Pierre Montagné

Monique Tillard Agrégats

Monique Tillard
email: mtillard@univ-montp2.fr

On the adaptability of 1/1 cubic approximant structure in the Mg-Al-Zn system with the particular example of Mg32Al12Zn37

Keywords: MgAlZn, intermetallics, crystal structure, X-ray diffraction, quasicrystal approximant, computer simulations, disordered systems

Intermetallic Mg alloys are of interest in the development of light materials with specific properties and some of them are crystalline approximants of quasicrystalline materials.

Mg32Al12Zn37 single crystal structure, cubic, Im 3 , a = 14.1845(1) Å, is disordered and belongs to the well known 1/1 approximant structural type commonly called Mg32(Al1-xZnx)49 or T-phase. In this family, Mg32Al12Zn37 is a new member remarkable for its very special composition nearby intersection of the 2/1 and 1/1 lines that mark composition domains of cubic 2/1 and 1/1 approximants. Structural features are discussed comparatively with literature data to emphasize the great adaptability through atom disorder of the 1/1 structure.

Introduction

The magnesium alloys are attractive for their special properties and have been of great interest in the development of light and high strength alloys to meet industrial or commercial requirements. Systems based on these elements have been thus investigated many times with the aim of understanding processes and better controlling the casting techniques. The discovery of quasicrystalline compounds in various Al-based systems has further motivated many studies, particularly explorations in the ternary Mg-Al-Zn system. The recently reported diagram [1] shows quite complex thermal equilibriums and two ternary phases are identified in this system.

The first, labeled Φ, has been approximated to a stoichiometric compound of formula Mg5Al2Zn2 or Mg6(Al1-xZnx)5 [2][3][4]. Its composition domain is quite narrow with a limited Mg-content (53 to 55 at.%) but spreads over a wide range of Al/Zn ratio (0.64 -1.7). The second phase, reported there as T-phase but elsewhere as R-phase [5], extends over a larger domain ranging from 32 to 42 at.% Mg, 13 to 49 at.% Al and 10 to 49 at.% Zn. Even though Mg-content is known to vary by a few percents, this phase is commonly reported as Mg32(Al1-xZnx)49. As it was first identified by Bergmann [6], the term "Bergman line" has been used to designate its composition domain. At that time, this cubic structure was described by means of atom coordinations (12 to 16) and polyhedral duality. Further investigations established its relationship with icosahedral quasicrystals and shown that it should be considered as a crystalline approximant of type 1/1 [7].

Many stoichiometries are reported in the literature for Mg-Al-Zn alloys, of which some correspond to quasicrystalline materials [8]. In most cases, crystalline and quasicrystalline compounds are identified based on their powder diffraction patterns [9,10], more rarely single crystal structures are provided [11,12]. As evidenced, quasicrystals may form in this system over a wide range of compositions, those formed along the Bergman line are regarded as metastable while those, Mg-rich with low Al-contents (12-15 at.%) are found thermally more stable [13,14].

Quasicrystals of the former class convert into crystals with the 1/1 structure type (T-phase) under heating at ~300 °C [9,13,14]. Instead members of the second class are stable up to ~450 °C and are located in the composition diagram near the 2/1-type domain along a line that we will call the 2/1 line. Cubic structure of type 2/1 was established for a few compositions but complete results are not always reported [15,16]. Other Mg-rich crystalline compounds displaying the orthorhombic symmetry are reported as Φ-phase, the stability of which was studied together with its relationship with icosahedral quasicrystals [2][3][4].

This work brings reliable structural data for a disordered compound having a very special chemical composition. With a weak Al-content as 2/1 approximants, Mg32Al12Zn37 however belongs to the well known 1/1-type Mg32(Al1-xZnx)49. It lies at one extreme of what has been reported before for T-phase, nearby intersection of 2/1 and 1/1 lines. Several structural features are discussed to emphasize the structural adaptability, unravel complexity of structures and contribute to better understanding of the ternary Mg-Al-Zn system.

Experimental section

Metal powders (Mg, 99.8%, 20-100 mesh; Al, 99.5%, 325 mesh; Zn, 99.9%,140-325 mesh, Alfa Aesar) kept and handled in Ar atmosphere were weighed in appropriate amounts (45:15:40), intimately mixed and inserted in a Ta tube weld-sealed under argon protected from oxidation in an Ar-filled sealed stainless steel jacket. Samples were heated to 800 °C for 16 h and submitted to a continuous rotation to ensure a good homogenization. A subsequent thermal treatment, based on that reported for 2/1 approximant [16] was applied: cooling to 360 °C at 10 °/h, keeping temperature for 100 h before quenching in cold water. A part of the grey and shiny product, seemingly homogeneous and well crystallized, was ground and sieved for powder X-ray diffraction (XRD) experiments. Several crystals were selected under a microscope inside the glove box and sealed in capillaries to be checked for crystallinity. They all displayed cubic symmetry (a = 14.2Å) and the best diffracting one was chosen for data collection on the Xcalibur CCD (Oxford Diffraction) four-circle diffractometer using MoK radiation (table 1). EDX analyses were performed using an Oxford Instrument Environmental Scanning Electron Microscope, equipped with an X-Max large area SDD sensor that allows excellent sensitivity/precision/resolution. For several large pieces, noticeable variations in composition indicated that the final product was not obtained as a single phase. On the other hand, the Mg:Al:Zn compositions of several identified single crystals were found very close to 39.7:14.9:45.6, that of the single crystal used for data collection. XRD patterns were recorded on a Philips analytical X'pert diffractometer equipped with a Cu tube and a hybrid monochromator (parabolic multilayer mirror and two-crystal). Treatment using the Rietveld procedure included in program Jana2006 [17] confirmed the presence of at least two components. Geometry optimizations have been performed at the DFT level with program CASTEP [18,19] using GGA-PW91 exchange correlation functionals [20], ultra-soft pseudo-potentials [21] and a densitymixing scheme. Kinetic energy cut-off was set at 340 eV and a Monkhorst-Pack grid of k-points was used for numerical integration in Brillouin zone [22].

Results and discussion

Structural characterization

The XRD pattern analysis with cubic T-phase leaves unindexed lines that correspond to the orthorhombic Φ-phase (Pbcm, a = 8.89, b = 16.94 and c = 19.29 Å). In a two-phase profile refinement, cell parameters satisfactorily converge to values that are consistent with those previously published for the two compounds. Nevertheless, presence of a third unidentified component in very small amount cannot be rejected (very weak non indexed lines). Single crystal data clearly indicated centrosymmetry and I-centering, they are indexed within a cubic Im 3 lattice of parameter a = 14.1845(1) Å. Absorption effects were corrected using the procedure included in CrysAlis software [START_REF]CrysAlis'Red' 171 software package[END_REF] and the final data set used for full-matrix leastsquares refinements (SHELXL97 [START_REF] Sheldrick | SHELXL97: A Program for Refining Crystal Structures[END_REF]) consisted of 901 (857 with I > 2(I)) unique reflections.

