most human readable and understandable RDF mapping syntax for users that are not familiar with LD. Source code of out tool is available at GitHub 9 under the MIT license. Our tool is available as a web service at https://chatbot.opendatasoft.com/. During the demonstration, attendees will be able to use the tool to generate RDF mappings for any structured dataset of the network as LD (e.g., Roman Emperors 10 ).


F Ruffier
email: franck.ruffier@univmed.fr

G Portelli

J Serres

T Raharijaona

N Franceschini

From Bees' surface following to Lunar landing

To better grasp the visuomotor control system underlying insects' height and speed control [START_REF] Srinivasan | Honeybee navigation en route to the goal: visual flight control and odometry[END_REF], Portelli et al. 2010a), we attempted to interfere with this system by producing a major perturbation on the free flying insect and observing the effect of this perturbation. Honeybees were trained to fly along a high-roofed tunnel, part of which was equipped with a moving floor. The bees followed the stationary part of the floor at a given height. On encountering the moving part of the floor, which moved in the same direction as their flight, honeybees descended and flew at a lower height (See Fig. 1 E-F). In so doing, bees gradually restored their ventral optic flow (OF) to a similar optic flow value to that they had perceived when flying over the stationary part of the floor. OF restoration therefore relied on lowering the groundheight rather than increasing the groundspeed (Portelli, Ruffier, Franceschini 2010b). (A) Vertical trajectory in the longitudinal plane. On the left, the operator simply pitched the MH forward rampwise by an angle of 10° (between arrowheads 1 and 2). The ensuing increase in groundspeed (up to 3 m/s; see [B]) automatically triggered a proportional increase in groundheight: the MH climbed and flew level at a groundheight of approximately 1 m, as dictated by the OF set-point (ω set = 3 rad/s, i.e., 172°/s, i.e., 2.5V as shown in [C]). After flying 42 m, the MH was simply pitched backward rampwise by an opposite angle of -10° (between arrowheads 3 and 4), and the ensuing deceleration (see [B]) automatically initiated a proportional decrease in groundheight until landing occurred. During the final approach, which started when the MH had regained its completely upright position (arrowhead 4), the robot can be seen to have flown at a constant descent angle, as also observed in bees' landing performances [START_REF] Mv Srinivasan | How honeybees make grazing landings on flat surfaces[END_REF]. Because the landing gear maintains the robot's eye 0.3 m above ground (dotted horizontal line), touchdown occurs shortly before the groundspeed v x has reached zero, and the MH ends its journey with a short ground run. This result can be accounted for by a control system called an optic flow regulator, that is, a feedback control system based on an OF sensor, which strives to maintain the ventrally perceived OF at a constant set point by adjusting the vertical lift (See Fig. 1 A-D) [START_REF] Ruffier | Optic flow regulation: the key to aircraft automatic guidance[END_REF][START_REF] Franceschini | A bio-inspired flying robot sheds light on insect piloting abilities[END_REF]. This visuo-motor control scheme may not only explain how honeybees land at a constant descent angle [START_REF] Mv Srinivasan | How honeybees make grazing landings on flat surfaces[END_REF] but also how they navigate safely along surfaces on the sole basis of OF measurements, without any need to measure either their speed or their distance from the ground, the ceiling or the surrounding walls [START_REF] Serres | A vision-based autopilot for a miniature air vehicle: joint speed control and lateral obstacle avoidance[END_REF], Portelli et al. 2010a), that is, without relying on any of the conventional avionic sensors such as velocimeters or rangefinders. Results obtained in neurophysiological, behavioural, and biorobotic studies on insect flight control were used to safely land a spacecraft on the Moon in a simulated environment. The optic flow regulator for automatic landing was tested in a realistic simulated Lunar environment [START_REF] Valette | Biomimetic optic flow sensing applied to a lunar landing scenario[END_REF]. Visual information was provided using the ESA's PANGU software program and used to regulate the optic flow sensed during the descent of a 2-DOF spacecraft. The results of the simulation showed that a single 2-pixel optic flow sensor coupled to an optic flow regulator was able to robustly control the autonomous descent of the simulated lunar lander (See Fig. 2). "Low gate" located approximately 10 m above the ground was reached with reduced vertical and horizontal speeds of 4m/s and 5m/s, respectively. It was also established that optic flow sensing methods can be used successfully to cope with temporary sensor blinding and poor lighting conditions [START_REF] Valette | Biomimetic optic flow sensing applied to a lunar landing scenario[END_REF], as typically occurs at the Moon south pole that the 2018 Next ESA mission is planning to explore.