Among the 7 positions provided by direct methods (SHELXS97 [START_REF] Sheldrick | SHELXS 97. A Program for Crystal Structures Solution[END_REF]), 4 were assigned to Mg and 3 to metal atoms. Refinement of the model with Al-filling at metal sites led to R1 of ~13%, instead R1 dropped to ~7% for Zn-filling. From here, considering Al/Zn mixing at these sites drastically improved agreement factors. Nevertheless the refined formula Mg52Al37Zn71

(normalized to 32.5:23.1:44.4) was too far from composition of the single crystal (EDX). On the other hand, one site over the four initially assigned to Mg showed a displacement parameter significantly higher than others. Some reports state that this site 12e (x,0,½) is affected by a less common Mg/Zn disorder (elements differing from almost 10% in their Pauling radii) [11,[START_REF] Lee | [END_REF] which was considered here. The position is found completely filled contrary to what is observed 1 CIF file CSD-429416 may be obtained from www.fiz-karlsruhe.de/request_for_deposited_data.html for the Mg2-y(Al1-xZnx)3+y series with only ~60% filling [11,[START_REF] Lee | [END_REF]. Note that refinements with similar atom distribution and 12e partial filling gave in this case unsatisfactory results, particularly regarding the compound composition. At remaining Mg sites, freely refined factors did not deviate, within standard limits, from the full site occupancy. It is worthy to mention that several studies considered the 2a position (0,0,0) either partially or even completely filled in isostructural compounds [6,[START_REF] Lee | [END_REF][27][28][29].

with analysis of the single crystal (39.7:14.9:45.6). Compound can be formulated Mg31.64Al11.62Zn36.74 with Z = 2 as well as Mg2-y(Al1-xZnx)3+y with x = 0.76, y = 0.022 and Z = 32.

In the following, the rounded formula Mg32Al12Zn37 (or Mg32(Al1-xZnx)49 for x = 0.76) will be used to refer to this compound. Fig. 2. The Bergman endohedral cluster Al1.5Zn10.5@Mg20@Al3.3Zn8.7@Mg11.3Al13.5Zn35.2 and polyhedral packing in Mg32Al12Zn37

Structural description and discussion

The M60 units join by sharing M4-M4 hexagon-edge (Fig. 2) and interstitial space is filled by wedge-shaped polyhedra Mg@Mg2.8Al2.3Zn5.9 (around A1) that allow packing of M60 units within a periodic 3-dimensional arrangement. These polyhedra are thus essential elements in formation of crystalline rather than quasicrystalline arrangements. It is at M60 shell and at a less extent at junction polyhedra which one observes the largest atom mixing phenomena that involves three atom species. This is quite a common feature for crystalline approximants of type 1/1 and Mg32Al12Zn37, with an electron to atom ratio e/a of 2.14, conforms to this general trend. However one must mention that perfectly ordered Bergman's polyhedra also exist, as for example the unit Ga12@Li20@Cu12@Ga60 at 2a empty center in isostructural Li13Cu6Ga21 (e/a = 2.05) [31]. On the contrary, M60 shell is strongly affected by atom disorder and sometimes additionally by occupation disorder in MgAlZn compounds. Samson's surface is the place where major disorder phenomena are observed, it is also the place where the failure in propagation of the 5-fold symmetry necessarily happens. This is a zone of important strain where frustrations must be relaxed through distortions or deviations from perfect 5-fold symmetry to allow the packing of polyhedral units (close to icosahedral symmetry) within a cubic 3-D periodic arrangement. Atom deviations from the ideal positions [7], here 0.03-0.07Å except 0.2 Å for M4, give a measure of this distortion. Stabilization into a crystalline network may depend upon atom distribution in this boundary region. Such an interpretation is supported by the theoretical analysis of hypothetical models Mg32Al49 [11] which states that vacancy formation and atomic Al/Zn adjustment are crucial in tuning the VECs (valence electron counts or e/a) and in reducing antibonding interactions. This can be illustrated with the Zn-content at each metal site plotted in Fig. 3 vs. the total Zn-content, for Mg32Al12Zn37 and isostructural compounds [11,12]. At first sight, Zn proportion at M1 to M3 displays an overall trend to increase with the total Zn-content but things are not as neat for M4. The M1 site on the internal icosahedron is mainly filled by Zn, in proportions limited between 62-100 at.%. On larger icosahedral and M60 shells, Zn-contents at M2 and M3 are curiously very similar over the whole composition range and regularly increase from 13 to 77 at.%. It becomes clear that these sites will be important in the structure capability to absorb the Al/Zn compositional variations. In most cases M4 site is entirely filled and Zn-free but examples are known where Zn is present in well defined proportions (27 or 40%) at this non-fully filled Mg site therefore complemented by vacancies [11]. This suggests that Mg for Zn substitution process would occur by steps. In Mg32Al12Zn37 (or Mg39.6Al14.5Zn45.9), the M4 entire filling is achieved with a low but non-zero Zn-content of 6 at.% (i.e. 11.3 Mg and 0.7 Zn) tending to invalidate such an assumption. It should be noticed that a rather similar situation, with 10.6 Mg and 1.4 Zn, is reported for the Zn-rich compound Mg39.5Al9.2Zn51.5 [12]. Such analysis of the distribution of atoms different in nature over the various sites is a helpful tool to better understand the great adaptability of the 1/1 cubic structure. Thus, sites M3 and M4 on M60 shell, take a significant part in phenomena that lead to accommodate the structural arrangement with different atomic and/or electronic contents.

Structural preferences

Compound Mg32Al12Zn37, the object of this work, is remarkable by its particular chemical composition, with both a weak Al-content as in approximants of type 2/1 and a weak Mg-content as in approximants of type 1/1, the two well-known structures for crystalline cubic arrangements in relation to quasicrystals. Let us compare stoichiometry Mg32Al12Zn37 with Mg46Al17Zn37 [16], that of a 2/1 approximant (Pa 3 , a ~23.1 Å). Even though they have close Al/Zn ratios, they significantly differ in their Mg-content. Based on a general review of literature reports, it is obvious that Mg-richest compounds adopt the 2/1 structure type. Instead, type 1/1 is favored for low Mg-contents. To better visualize their respective domains, we have reported in a composition diagram a series of compounds quoted in literature (Fig. 4). Crystalline approximant materials of types 1/1 or 2/1 are represented as circles and triangles while quasicrystalline materials are marked as rhombuses. The 1/1 line, also called Bergman line, represents the existence domain of the 1/1 structure type (T-phase, Im 3 , a ~14.2 Å) and the 2/1 line delimits the region where occurs the 2/1 structure type (Pa 3 , a ~23.1 Å). Note worthy are the positions distributed along these lines over a large composition domain, of various stoichiometries of materials identified as quasicrystals. The orthorhombic Φ-phase (square marks) is observed for Mg-rich compositions (~55 at.%) and Zn-contents from 20 to 40 at.%. In other words, one can consider the Φ-phase to mainly occur along a parallel with the 1/1 line. This structure is adopted by several compounds which can be formulated Mg55-ε(Al1-xZnx)45+ε, x ranging from 0.44 to 0.89 [2][3][4]. The metastable phase Mg4AlZn11 known as η' phase [32] which is of little interest in the current context (far from the focused region) is not reported in the diagram. Looking at Fig. 4, it is clear that all phases that display the 2/1 structure have very similar Al-contents (close to 15 at.%) but variable Mg/Zn proportions. The Zn-content therein ranges from 14 to 56 at.%, thus 2/1 approximants can be formulated as Al15-ε(Mg1-xZnx)85+ ε with 0.16  x  0.66. With close Mg-contents and Al/Zn variable proportions, the 1/1 structure type has many representatives in an extremely large composition domain [6,9,12,27]. It is commonly called Mg32(Al1-xZnx)49 for 0.16  x  0.85, but it is also appropriate to designate this family using the normalized equivalent formula Mg39(Al1-xZnx)61 in which Zn-content ranges from 9.8 to 51.5 at.%. Compound Mg32Al12Zn37, otherwise formulated Mg39Al15Zn46, is one of the Zn-richest compounds displaying this structural type. Its position in the diagram, marked with a large red circle, is particularly interesting since it lies at intersection of the 1/1 and 2/1 lines associated with corresponding structural types. As a consequence of its very particular position it can be formulated Mg39(Al1-xZnx)61, x = 0.754 or Al15(Mg1-xZnx)85, x = 0.541, and one can expect for Mg32Al12Zn37 an ability to crystallize within the two structural types.

Let us return now to the experimental conditions of synthesis. Mg32Al12Zn37 was obtained from Mg:Al:Zn in proportions 45:15:40, a stoichiometry taking place on the 2/1 line just between lines associated with T-phase (type 1/1) and Φ-phase. Note the complexity in this region where some imprecision remain [2] and where stoichiometries could correspond to crystalline approximants 1/1 or 2/1, to quasicrystalline materials and even to crystalline Φ-phase (also in relation with icosahedral quasicrystals [4]). Therefore it is easy to understand that experimental conditions will play a decisive part in the formation of compounds. What could seem an obvious assertion takes a huge importance here, in a case where very subtle changes can have a major impact on both composition and structure of the final alloy. In this chemical composition domain, a given combination might yield each of the structural types mentioned above. Moreover, crystalline and quasicrystalline materials with almost identical stoichiometries can coexist. One could simply state that a kind of competition exists between the various structures and that extremely small variations in experimental conditions will favor either one or the other atomic/structural arrangement. Under these conditions, reproducing an experiment becomes a quite difficult task. This is well illustrated by the different kinds of marks -triangles, circles, rhombuses-observed in a very limited composition domain (Fig. 4). It is then likely that the prepared alloy contains a small amount of 2/1 crystalline or even of quasicrystalline phases, which would explain the weak lines remaining unindexed in XRD pattern refined with T-phase as major component and Φ-phase as side product. This is also in agreement with the possibility to formulate Mg32Al12Zn37 (Mg39Al15Zn46) both using the 1/1 or 2/1 approximants general formula. If present in the alloy, a quasicrystalline component can belong to one or the other kind of quasicrystals.

Only four binary phases selected for their singular positions are reported in the diagram (Fig. 4).

Except for Mg2Al3, they are described with totally ordered atomic arrangements. The Bergman line would be extended to Mg4Zn7 (or Mg36Zn64) [33] and Mg2Al3 (or Mg37.3Al62.7, according to its true formula [34,35]). Similarly, Mg21Zn25 (or Mg45.7Zn54.3, e/a = 2) [36] and Mg17Al12 (or Mg58.6Al41.4, e/a = 2.41) [37] are located at both ends the Φ-phase domain. Moving along the Bergman line from Mg4Zn7 to Mg2Al3, the e/a ratio increases from 2 to 2.66. Between these limits the T-phases are characterized by values in range 2.09-2.49, close to the e/a ratios of 2.18 and 2.55 given for the Mg2-y(Al1-xZnx)3+y phase width [11]. On the other hand, the 2/1 structural type is observed in a narrow domain from 2.12 to 2.20, just as the Φ-phase from 2.10 to 2.25. In spite of such special positions, no structural relationship can be established as for instance a structural change with the progressive substitution of Zn for Al along the concentration range. Despite a metric relationship in lattice dimensions (doubled parameter), Mg2Al3 cannot be considered as a boundary of the T-phase domain even though its composition, in fact Mg37.3Al62.7, does not deviate much from the general formula Mg39(Al1-xZnx)61 and perfectly fits the alternate formula Mg2-y(Al1-xZnx)3+y for y = 0.13. Actually, Mg37.3Al62.7 is characterized by a high level of positional and atomic disorder and it can be assumed that occurrence of such a disorder is the result of packing frustrations and electronic constraints (too high e/a of 2.66) that prevent and make unrealizable the atom arrangement in the T-phase structure type. Subsequently the adaptability of the 1/1 structure is apparently restricted, with variable atom composition and consequently electron concentration, to a domain which does not expand to the most distant binary compositions. Within its whole composition domain the T-phase would be considered as a solid solution, then its lattice parameter is expected to vary with Al/Zn ratio according to Vegard's law.

Cell parameter of Mg32Al12Zn37 fairly well agrees with the available data for a series of compounds characterized with a filled M4 position (circles, Fig. 5) [12]. With M4 partial filling, Mg2-y(Al1-xZnx)3+y have slightly lower parameters (triangles, X-ray and rhombuses, neutron) suggesting that a lower Mg-content could be accountable for a cell contraction. Large red circle: Mg32Al12Zn37, blue circles [12], green triangles and rhombuses [11], black square [27].

With the intention to provide supplemental elements to the structural analysis, DFT calculations were carried out for ordered models established starting from the experimental structure of Mg32Al12Zn37. Accurate geometries were optimized within cubic symmetry and unit cell dimensions did not deviate from the experimental by more than 3.5%. Since Mg predominates at Deformation charge density computed by subtracting densities of isolated atoms from the total electron density is mapped at M60 surface for models Mg32M48 and Mg26M54 (Fig. 6). As it represents the electron redistribution due to chemical bonding, it is very informative on the nature of interactions. Highest positive values are associated with bond formation and lowest negative values point out electron losses. A glance at these maps clearly shows changes in the electron redistribution with the nature of atoms. This is particularly well illustrated at specific M4-M4 pair with clearly non bonding interactions for Mg (Mg32M48), bonding for Zn (Mg26Zn54) and strongly bonding for Al (Mg26Al54). Optimized distances (table 3) indicate a significant enlargement of the central icosahedron but no effects on the M60 shell for 2a-filling. On the contrary, changing nature of atoms at M4 not only affects distances but also create distortion, as exemplified in Mg26Zn54 with the hexagon which clearly deviates from flatness. This provides a graphic illustration of the constraints mentioned above that occur at this shell where atomic and electronic adjustments take place.

Conclusion

Mg32Al12Zn37 prepared and characterized displays a special composition. With a weak Al-content, it could be described with both generic formula of 1/1 and 2/1 crystalline approximant, but its belongs to the 1/1 structure type well known as Mg32(Al1-xZnx)49. A comparative analysis with regard to a series of compositions taken from the literature highlights the complexity of this ternary system, particularly in an area where subtle changes in experimental conditions could favor one or the other crystalline or even quasicrystalline forms. The structural study brings a proof of vacuity at the cell origin in Mg32Al12Zn37, a result supported by DFT calculations. Atom disorder is not evenly distributed on the whole structure, innermost shells mainly consist of Mg and disorder preferentially occurs at outer shells. This is reflected by the formula Al1.5Zn10.5@Mg20@ Al3.3Zn8.7@Mg11.3Al13.5Zn35.2 of the endohedral 104-atom cluster. The diversity of arrangements on the M60 shell, sometimes including vacancy defects, accounts for the great adaptability of the structure which is able to accommodate variable atomic and electronic compositions.

Mg32Al12Zn37, cubic, Im 3

 3 , a = 14.1845(1) Å, Z = 2 represents a new chemical composition for the already known structure Mg32(Al1-xZnx)49 or T-phase. Its close-packed structure is dominated by icosahedral concentric shells around the 2a empty site. Polyhedra occur in the characteristic sequence of Bergman cluster: M12 icosahedron, A20 pentagonal dodecahedron, M12 icosahedronand M60 Samson polyhedron. The icosahedral 5-fold symmetry could extend within this 104-atom cluster Al1.5Zn10.5@Mg20@Al3.3Zn8.7@Mg11.3Al13.5Zn35.2 (Fig.2). Al and Zn are mixed at both innermost (M1) and larger (M2) icosahedra, respectively at 2.52 and 5.05 Å from 2a center.Between the two, the dodecahedral shell (8×A2+12×A3) at 4.57 Å is only formed by Mg. From 6.68 to 7.26 Å, on the outer M60 shell (12×M4+48×M3), Zn is partially replaced by Mg at M4 and by Al at M3 sites. Consequently, Mg/Zn (M4-M4) edges are somewhat lengthened, 2.77 Å, compared to Al/Zn (M3-M3) edges of 2.67 Å.

Fig. 3 .

 3 Fig. 3. Zn-content at M sites for Mg-Al-Zn T-phases containing 14 to 51 at.% of Zn.

Fig. 4 .

 4 Fig. 4. Composition diagram reporting various compounds characterized and identified in literature. Circle: T-phase (cubic 1/1 approximant); triangle: cubic 2/1 approximant, rhombus: quasicrystalline materials, square: orthorhombic Φ-phase.