Figure 2: Automatic landing based on a biomimetic OF sensor combined with a bio-inspired control scheme. The automatic approach lasted 58.4s, starting from an initial height of 500m, with an initial ground speed of 150m/s and an initial vertical speed of 50m/s. (A) Vertical trajectory in the longitudinal plane. At t l -58.4s (58.4s before reaching "low gate" at time t l ), the lander's pitch angle θ pitch was equal to -60° and decreased exponentially to -30° at t l -10s. The ensuing decrease in the ground speed (150m/s to less than 10m/s at t l -10s) led to an automatic decrease in the ground height and the vertical speed, which kept the measured OF near the set-point. (B) Lunar surface as presented to the simulated lander during the entire landing phase. (C) Ground speed v x (black) and Vertical speed v z (green) monitored throughout the landing phase. (D) Output ω meas of the OF sensor was also monitored during the landing phase and the data recorded show that the OF remained fairly constant during the landing phase, ω set = 1V (0.3rad/s = 17.2°/s). (E) Sketch of the OF-based lunar landing autopilot. The digital autopilot received the following inputs: the pitch angle θ pitch (given by an IMU), the measured OF ω meas (given by an EMD) and the vertical acceleration (given by an accelerometer). In addition, the controller imposed the thrust level and the lander's pitch. Adapted from [START_REF] Valette | Biomimetic optic flow sensing applied to a lunar landing scenario[END_REF]

Figure 1 :

 1 Figure 1: Honeybees altitude control accounting for the ventral optic flow regulator demonstrated here on a robotic Micro-Helicopter (MH) (A-D) Flight Parameters Monitored during a 70 m Flight performed by the robotic micro-helicopter equipped with an Optic-Flow Regulator. The complete journey (over the randomly textured pattern shown in [A]) includes take-off, level flight, and automatic landing. (A) Vertical trajectory in the longitudinal plane. On the left, the operator simply pitched the MH forward rampwise by an angle of 10° (between arrowheads 1 and 2). The ensuing increase in groundspeed (up to 3 m/s; see [B]) automatically triggered a proportional increase in groundheight: the MH climbed and flew level at a groundheight of approximately 1 m, as dictated by the OF set-point (ω set = 3 rad/s, i.e., 172°/s, i.e., 2.5V as shown in [C]). After flying 42 m, the MH was simply pitched backward rampwise by an opposite angle of -10° (between arrowheads 3 and 4), and the ensuing deceleration (see[B]) automatically initiated a proportional decrease in groundheight until landing occurred. During the final approach, which started when the MH had regained its completely upright position (arrowhead 4), the robot can be seen to have flown at a constant descent angle, as also observed in bees' landing performances[START_REF] Mv Srinivasan | How honeybees make grazing landings on flat surfaces[END_REF]. Because the landing gear maintains the robot's eye 0.3 m above ground (dotted horizontal line), touchdown occurs shortly before the groundspeed v x has reached zero, and the MH ends its journey with a short ground run. (B) Groundspeed v x was monitored throughout the journey. (C) Output ω meas of the OF sensor was monitored throughout the journey and shows the relatively small deviation from the OF set-point ω set (in red); even during the transient initial and final stages where the groundspeed varies considerably, see[B]. (D) Ventral optic flow generated ω (calculated as v x /h), output of the feedback loop. This ventral OF resulting from the MH flight pattern was held relatively -but not perfectly -constant throughout the journey. Although a single trajectory is shown in (A), all the take-offs, level flights, and terrain-following and landing maneuvers analyzed were found to be extremely reliable and never led to any crashes.(E-F) Side view of the trajectories of 21 individual honeybees flying freely along a high-roof tunnel under two conditions: over a stationary or a partially moving part of the floor. The horizontal visual field of the camera (20 cm < x < 180 cm) covered the transition between the stationary and moving parts. The latter extended up to x = 210 cm. The blue trajectories were recorded over the stationary floor, and the green trajectories, over the part of the floor set in motion. All error bars are ±SEM. (E) When the floor was stationary, the honeybees flew at a height of 16 ± 1.3 cm above the floor. (F) When the floor was set in motion (at a speed of V Floor = 0.5 m/s) in the same direction as the honeybees' flight, the insects descended and flew at a height of only 10.9 ± 0.7 cm above the floor, restoring the initial ventral optic flow. Adapted from Franceschini, Ruffier, Serres 2007 and from Portelli, Ruffier, Franceschini 2010b