Fig. 5 .

 5 Fig. 5. Lattice parameter vs. Al-content in T-phases. Large red circle: Mg32Al12Zn37, blue circles

Fig. 6 .

 6 Fig. 6. CASTEP electron density difference at M60 shell showing a clearly non bonding character at Mg pairs in Mg32Al48 (up left) and Mg32Zn48 (up right) and bond formation (red zones) at Al and Zn pairs in Mg26Al54 (down left) and Mg26Zn54 (down right)

Table 1 .

 1 Crystal data and structure refinement 1 for Mg32Al12Zn37

	____________________________________________________________
	Refined formula	Mg31.64Al11.62Zn36.74
	Space group, Z	Im 3 (n° 204), 2
	Cell dimensions (Å)	a = 14.1845(1)
	Crystal size (mm), µ (mm -1 )	0.14 x 0.15 x 0.16, 15.638
	Theta range (°), completeness	3.52 to 31.78, 99.6%
	Data collected / independent	29599 / 901 [R(int) = 0.0374]
	Refined parameters	43
	Final indices I>2sigma(I) (all data)	R1 = 0.0205 (0.0227), wR2 = 0.0456 (0.0460)
	Largest diff. peak and hole (e.Å -3 )	0.399 and -0.984

Table 3 .

 3 M4, an Mg atom was placed at this position and then filling Al/Zn sites either with Al or Zn led to the models Mg32Al48 and Mg32Zn48. From here, models Mg26Al54 and Mg26Zn54 were built with Al or Zn at M4 and models Mg32Al49 and Mg32Zn49 with Al or Zn additional centering at site 2a. The optimized cells are significantly smaller for Zn models and volumes both increase with Mgcontent and with icosahedron centering (table3). Such behavior is in good agreement with experimental findings, i.e. parameter is reduced when Mg-content decreases or when Al-or Zncontent increases. The formation enthalpy, defined as the difference in total energies calculated for compound and for elements in their solid state structures, is a useful tool to evaluate relative Optimized lattice parameter (Å), distances (Å) and formation enthalpy (meV/atom)

	stabilities. Negative values of about -50 and -170 meV/atom (table 3) indicate that compounds
	may form with such stoichiometries. Because of small differences in energy, stabilization effects
	would be quite weak. Nevertheless, the slightly higher enthalpies computed for models Mg32M49

offer some support for non-centering of the Bergmann unit. The transformation of Mg32M48 into Mg26M54 (by Mg replacement at M4) leads to opposite effects in Al and Zn models, respectively destabilized by 7 or stabilized by 10 meV/atom. This would account for the numerous variants of ternary compounds reported with this structural type because stability would be reached for a balance between stoichiometry and atom distribution. Therefore a compromise has to be found between various parameters to fulfill stability and structural requirements. .

01

2.76 2.76 2.77 3.01 2.61 2.74 14.5130 -0.048 Mg32Al48 2.87 3.03 2.75 2.81 2.75 3.03 2.62 2.74 14.6195 -0.055 Mg32Al49 2.89 3.05 2.76 2.82 2.76 3.05 2.70 2.82 14.6835 -0.045 Mg26Zn54 2.49 2.94 2.66 2.79 2.66 2.94 2.47 2.59 13.9262 -0.174 Mg32Zn48 2.76 2.92 2.64 2.77 2.64 2.92 2.49 2.61 14.0848 -0.164


Mathilde Pelosi

Monique Tillard
email: mtillard@univ-montp2.fr

David Zitoun
email: david.zitoun@biu.ac.il

Ge nanoparticles by direct oxidation of Zintl alloys and their electrochemical behavior as anodes of Li-ion batteries

Keywords:

Germanium nanoparticles (NPs) are obtained by direct oxidation of a solid-state Na12Ge17 Zintl phase used as germanium precursor. Alcohols are chosen as oxidizing agents in a room-temperature process. The materials are characterized by X-ray diffraction, trans-mission electron microscopy, and Raman spectroscopy. The nature of the alcohol influences the final products and their crystallinity. The germanium NPs, tested as anodic materials for lithium ion batteries, display interesting behaviors with initial capacities up to 1,530 mAh g -1 .

Introduction

Germanium nanoparticles (NPs) have attracted attention as potential competitors for optoelectronic devices, elaborated by a bottom-up approach. Nano-crystals of germanium have also been studied for the quantum size effect on their optical properties [START_REF] Heath | Germanium quantum dots: optical properties and synthesis[END_REF][START_REF] Bostedt | Strong quantum-confinement effects in the conduction band of germanium nanocrystals[END_REF]. Elemental Ge has important applications in transistors, photodetectors, sensors, and batteries. As anodic material for Li-ion batteries, germanium displays a high theoretical capacity (1,600 mAh g -1

) and allows faster lithium diffusivity than silicon (extrapolated to be 400 times higher at room temperature) [START_REF] Fuller | Mobility of impurity ions in germanium and silicon[END_REF]. Nevertheless, the use of group 14 elements like Ge, Si, or Sn in anodes has been hampered by the poor cyclability of bulk materials, a drawback that could be overcome at the nanometric scale [START_REF] Armand | Building better batteries[END_REF]. The success in colloidal bottom-up approach synthesis of Ge is more limited than the textbook examples of II-VI and even IV-VI and III-V semiconductors and synthesis pathways deserve further investigations.

Gas-phase syntheses have been reported for the preparation of well-defined germanium nanocrystals [START_REF] Medeiros-Ribeiro | Shape transition of germanium nanocrystals on a silicon (001) surface from pyramids to domes[END_REF][START_REF] Min | The role of quantum-confined excitons vs defects in the visible luminescence of SiO2 films containing Ge nanocrystals[END_REF] and nanowires [START_REF] Morales | A laser ablation method for the synthesis of crystalline semiconductor nanowires[END_REF][START_REF] Wu | Germanium nanowire growth via simple vapor transport[END_REF]Wang andDai 2002, 2006;[START_REF] Wang | Oxidation resistant germanium nanowires: bulk synthesis, long chain alkane-thiol functionalization, and Langmuir-Blodgett assembly[END_REF]. The common precursors used in chemical vapor deposition are germanes (like Ge2H6), chlorogermanes, or phenylgermanes, which undergo thermal decomposition or hydrogenolysis; in the presence of a metal catalyst, Vapor-Liquid-Solid mechanism leads to the formation of anisotropic nanostructure. These routes have been adapted to the synthesis of Ge nanocrystals in high boiling point solvents [START_REF] Gerion | Solution synthesis of germanium nanocrystals: success and open challenges[END_REF][START_REF] Zaitseva | Germanium nanocrystals synthesized in high-boiling-point organic solvents[END_REF][START_REF] Chockla | Colloidal synthesis of germanium nanorods[END_REF]or supercritical fluids [START_REF] Lu | Synthesis of germanium nanocrystals in high temperature supercritical fluid solvents[END_REF][START_REF] Collins | Germanium nanowire synthesis from fluorothiolate-capped gold nano-particles in supercritical carbon dioxide[END_REF] and to Solution-Liquid-Solid mechanism toward the formation of Ge nanorods [START_REF] Hanrath | Nucleation and growth of ger-manium nanowires seeded by organic monolayer-coated gold nanocrystals[END_REF]Lu et al. 2005a, b).