  Figure 1: Honeybees altitude control accounting for the ventral optic flow regulator demonstrated here on a robotic Micro-Helicopter (MH) (A-D) Flight Parameters Monitored during a 70 m Flight performed by the robotic micro-helicopter equipped with an Optic-Flow Regulator. The complete journey (over the randomly textured pattern shown in [A]) includes take-off, level flight, and automatic landing. (A) Vertical trajectory in the longitudinal plane. On the left, the operator simply pitched the MH forward rampwise by an angle of 10° (between arrowheads 1 and 2). The ensuing increase in groundspeed (up to 3 m/s; see [B]) automatically triggered a proportional increase in groundheight: the MH climbed and flew level at a groundheight of approximately 1 m, as dictated by the OF set-point (ω set = 3 rad/s, i.e., 172°/s, i.e., 2.5V as shown in [C]). After flying 42 m, the MH was simply pitched backward rampwise by an opposite angle of -10° (between arrowheads 3 and 4), and the ensuing deceleration (see[B]) automatically initiated a proportional decrease in groundheight until landing occurred. During the final approach, which started when the MH had regained its completely upright position (arrowhead 4), the robot can be seen to have flown at a constant descent angle, as also observed in bees' landing performances[START_REF] Mv Srinivasan | How honeybees make grazing landings on flat surfaces[END_REF]. Because the landing gear maintains the robot's eye 0.3 m above ground (dotted horizontal line), touchdown occurs shortly before the groundspeed v x has reached zero, and the MH ends its journey with a short ground run. (B) Groundspeed v x was monitored throughout the journey. (C) Output ω meas of the OF sensor was monitored throughout the journey and shows the relatively small deviation from the OF set-point ω set (in red); even during the transient initial and final stages where the groundspeed varies considerably, see[B]. (D) Ventral optic flow generated ω (calculated as v x /h), output of the feedback loop. This ventral OF resulting from the MH flight pattern was held relatively -but not perfectly -constant throughout the journey. Although a single trajectory is shown in (A), all the take-offs, level flights, and terrain-following and landing maneuvers analyzed were found to be extremely reliable and never led to any crashes.(E-F) Side view of the trajectories of 21 individual honeybees flying freely along a high-roof tunnel under two conditions: over a stationary or a partially moving part of the floor. The horizontal visual field of the camera (20 cm < x < 180 cm) covered the transition between the stationary and moving parts. The latter extended up to x = 210 cm. The blue trajectories were recorded over the stationary floor, and the green trajectories, over the part of the floor set in motion. All error bars are ±SEM. (E) When the floor was stationary, the honeybees flew at a height of 16 ± 1.3 cm above the floor. (F) When the floor was set in motion (at a speed of V Floor = 0.5 m/s) in the same direction as the honeybees' flight, the insects descended and flew at a height of only 10.9 ± 0.7 cm above the floor, restoring the initial ventral optic flow. Adapted from Franceschini, Ruffier, Serres 2007 and from Portelli, Ruffier, Franceschini 2010b


F Goutenoire

R Retoux

E Suard

P Lacorre

Ab initio determination of the novel perovskite-related structure of La 7 Mo 7 O 30 from powder diffraction

Keywords: ball milling, controlled reduction, reduced lanthanum molybdate, perovskite type rods, structural determination, X-ray and neutron powder diffraction, electron microscopy

A new mixed valence molybdate, La 7 Mo 7 O 30 , first prepared by high energy ball milling, has been successfully synthesized by controlled hydrogen reduction of La 2 Mo 2 O 9 . Its original crystal structure was determined from X-ray and neutron powder diffraction (space group R-3; a = b = 17.0051(2) Å, c = 6.8607(1) Å; Z = 3; reliability factors: R p = 0.081, R wp = 0.091, χ 2 = 3.1, R Bragg = 0.049, R F = 0.033). It consists in the hexagonal stacking of individual cylinders of perovskite-type arrangement. These cylinders are built up from perovskite cages sharing corners in trans-position along their diagonal axis. Two different mixed-valence molybdenum sites coexist, with more (Mo +5.75 ) Or less (Mo +4.5 ) distorted octahedral environments. Lanthanum atoms are located within the perovskite cages and around them, very close to their regular positions in the perovskite structure. Lanthanum and molybdenum atoms thus form two rows of almost perfect cubes, shifted from each other by c/2. An electron microscopy study revealed the defect-free cationic and octahedral arrangements in the (a,b) plane.

performed in a thermogravimetric Setaram TG92 apparatus at a constant temperature of 760°C, under a flow of 6%H and 94%N

. The TGA curve (see Fig. 1) shows an intermediate stage in the reduction process around 1.5% weight loss, suggesting the existence of a definite phase that is slightly reduced compared to La Mo O . The X-ray diffraction pattern of this phase was found to be identical to that of the phase obtained through ball milling. For the neutron diffraction study, a large sample was prepared from several smaller batches. Each reduction was performed with 0.8-1 g of starting oxide heated to 760°C, the temperature being rapidly decreased when the weight loss was approximately 1.5%. This reduction process was performed 10 times to produce 10 g of sample. The sample was finally mixed and heated for homogenization in an evacuated furnace (+10\-10\Pa) for one night at 800°C. In order to check the stoichiometry of the annealed reduced compound, subsequent re-oxidation in air of a small sample was carried out on a TA Instruments DTA-TGA apparatus (see insert Fig. 1).

Structural Characterization

The X-ray and neutron powder diffraction patterns were recorded on Bragg-Brentano diffractometers (Siemens D500 and ILL D2B, respectively); details of data collection are presented in Table 1. The program Electron Diffraction v3.5 [START_REF] Morniroli | Electron Diffraction[END_REF], kindly provided by J. P. Morniroli, was used to find the real symmetry of the structure. The atomic arrangement was determined through Patterson function analyses and Fourier syntheses, using alternatively the programs FullProf [START_REF] Rodriguez-Carjaval | FullProf[END_REF], ShelxS86, and ShelxL93 [START_REF] Sheldrick | ShelxS86[END_REF]. The electron diffraction study (high-resolution and diffraction modes) was performed with a JEOL 2010 electron microscope (tilt $30°) fitted with a eucentric goniometer (60°), and equipped with an EDX analyzer. A few droplets of a suspension of microcrystals (obtained from thorough grinding of a powdered sample) in alcohol were deposited on a carbon-coated holey film. The simulated projected potential of the structure was calculated with the series of programs written by P. A. Stadelman [START_REF] Stadelman | [END_REF].

CRYSTAL SYMMETRY

Both X-ray and neutron diffraction patterns of the wellcrystallized phases could be perfectly fitted on the base of the monoclinic cell determined from electron diffraction of a poorly crystallized sample (9), with space group C2/m and cell parameters a"10.831 A s , b"17.005 A s , c"6.709 A s , "112.03°. However, a careful examination of both patterns showed that the h and k indices of all observed hkl reflections were even, so that the patterns could be fitted as well into a smaller monoclinic cell with parameters a" 5.4099(2) A s , b"8.5026(2) A s , c"6.7075(2) A s , "112.021° (1). No extra reflection in the X-ray and neutron diffraction patterns could justify the doubling of parameters a and b as observed on electron diffraction patterns.