Examples of solution routes are sparser and involve the reduction of a germanium halide (GeCl4 and RGeCl3) by a strong reducing agent alkalides [START_REF] Heath | Germanium quantum dots: optical properties and synthesis[END_REF], butyl-lithium, naphthalides [START_REF] Kornowski | Advanced laser technology[END_REF][START_REF] Chiu | Investigation of reaction conditions for optimal germanium nanoparticle production by a simple reduction route[END_REF], or hydrides [START_REF] Wilcoxon | Synthesis and optical properties of colloidal germanium nanocrystals[END_REF][START_REF] Chou | Colloidal synthesis of germanium nanocrystals using room-temperature benchtop chemistry[END_REF], in the presence of a surfactant to limit the NPs growth and coalescence. Very recently, oleylamine has been successfully used as reducing agent in a microwave-assisted synthesis of Ge NPs [START_REF] Muthuswamy | Facile synthesis of germanium nanoparticles with size control: microwave versus conventional heating[END_REF]. Alternative routes consist in the thermal decomposition of chlorogermane [START_REF] Henderson | Synthesis and photoluminescent properties of size-controlled germanium nanocrystals from phenyl trichlorogermane-derived poly-mers received much attention over the past several dec-ades, due in[END_REF], germanium alkoxide [START_REF] Gerung | Solution synthesis of germanium nanowires using a Ge alkoxide precursor[END_REF], or Ge{N(SiMe)3}2 [START_REF] Lambert | Water-soluble germanium (0) nanocrystals: cell recognition and near-infrared photo-thermal conversion properties[END_REF][START_REF] Gerung | Anhydrous solution synthesis of germanium nanocrystals from the germanium(II) precursor Ge[N(SiMe3)2]2[END_REF]. As shown recently, the use of GeI4 and/or GeI2 as precursors allowed a better control over the particle mean size and the size dispersion of the colloidal product (Lu et al. 2005b;[START_REF] Ruddy | Size and bandgap control in the solution-phase synthesis of near-infrared-emitting germanium nanocrystals[END_REF][START_REF] Vaughn | Colloidal synthesis of air-stable crystalline germanium nanoparticles with tun-able sizes and shapes[END_REF]. The stable colloidal suspension is suitable for spectroscopic studies [START_REF] Lee | Colloidal synthesis of infrared-emitting germanium nanocrystals[END_REF]) but the surfactant shell impedes its use for electronic/electro-chemical applications.

Therefore, the use of germanium Zintl phases (Ge anionic clusters with alkaline or alkaline earth cations) has been standing as a unique room-temper-ature oxidative route toward Ge nanostructures. The Kauzlarich group has first developed this pathway by varying the Zintl phase (NaGe, KGe, and Mg2Ge), the oxidative agent GeCl4 or GeCl2, and the capping agent (Grignard reagent or alkyl lithium) [START_REF] Ma | Alkyl-terminated crystal-line Ge nanoparticles prepared from NaGe: synthesis, functionalization and optical properties[END_REF][START_REF] Taylor | Solution synthesis of germanium nanocrystals demon-strating quantum confinement[END_REF][START_REF] Taylor | Solution synthesis and characterization of quantum con-fined Ge nanoparticles[END_REF][START_REF] Taylor | Solution preparation of Ge nano-particles with chemically tailored surfaces[END_REF][START_REF] Tanke | Synthesis of germanium nanoclusters with irreversibly attached functional groups: acetals, alcohols, esters, and polymers[END_REF][START_REF] Chiu | Phase changes in Ge nanoparticles[END_REF]. Such Zintl precursors have been success-fully used as building blocks for Ge mesoporous materials (Armatas and Kanatzidis 2006a, b;[START_REF] Sun | Hexagonal nanoporous germanium through surfactant-driven self-assembly of Zintl clusters[END_REF]) and anodic materials for Li-ion batteries [START_REF] Lee | Surface-stabilized amorphous germanium nanoparticles for lithium-storage material[END_REF][START_REF] Park | Flexible dimensional control of high-capacity Li-ion-battery anodes: from 0D hollow to 3D porous germanium nanoparticle assemblies[END_REF]. Soft oxidation of Zintl phases also yields a guest-free clathrate and an allotropic form (metastable allo-Ge) of germanium [START_REF] Guloy | A guest-free germanium clathrate[END_REF][START_REF] Gruttner | Novel meta-stable germanium modifications allo-Ge and 4H-Ge from Li7Ge12[END_REF][START_REF] Kiefer | Bulk synthesis and structure of a microcrystalline allotrope of germanium (m-allo-Ge)[END_REF]. The appealing ability to perform a room-temperature synthesis of germanium nanostructures starting from Zintl precursors, and the need for better comprehension of the reaction pathways, drive us to study the reactivity of germanium anionic clusters under oxidation with mild reducing agents.

In this paper, we report on the preparation of Ge NPs by direct oxidation of Ge Zintl anions by alcohols. The Na12Ge17 Zintl phase has been used as germanium precursor either in solid state or after dissolution in ethylene diamine. Transmission electron microscopy (TEM) shows that the chain length of the alcohol and the reaction pathway have some influence on the mean size, the crystallinity, and the size dispersion of particles. The crystallinity has been carefully studied by Raman spectroscopy. The electrochemical tests of NPs as anodic materials for Li-ion batteries emphasize the relationship between synthesis pathway and properties.

Experimental section

Reagents are handled in a glove box under controlled argon atmosphere. Before the reaction with Ge powder (150 lm, Sigma Aldrich, 99.999 %), the alkali metal (Na, ingots,Merck, 99 %) is scraped to remove surface impurities. Benzyl alcohol (C6H5CH2OH, 99 %, anhydrous, Acros-Organics) is used without further purification and anhydrous n-butanol (C4H9OH, Sigma Aldrich) is dried on a Schlenk line. The Zintl phase Na12Ge17 is prepared from stoichiometric proportions of the elements mixed in a weld-sealed Ta-container, enclosed in its turn within an evacuated silica or stainless steel jacket to avoid any oxidation at high temperature. The Na/Ge mixture is heated to 1,230 K for 2 days during which the reactor is shaken several times to improve homogenization of the melt. The air-sensitive products of reactions are handled in the glove box.

NPs are obtained by direct oxidation of the solid precursor by an alcohol. In such a reaction, called route I reaction, the alcohol is acting as oxidizing agent as well as solvent. The Na12Ge17 powder (100 mg) is slowly added to 10 mL alcohol under stirring in a glove box or on a vacuum line. The reaction time is optimized for completion of the reaction, and the solution is left to stir for 16 h, a time beyond which no homogeneity improvement in the product is observed. At the end of the reaction, the black powder (Ge NPs) is separated by centrifugation and washed with hexane.

NPs are also obtained from a different pathway in the route II reaction: the solid Na12Ge17 (100 mg) is initially dissolved in ethylene diamine (10 mL) before adding the alcohol (10 mL). For this alternate route, the total reaction time should be divided into two parts: the time required for the solubilization which is set to 72 h and the oxidation time which is fixed to 16 h (as in the route I reaction). Temperature change from 273 to 473 K has no noticeable effect on the reaction products; therefore, all the experiments are carried out at room temperature.

A modified pathway named route III is used in order to avoid the high-temperature synthesis of the Zintl phase. The elements, Na and Ge, were taken in 12/17 atomic proportions and directly mixed in ethylene diamine before the oxidation reaction.

Note that similar reactions are successfully carried out starting from K4Ge9, a Zintl phase having a weaker reducing character (attributed to the weaker negative mean charge per Ge atom [START_REF] Goicoechea | Ligand-free deltahedral clusters of silicon in solution: synthesis, structure, and electrochemistry of Si9[END_REF] and consequently a lower reactivity toward the alcohol.

TEM is performed with a JEOL 1200 EX II microscope operating at 100 kV accelerating voltage. The X-ray diffraction (XRD) patterns are recorded on a Philips analytical X'pert diffractometer equipped with a copper tube and a hybrid monochromator (parabolic multilayer mirror and two-crystal, Cu Ka radiation). The Raman spectroscopy is carried out using a LAbRam Aramis IR 2 spectrometer equipped with a helium neon laser (633 nm). The intensity of the laser beam is estimated to a maximal value of 6 mW very close to the sample. The use of various filters (named D0.6, D1, and D2) allows a reduction of intensity to 200, 70, and 50 μW, respectively.