Direct methods were then applied to the observed intensities as extracted [START_REF] Rodriguez-Carjaval | FullProf[END_REF] by the program FullProf (Profile Matching mode) on the basis of the small monoclinic cell, using ShelxS86 (13) and Sirpow92 (15). No satisfying solution resulted from the calculation, either in the small or in the large monoclinic cell. This prompted us to reexamine the diffraction patterns. We noticed that some reflections with equal interreticular distances (such as 020, 111 , and 1 11 with d"4.2503 A s ) had interplanar angles exactly equal to 120°. It strongly suggests that the lattice is hexagonal rather than monoclinic. As a matter of fact, the X-ray and neutron diffraction patterns could be indexed in a hexagonal cell with parameters a"17.0064(2) A s and c"6.8613(1) A s .

Several new reciprocal lattice reconstructions from electron diffraction patterns were made in order to confirm the symmetry. The new reflection conditions found are hkl: !h#k#l"3n, which imply a rhombohedral symmetry. The The electron diffraction pattern presented on Fig. 2b has been indexed with the parameters preliminary found in the X-ray and neutron diffraction study. In that figure, reflection dots can clearly be seen, which imply a doubling of the hexagonal c parameter (small horizontal arrows). Such a doubling could be seen, in the reciprocal lattice reconstructions, on every electron diffraction pattern involving the c axis. However, powder diffraction patterns did not show any notable extra reflection that would induce a doubling of the c axis. Only one or two very faint extra peaks in the X-ray pattern could possibly justify such a doubling, not enough, however, to be taken seriously into account in the search procedure for atomic positioning.

Reflection conditions in the single hexagonal cell suggested a rhombohedral symmetry (cell parameters a"10.0807 A s , "115.012°) with no extra conditions, thus suggesting R , R 3 , R , R K , or, R 3 m as possible space groups.

STRUCTURAL DETERMINATION

The ab initio structural determination was carried out in the hexagonal setting of the rhombohedral single cell, start-ing with the centrosymmetric space groups R 3 m and R 3 . The Patterson function was calculated using program ShelxS86 with integrated intensities as extracted, by the full pattern decomposition method of program FullProf, from the X-ray diffraction pattern in order to locate heavy atoms first. The analysis of the Patterson functions and consideration of distances between cations gave us evidence that space group R 3 was the most appropriate. It allowed us to find two atomic positions M +0.80, 0.00, z#0.25, and M +0.20, 0.00, z. The rest of the atomic sites were located after subsequent cycles of refinement and Fourier syntheses with programs ShelxL93 and FullProf (refinement mode). At the final stage, all the atomic sites and the isotropic thermal factors of the heavy atoms were refined. In these conditions the lowest R reliability factor (7.16%) was obtained for the atomic positions listed in Table 2 (see Fig. 3a). In order to get a better precision on the oxygen atoms location, a structural refinement was performed using the neutron diffraction pattern. With all atomic positions and isotropic thermal factors refined, the reliability factor was lowered to R "4.88% (see Table 2 and Fig. 3b). A refinement in the acentric subgroup R did not improve this result significantly.

The compound's formula proved to be La Mo O , with one formula unit in the rhombohedral cell (rhombic description). Thermogravimetric measurements of weight loss and gain from or toward the parent compound La Mo O appeared to be slightly higher (&1.5%) (see Fig. 1) than expected from the formula ( &1.1%). This slight discrepancy might be due either to the measurement precision or to a slight deficiency on the oxygen lattice. However, subsequent refinements of the occupation rate on oxygen sites (neutron diffraction data) did not show any oxygen deficiency. ? Cell parameters and atomic coordinates (from X-ray data: in italic) are given in the hexagonal setting (full occupancy for all sites). @ The oxygen thermal factor is arbitrary fixed to 1.0 A s .

CATIONS COORDINATION AND VALENCE

The analysis of interatomic distances and angles (Table 3) was based on the atomic parameters as refined from the neutron diffraction data. The La1 coordination polyhedron is a quasi-regular icosahedron (see Fig. 4a), with two groups of close La-O distances: La1-O3"2.59 A s and La1-O5" 2.70 A s . The La2 coordination polyhedron (see Fig. 4a) is more distorted, La2 being surrounded by nine oxygen atoms at distances ranging from 2.44 to 2.85 A s . This polyhedron can be considered as a very distorted monocapped antiprism, but is better described as a nonregular pentagon, dicapped on both sides like in LaPO . The two capping dioxygen groups form two edges almost perpendicular to each other. Valence calculations for lanthanum atoms give #3.35 and #3.08 for La1 and La2, respectively, slightly in excess of the expected #3.

The Mo1 coordination polyhedron is an almost regular octahedron (Fig. 4b) formed by six oxygen neighbors at equal distance (d(Mo1-O5)"1.99 A s ), with 180°trans-oxygen O-Mo-O angles and cis-oxygen O-Mo-O angles between 85°and 95°. The Mo2 coordination polyhedron is a more distorted octahedron (Fig. 4b), with Mo-O distances ranging from 1.77 to 2.21 A s , trans-oxygen O-Mo-O angles from 152°to 170°, and cis-oxygen O-Mo-O angles from 78°t o 105°. Such an environment is comparable to that of molybdenum in MoO , where distances range from 1.67 to 1.95 A s forming a distorted tetrahedron, with two longer distances at 2.25 and 2.33 A s , altogether forming a very distorted octahedron. Quite similarly, the strongly distorted Mo2 coordination octahedron is formed by four short distances between 1.77 and 1.94 A s , and two longer ones at 2.20 and 2.21 A s .

The more or less distorted octahedral coordination most probably reflects a difference in the oxidation states of the corresponding molybdenum atoms. For instance, Mo> in oxides is known to have a regular octahedral coordination, while smaller Mo> has either a very distorted octahedral coordination or a tetrahedral coordination. It is thus tempting to attribute a lower valence to Mo1 than to Mo2, the latter being probably close to #6. In order to estimate the mean oxidation state on each Mo site, a Mo valence calculation was undertaken. Unfortunately, only Mo4' appears in the list of bond-valence parameters compiled by Brese and O'Keeffe in their reference paper (16).

The bond-valence parameters for lower oxidation states of Mo had therefore to be calculated from known structures of reduced molybdates. For the purpose, only known structures with molybdenum sites where an integer oxidation state has been clearly evidenced were considered. This is the case, for instance, of MoO (17) 3, the Mo theoretical valence was computed as a function of x from the bond-valence parameters of Mo4', Mo4, and Mo'4 as determined above.

Figure 5 gives the result of such a calculation, as the evolution with x of the difference between the calculated valence and the corresponding formal charge (4#x for Mo1, (35!x)/6 for Mo2). The most appropriate distribution is obtained for a null difference, which appears to be close to x"0.5. The probable mean valences of Mo1 and Mo2 are thus #4.5 and #5.75, respectively.

STRUCTURE DESCRIPTION

The most interesting feature of this structure is revealed by the connection between Mo atoms. The closest Mo atoms are very regularly spaced (d +\+ "4.009 A s ), with interatomic Mo-Mo-Mo angles very close to 90°(between 89.3°and 90.7°), thus forming almost regular cubes with 4.01 A s edges. The same is true for La atoms with a c/2 shift: they form slightly distorted cubes with edges between 4.25 and 4.27 A s and angles between 86°and 94.2°. In each Mo or La set, cubes share corners along their long diagonal, thus forming infinite chains along the c axis of the structure (see Fig. 6a). A close examination of the positioning of oxygen atoms reveals that the molybdenum coordination octahedra form distorted perovkite cages with La1 lanthanum atoms at their center (see Fig. 6b). La2 lanthanum atoms surround the infinite files of perovskite cages in almost the same manner as in the perovskite structure, the lanthanum cubes being only slightly (6.5%) bigger than the molybdenum cubes. Note that alkaline earth molybdates AMoO (A"Ca,Sr,Ba) adopt a regular perovskite structure with Mo>-Mo> distances close to 4 A s (see [START_REF] Goodenough | Landolt-Bo¨rnstein[END_REF] and references therein) and that the lanthanum molybdate Mo O (8) exhibits perovskite-type layers. The structure of La Mo O is built up from a hexagonal array of cylinders or rods of a perovskite-type arrangement along its three-fold axis (see Fig. 7). A high-resolution electron microscopy image (see Fig. 8) along the c axis of the structure gives a vivid illustration of the very regular cationic arrangement (no defect was observed). Each rod of formula La