The electrochemical behavior of nanostructured Ge powders is evaluated in Li/LiPF6 (EC, PC, DMC)/Ge electrochemical cells assembled under argon in a glove box. Electrodes incorporated 85 wt. % active material intimately mixed with 15 wt. % carbon black. A 1 M solution of LiPF6 in a 1:1:3-mixture of ethyl, propyl, and dimethyl carbonates (Merck Laboratories) is used as electrolyte. Charge-discharge experiments are monitored in standard conditions using a MacPile multichannel galvanostat-potentiostat. Li insertion/ extraction is limited within a 0-2.5 V (vs. Li + /Li) window voltage. The polarization between charge and discharge voltage upon cycling did not exceed 0.5 V. The voltage versus composition curves are plotted for the first cycles of galvanostatic experiments per-formed at a regime corresponding to the exchange of one Li atom in 10 h.

Results and discussion

Ge NPs have been prepared from the oxidation of a Zintl phase used as Ge precursor (Fig. 1). Alcohols are chosen as oxidizing agents for their especially mild reducing character. Furthermore, they can be easily removed from the nanostructure surface after synthesis. The alcoholic proton reacts with the Zintl phase to give gaseous H2 and alkoxide (which protonates upon reaction in air). Indeed, the reaction occurs with gas evolution and no other by product has been noticed from 1 H NMR spectrum of the supernatants. The route I reaction is a direct oxidation of the solid phase by a liquid oxidant also acting as solvent. Instead, route II reaction takes advantage of the special property of Ge Zintl phases to be soluble in basic polar solvents, as for example ethylene diamine. Solubilization releases free Zintl anions in the solution that are ready to react directly with the oxidant in the liquid phase. 4-and Ge 9 4-, packed together with sodium cations (Fig. 2c). Then, the Na12Ge17

Zintl compound can be formally described as

[Na + ]12 [Ge 4 4-]2 [Ge 9 4-].
Raman spectroscopy is a complementary technique for the full identification of Na12Ge17. According to literature reports [START_REF] Hoch | Synthesis and crystal structure of the tetrelides A12M17 (A=Na, K, Rb, Cs; M=Si, Ge, Sn) and A4Pb9 (A=K, Rb)[END_REF][START_REF] Kliche | The internal vibrations of the tetrahetero-tetrahedrane anions Ge4 4-, Sn4 4-, and Pb4 4[END_REF][START_REF] Somer | Vibrational spectra of the cluster anions [E4] 4in the metallic sodium and barium compounds Na4E4 and Ba4E4 (E = Si, Ge)[END_REF], Ge The Raman spectrum recorded for Na12Ge17 is composed of three bands (Fig. 2b), those at 276 and 224 cm ). Raman spectroscopy brings information that, in addition to XRD results, allows the complete and unambiguous identification of compound Na12Ge17.

The route I solid-liquid reaction

The route I solid-liquid reaction is carried out using solid Na12Ge17 and either benzyl alcohol or butanol as oxidizing agents. Powder XRD analyses of products haveshownthatNa12Ge17reactscompletelyduringthe oxidation reaction. In all cases, products of reaction are crystalline as proved by the electron diffraction patterns indexed within the cubic diamond-type structure of germanium (JCPDS files no. 04-0545, a = 5.657 Å).

As clearly illustrated in Fig. 3, particle size depends on the nature of the oxidizing agent: small NPs of 5.4 ± 2.3 nm in diameter are obtained from benzyl alcohol while particles of 19.1 ± 8.5 nm in diameter, with a larger size distribution, are obtained from butanol. In the following, these samples will be referred to as 5 and 20 nm NPs.

Raman spectroscopy has proven its efficiency to characterize these materials, even those being poorly crystallized or amorphous. Actually, Raman lines are characterized by their shape (width, symmetry) and their position, parameters that are strongly correlated with the structure/morphology (crystalline, amorphous) and with the particle size (micrometric, nanometric).

Germanium nanopowders from route I reactions are analyzed by Raman spectroscopy. Their spectra, shown in Fig. 4, present a large and asymmetric band clearly shifted from the position of the Raman line attributed to crystalline germanium (Ge-Ge stretch at 302 cm -1

). The Raman band position is dependent on the particle size, shifted toward low wavenumbers when the average particle size decreases, and broadened when the amorphous character increases [START_REF] Muthuswamy | Facile synthesis of germanium nanoparticles with size control: microwave versus conventional heating[END_REF][START_REF] Schlecht | Synthesis and Raman spectroscopy of nanoparticles of crystalline and X-ray amorphous germanium within mesoporous SiO2[END_REF]Gorokhovet al. 2005). After background correction, the experimental Raman spectrum is fitted using two Gaussian functions centered at 278 and 261 cm -1 (sample from benzyl alcohol). The contribution at 278 cm -1 is attributed to the crystalline core of the particles while that at 261 cm -1 corresponds to the amorphous part of the sample (surface and/or amorphous NPs). For samples prepared in butanol, Gaussian parameters are found slightly closer to values for bulk Ge, the Raman shifts of 288 and 271cm -1 are in agreement with an increase in the coherence length of the particles crystalline core. These values of Raman shifts can be compared with 273 cm -1 , a value reported for nanocrystals [START_REF] Ma | Alkyl-terminated crystal-line Ge nanoparticles prepared from NaGe: synthesis, functionalization and optical properties[END_REF].

The route II liquid-liquid reaction

Ge-nanosized samples are also prepared by the liquid-liquid route II reaction, which proceeds through the solubilization in ethylene diamine of the Na12Ge17 Zintl phase prior to oxidation. In that case, the individualized Ge 4 4and Ge 9 4-Zintl units already present in the solid state are released as free anionic species in the solution. The intense brown-violet color of the solution was indicative of the presence of Ge anionic clusters able to react immediately with the alcohol. The route II reaction takes place in specific conditions (kinetics, mechanism…) that might be significantly different from route I reaction conditions. Powders obtained in that way with benzyl alcohol or butanol are studied using Raman spectroscopy. The recorded spectra, typical of Ge nanosized particles, showed large and asymmetric bands with Raman shifts of 264 cm -1 (benzyl alcohol) and 273 cm -1 (butanol). The displacement of the band from 264 to 273 cm -1 may be interpreted as the consequence of an increase in the long-distance order within the material, associated with a particle size enlargement or/and a crystallinity improvement. Comparison with the Raman shifts (278 and 288 cm -1

) observed for route I products indicates that route II products displayed a shorter coherence length.

Sample morphology and particle size distribution are examined by electron microscopy, the TEM micrographs of two samples prepared from benzyl alcohol and butanol are given comparatively in Fig. 5.

Whatever oxidizing agent is used, individualized NPs are found embedded in an amorphous germanium matrix. With benzyl alcohol, particles are below 7 nm in diameter and amorphous; while with butanol, crystalline particles range between 10 and 15 nm. The difference in morphology between routes I and II products may be attributed to the ability of Zintl anions to react with the alcohol. In the route I reaction, the Zintl anionic units (Ge 4 4-and Ge 9 4-) confined in the solid phase would react rather slowly with the alcohol. Instead, in the route II reaction, these units already present in the solution are immediately available for reaction with alcohol, leading to embedded NPs. Results acquired from microscopy well agree with findings from Raman spectroscopy and XRD. In route II experiments, as already observed for route I, light alcohols are the most reactive oxidizing agents. Butanol is found to react faster than benzyl alcohol and to produce larger particles with higher crystallinity. The exothermic character of the reaction causes a local elevation of temperature, a factor that could favor the crystallization of Ge particles.

The route III reaction

In order to avoid the Zintl-phase high-temperature synthesis, elements are directly mixed in ethylene diamine. Though soluble in ethylene diamine, some solid germanium is still present at the bottom of the reactor after stirring the solution during 3 days. The dark-violet coloration of the solution indicated the presence of germanium anions. After filtration, the solution is added to benzyl alcohol or butanol to perform the oxidation reaction. No effect related to the nature of the alcohol can be detected. The black powder collected, washed, and dried is analyzed by XRD, its pattern indicated the presence of a small quantity of a sodium hydrogen germanium oxide hydrate, identified as Na3HGe7O16(H2O)4 (ICDD no. 01-076-0102), in addition to crystallized Ge. Diffraction lines marked with a star correspond to the germanium hydrate (Fig. 6b).

The presence of germanium hydrate supports the proposed mechanism: Ge anionic clusters can be oxidized either to Ge in anhydrous conditions or to sodium hydrogen germanium oxide hydrate in the presence of residual water (several ppms). The hydrate is easily eliminated by acid wash with dilute nitric acid. Unlike powders from routes I and II, the Ge powder from route III displays very narrow XRD lines suggesting the presence of large crystalline particles. The Raman band is observed at 300 cm -1 , the same position as for Ge commercial powder (150 lm). Nevertheless, with a half-maximum full-width of 11 cm -1 instead of 4 cm -1 , the band is slightly wider. The asymmetry would be assigned to a non-homogeneous size distribution and broadening explained by a reduction in the particle size.

These interpretations are consistent with the large crystalline aggregates of 50-100 nm observed in TEM images for samples prepared either from benzyl alcohol or from butanol (not shown). The observed particle size in product is at least two orders of magnitude below the powder used as reagent, another argument for the reaction mechanism proposed. At first sight, the routes II and III reactions would appear very similar. Nonetheless, they are quite different since their final products do not display close characteristics. It is likely that solubilization in ethylene diamine of Na12Ge17 on one hand, and Na and Ge (in 12/17 atomic proportions) on the other hand, do not produce identical solutions. Raman studies of these solutions could indicate which species were present in the solutions, but they do not give workable data. Finally, no conclusive proof of the formation of Ge Zintl anions in the solution can be provided, anyway consideration of results from route III stresses the need to retain the high-temperature synthesis of the Zintl phase and underlines how complex the reaction media could be.

Raman spectra and samples stability

For each sample, Raman spectra are first acquired using the lowest intensity for the laser beam (50 μW, D2 filter), and then the intensity can be increased to 75 μW (D1) or 200 μW (D0.6). Depending on experimental conditions, a displacement of the Raman bands is observed that could be correlated with the sample stability under irradiation. To better understand the origin and try to quantify the amplitude of such displacements, Raman experiments are performed for Ge wafers and Ge commercial powder. For these two samples, Raman lines occur at 300 cm -1 in the first spectrum recorded with the lowest intensity (D2 filter).

As shown in Fig. 7, a progressive increase of the beam intensity using successively D1 and D0.6 filters does not induce any change in the band position for the Ge wafer. Instead, the Raman band of the Ge powder is continuously shifted toward weak displacement, as the beam intensity is increased (D1 then D0.6 filter). After a subsequent intensity lowering (D2 filter), the Raman line recovers its initial position. A similar phenomenon has been reported for small silicon crystalline nanowires of about 2 nm in diameter [START_REF] Gupta | Laser-induced Fano resonance scattering in silicon nanowires[END_REF]. This reversible event results from thermal expansion: crystal cell inflates with the elevation of local temperature due to the beam energy and then contracts upon cooling. In the meantime, the Raman band shifts to low displacement and retrieves its initial position. This effect is only observed for powders and then called ''powder effect.'' Owing to higher structural constraints, it does not occur for Ge wafers. This phenomenon, sensitive to the sample morphology, would be also associated with a decrease in thermal conductivity caused by the weakening of inter-grain contacts in finely divided powdered samples.

Similar experiments are carried out with the nano-sized Ge powders prepared in this work. As expected, the Ge powder obtained from route III reaction, composed of large particles of *100 nm, gives rise to the powder effect, like the commercial powder. The Raman band (D2 filter) of particles prepared from route I in benzyl alcohol is very intense (Figs. 4,7). This intensity is characteristic of a loss in the long-distance order that causes suppression of the selection rules and allows all phonons participation [START_REF] Richter | The one phonon Raman spectrum in microcrystalline silicon[END_REF]. The raise in intensity occurs together with a narrowing of the band strongly moved toward high displacement values, sign of some local ordering (coalescence/crystallization). After a subsequent come back to low intensity (D2 filter) the band is observed at 300 cm -1 (Fig. 7). Based on results previously reported for silicon with shifts increasing with the particle size [START_REF] Iqbal | Polycrystalline silicon films deposited in a glow discharge at temperatures below 250 C[END_REF][START_REF] Morhange | Raman study of laser annealed silicon[END_REF], this irreversible phenomenon is assimilated to a thermal annealing.

Particles prepared from route II (16 h reaction) display a Raman band at 300 cm -1 , which shifts reversibly to low displacements demonstrating a powder effect. In case of powders collected after only 2 h of reaction (smaller amorphous particles), dis-placements of the Raman band initially occurring at 264 cm -1 indicate that the material first undergoes a thermal annealing and then a powder effect.

Electrochemical experiments

The Ge powders prepared in the present work are tested against a pure lithium electrode in electrochemical cells. Starting with a discharge of the battery, the maximum lithium insertion that can be achieved led to nominal compositions Li4.2Ge and Li3.4Ge for, respectively small particles (5 nm) from benzyl alcohol and large particles (20 nm) from butanol, corresponding to the respective specific capacities of 1,530 and 1,250 mAh g -1 (Fig. 8). The limits in stoichiometry reached at the end of the subsequent charges of the cells are Li1.7Ge and Li1.2Ge, leading to irreversible capacities of 600 and 380 mAh g -1 that would include solid-electrolyte layer occurring at first discharge. The reversible specific capacity for the complete first cycle is then about 900 mAh g -1 but the capacity rapidly fades upon cycling. The voltage versus composition curves are represented in Fig. 8 for the first cycles of galvanostatic experiments. The first discharge is characterized by a pseudo plateau representing the continuous insertion of Li + into the electrode [START_REF] Graetz | Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities[END_REF]). After the expected and usual small insertion of lithium in carbon black around 0.75 V, the Li insertion in Ge particles proceeds through two distinct processes better visualized in the derivative plots dx/dV as two peaks occurring at 0.35 and 0.18 V (Fig. 8a). During the following charge, the large and intense peak observed at 0.5 V is associated with lithium extraction. The insertion mechanism of Li in small particles looks more complex, as attested by the additional and not fully separated peaks observed at 0.5 V in discharge and at 0.4 V in charge on potential derivative plots (Fig. 8a). Li insertion into larger particles of 50-100 nm prepared from route III also occurs through a pseudo plateau up to a maximum of ~4Li + inserted per Ge, leading to a specific capacity of 1,405 mAh g -1

. The subsequent charge does not allow the extraction of the total Li amount, thus the electrode shows a rather large irreversible capacity of 510 mAh g -1

. Nevertheless, the capacity retention is found around 60 % over 10 cycles, which is much better than in case of small particles.

These exploratory experiments clearly evidence that the electrochemical behavior is related to the structure/ morphology of the electro-active material. Small-sized particles are found not only to improve the initial capacities but also to increase the irreversible part of the capacity while larger particles are favorable to good capacity retention upon cycling. In these nanosized Ge materials, lithium insertion occurs through a continuous process without steps corresponding to the formation of intermediate intermetallic compounds. Improvement of the electrochemical performances would be reached by finding a compromise and optimizing the particles size. On the other hand, materials conditioning and experimental parameters need to be refined (binder, conducting matrix, exchange rate, potential windows,…) to improve the characteristics and the cycling lifetime of these batteries.