SUMMARY AND CONCLUSION

An unprecedented structural type has been obtained in the La-Mo-O system either by energetic ball milling of La O #2MoO in WC, or by controlled reduction of La Mo O under hydrogen. To our knowledge, this is the first time that an entirely new crystal structure type has been prepared by ball milling, at least in the oxide field. This structure is built up from hexagonal stacking of isolated perovskite type building units, which can be described as cylinders along the cubic perovskite [111] axis. This is at variance from perovskite-related structures with fourling units (24), whose main axes run along the perovskite [ Concerning the charge distribution on molybdenum atoms, one can wonder whether the c axis doubling observed in the electron diffraction study does not reflect a possible charge ordering along the c axis between Mo'4 and Mo4 on the Mo1 site. Such a partial charge ordering would be consistent with the #4.5 mean oxidation state found on this site, and furthermore it would be, as actually evidenced, very hard to detect on powder diffraction patterns. Such a charge ordering could also explain the insulating property of the compound.

FIG. 1 .

 1 FIG. 1. Thermogravimetric curve of the reduction of LaMo O under diluted hydrogen (6% H -94%N). In insert: detail of the same curve, together with the thermogravimetric curve of the oxidation of La Mo O under a flow of atmospheric air.

FIG. 2 .

 2 FIG. 2. Electron Diffraction pattern of La Mo O along [001]* (a) and [11 0]* (b) of the hexagonal cell. The ED pattern obtained along [11 0]* has been indexed with the parameters found in the X-ray and neutron diffraction study. Small horizontal arrows point lines of dots suggesting a doubling of the hexagonal c parameter (c+13.7 A s ) which has not been taken into account in the structure calculations (see text).

  [001]* and [11 0]* electron diffraction patterns of La Mo O shown in Fig. 2 evidence the hk0: !h#k "3n, and hhl: l"3n conditions.

FIG. 3 .

 3 FIG. 3. Result of the structural refinement of La Mo O : observed (crosses), calculated (line) and difference (lower) profiles for X-ray (a) and neutron (b) diffraction data.

FIG. 4 .FIG. 5 .

 45 FIG. 4. Coordination polyhedra of the different cations: (a) [La1O ] and [La2O ], (b) [Mo1O ] and [Mo2O ] octahedra.

FIG. 7 .

 7 FIG. 7. Projections of the cubic perovskite (left) and La Mo O (right) structures along the cubic [111] and hexagonal [001] directions, respectively: (a) octahedral network; (b) cationic network. Large circles represent the sections of perovskite cylinders evoked in the text.

  Mo O is shifted by c/3 relative to its neighbors. The coordination polyhedron of lanthanum La2 is formed by oxygen atoms belonging to three neighboring perovskite cylinders, thus ensuring the consistency of the structure. On a formal point of view, this A B O structure can thus be obtained from that of a perovskite by conveniently inserting nine oxygen atoms around A B O cylinders (as previously defined along the perovskite [111] axis) in order to complete B coordination octahedra. The isolated cylinders are slightly rotated around, and shifted along, the cubic [111] or hexagonal c directions.

TABLE 1

 1 Conditions of X-Ray and Neutron Data Collection and Structural Refinement for La 7 Mo 7 O 30

	Diffractometer	Siemens D500 D2B (ILL)
	Radiation 2 range [°]	X-ray CuK 5 -130	Neutron 1.5938 A s 5-160
	Step-scan increment [°]	0.02	0.05
	Counting time	33 s/pt.	3h (total)
	Peak shape	Pseudo-Voigt	Pseudo-Voigt
	Number of reflections	710	820
	Number of refined parameters	35	46
	R	7.16%	4.88%
	R	11.4%	8.13%
	R	13.6%	9.10%
	R	4.18%	5.15%
		10.7	3.12

TABLE 2 Crystallographic

 2 Parameters? of La 7 Mo 7 O 30

	Atom	Site	x	y	z	B ( A s )
	La1	3a	0	0	0	0.79(9)
						0.62(9)
	Mo1	3b	0	0	1/2	0.5(1)
						0.4(1)
	La2	18f	0.7803(1)	!0.0176(2)	0.3345(4)	0.83(4)
			0.7800(1)	!0.0167(1)	0.3342(4)	0.54(3)
	Mo2	18f	0.1992(1)	0.0118(2)	0.1666(4)	0.60(4)
			0.2001(1)	0.0131(2)	0.1660(6)	0.42(4)
	O1	18f	0.2423(2)	0.0989(2)	0.3573(4)	0.97(5)
			0.248(1)	0.089(1)	0.343(2)	1.00@
	O2	18f	0.2931(1)	0.0454(2)	!0.0006(5)	0.87(5)
			0.293(1)	0.039(1)	!0.003(2)	1.00
	O3	18f	0.1719(2)	0.1153(2)	0.0404(4)	1.12(6)
			0.182(1)	0.124(1)	0.014(3)	1.00@
	O4	18f	0.2060(2)	!0.0716(2)	0.3094(4)	0.85(4)
			0.204(1)	!0.075(1)	0.319(2)	1.00@
	O5	18f	0.0344(2)	0.1043(2)	0.3205(5)	0.67(2)
			0.036(1)	0.104(1)	0.319(2)	1.00@

Note. Space group: R 3 (No. 148), rhombic Z"1, hexagonal Z"3. Cell parameters: a"17.0051(2) A s , c"6.8607(1) A s , a"17.0064(2) As , c" 6.8613

(1)

As .

TABLE 3 Selected

 3 Bond Distances (A s ) and Angles (°) for La 7 Mo 7 O 30 Note. Standard deviations are given in parentheses and the number of equivalent distances or angles is given in brackets.

		Molybdenum Mo1 (regular octahedron)
		Mo1-O5		1.991(3) [;6]
		O5-Mo1!O5		94.1(1) [;6]
		O5-Mo1-O5		85.8(1) [;6]
		O5-Mo1-O5		180.0(1) [;3]
		Molybdenum Mo2 (distorted octahedron)
	Mo2	O1	O2	O3	O3	O4	O5
	O1	1.832(4) 105.3(1)	78.1(2)	152.4(2)	95.3(2)	82.5(1)
	O2	2.893(5)	1.807(4) 90.5(2)	89.7(1)	101.4(2)	169.6(2)
	O3	2.564(5)	2.868(6) 78.8(1)	167.6(2)	83.3(1)
	O3	3.658(5)	2.642(4) 2.639(5)	1.935(4) 104.4(2)	80.4(1)
	O4	2.666(5)	2.771(4) 3.962(6) 2.932(5)	1.774(5) 84.4(1)
	O5	2.674(5)	3.991(5) 2.676(5)	2.961(4)	2.689(6)	2.200(4)
			Lanthanum La1 (icosahedron)
		La1-O3			2.595(3) [;6]
		La1-O5			2.699(3) [;6]
			Lanthanum La2
		La2-O1			2.474(4) [;1]
		La2-O1			2.449(4) [;1]
		La2-O2			2.545(4) [;1]
		La2-O2			2.541(4) [;1]
		La2-O3			2.564(4) [;1]
		La2-O4			2.847(4) [;1]
		La2-O4			2.824(4) [;1]
		La2-O4			2.534(5) [;1]
		La2-O5			2.529(4) [;1]

ACKNOWLEDGMENTS

The authors express special thanks to Dr. N. Randrianantoandro (Laboratoire de Physique de l'Etat Condense´Universite´du Maine, Le Mans, France) for access to the planetary mill.