Initial capacities reach without any optimization vary from 1,250 to 1,530 mAh g -1

, not so far from the theoretical capacity of 1,600 mAh g -1 calculated for the nominal composition Li22Ge5. These capacities are in the same range as initial capacities reported in the literature for other kinds of nanosized germanium [START_REF] Graetz | Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities[END_REF]) like Ge nanowires with an initial capacity of 1,141 mAh g -1 retained over 20 cycles [START_REF] Chan | High capacity Li ion battery anodes using Ge nanowires[END_REF]. Ge amorphous NPs display a capacity of 1,470 mAh g -1 [START_REF] Lee | Surface-stabilized amorphous germanium nanoparticles for lithium-storage material[END_REF] while after annealing, the resulting crystallized particles inserted in a carbon matrix display a capacity of 1,200 mAh g -1 of which 88 % is retained over 30 cycles [START_REF] Lee | Synthesis and optimization of nanoparticle Ge confined in a carbon matrix for lithium battery anode material[END_REF].

Conclusion

Nanostructured Ge powders are prepared using three different routes; the two first routes are based on the oxidization by an alcohol of a Zintl phase used as Ge precursor. The third route consists in the in situ Zintl anions formation and oxidation. Results clearly emphasize the crucial and unique role of the Zintl phase as a precursor of small-sized Ge NPs. One-phase reactions (route II) allow preparing crystalline particles embedded in amorphous matrix while two-phase reactions (solid/liquid, route I) yield well-defined isolated NPs. The difference in the product morphology may depend on the ability of the Zintl anions Ge 4 4-and Ge 9 4-to react with the alcohol. The free anions in the liquid phase (Na12Ge17/ethylene diamine solution) react more quickly than the anions frozen in the solid Na12Ge17 Zintl phase.

The results obtained in the present work show that the size of Ge NPs could be tuned by changing the chain length of the oxidizing agent, butanol and benzyl alcohol leading, respectively, to 5 and 20 nm NPs.

The Ge NPs prepared using these original oxidation reactions are free of organic residues and, when tested in electrochemical cells, exhibited initial capacities up to 1,530 mAh g -1 with a capacity retention reaching 60 %. These results are interesting for Ge anodic materials prepared by solution-phase chemistry in so-far and could be greatly improved by optimization of the electrode preparation.

Fig. 1

 1 Fig. 1 Schematic representation of the synthetic routes I and II

  4 4-unit (symmetry Td) is characterized by three Raman active vibration modes observed at 274 symmetry, only five out of the 20 active modes are observed at 222

Fig. 2 X

 2 Fig. 2 X-ray diffractogram of Na12Ge17 (a), Raman spectrum of Na12Ge17 (b) and structure projection along b axis emphasizing the Ge 4 4-

  , respectively. The slight asymmetry of the band lying at 224 assigned to the m3 vibration mode of Ge 4 4-(164 cm -1 ) which overlaps with the Ge 9 4vibration modes occurring in the same domain (188, 164, and 147 cm -1

Fig. 3 Fig. 4 a

 34 Fig. 3 TEM micrographs of crystalline Ge nanoparticles from route I using benzyl alcohol (a) and butanol (b) as oxidizing agent (insets SAED)

Fig. 5

 5 Fig. 5 TEM micrographs of Ge nanoparticles obtained from route II in benzyl alcohol (a) and butanol (b), Raman spectra of the corresponding materials (c)

Fig. 7

 7 Fig. 7 Raman spectra under different irradiations for Ge wafers (a), Ge commercial powder (b), 5-nm Ge nanocrystals from route I (c)

Fig. 8

 8 Fig. 8 Derivative plot dx/dV of small (plain line) and large (dotted line) particles (a), specific capacity for large (b) and small particles (c). d Comparative cycling performances


Pierre Montagné

Monique Tillard
email: mtillard@univ-montp2.fr

Mg2Al3, a complex and disordered intermetallic compound as anode material for metal-air batteries

Keywords: Intermetallic compound, disordered structure, corrosion potential, metal-air batteries, anodic behavior, crystal structure

The intermetallic compound Mg2Al3 has been selected for this work to be tested as anode in a metal-air battery. The compound is prepared by a high temperature synthesis from elements and its cubic structure (Fd 3 m, a = 28.2341(3) Å) has been refined from X-ray single crystal diffraction data. It is characterized by a high level of disorder with a large number of partially filled atomic sites, split positions and atomic mixing. The compound displays an interesting behavior towards self-corrosion in alkaline and saline environments but the better electrochemical performances are obtained in saline NaCl electrolyte. It is shown that both Al and Mg are involved in the oxidation reaction that occurs at the Mg2Al3 electrode used as anode in a metal-air battery. The operating voltage of an Mg2Al3-air cell is remarkably stable over several hours and the Mg2Al3 anode efficiency is higher than that of a pure Mg anode in a saline environment.

INTRODUCTION

Numerous and exciting researches have been reported up to now in the field of energy storage and energy conversion. In order to fit requirements for various applications that range from portable electronics to electrical transports, a great quantity of materials has been explored. For a long time, lithium batteries are considered as particularly interesting and powerful systems. Yet widely commercialized, the Li-ion batteries still motivate scientists as indicated by the quantity of works aimed to improve their performances [START_REF] Goodenough | The Li-Ion Rechargeable Battery: A Perspective[END_REF][START_REF] Tarascon | Key challenges in future Li-battery research[END_REF]. Nevertheless, their energy densities of current do not meet high-energy requests in specific areas as hybrid electric vehicles. In today's world, where environmental considerations gain in importance, with climate changes and fossil energy supply limitations, there is a substantial interest to turn towards non-polluting devices and renewable energies. Therefore, the search for alternative energy storage and conversion devices and for new electrode materials is still a challenge.

Among energy sources, systems based on catalytic redox reactions such as fuel cells have acquired popularity due to their high efficiency and cleanliness. Currently the peculiar metalair fuel cells (MAFC) that generate electricity through reaction between a metal and oxygen create a renewed and increasing interest. The electrochemical reactions that occur are the metal oxidation on anode and the oxygen reduction on cathode, output cell voltage being the potential difference between cathode and anode. With very high theoretical energy densities because the cathode active material is not incorporated in the battery, these electrochemical power sources are considered as promising candidates. Even displaying a quite low specific capacity, the Zn-air system has been studied for many years and primary Zn-air batteries are commercially available [START_REF] Caramia | Materials science aspects of zinc-air batteries -A review[END_REF][START_REF] Chakkaravarthy | Zinc-air alkaline batteries -A review[END_REF]. Recent reviews provide an overview of fundamentals, progress and limitations in the area of Li-air batteries [START_REF] Kraytsberg | Review on Li-air batteries-Opportunities, limitations and perspective[END_REF][START_REF] Lee | Metal-Air Batteries with High Energy Density: Li-Air versus Zn-Air[END_REF][START_REF] Padbury | Lithium-oxygen batteries-Limiting factors that affect performance[END_REF]. It has been reported that capacity may depend on composition, porosity and morphology of the oxygen electro-catalyst employed for oxygen reduction at the cathode [START_REF] Wang | Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes[END_REF][START_REF] Younesi | Influence of the cathode porosity on the discharge performance of the lithium-oxygen battery[END_REF]. On the other hand, attention has also been paid to the nature of the electrolyte: aqueous saline or alkaline, with stabilizing additives or non aqueous, organic, ionic liquids [START_REF] Guadarrama-Muñoz | Electrochemical study on magnesium anodes in NaCl and CaSO4-Mg(OH)2 aqueous solutions[END_REF][START_REF] Khoo | The potential for ionic liquid electrolytes to stabilise the magnesium interface for magnesium/air batteries[END_REF][START_REF] Yoshimoto | Mixed electrolyte consisting of ethylmagnesiumbromide with ionic liquid for rechargeable magnesium electrode[END_REF]. Abundance on earth, low weight, low toxicity, high energetic theoretical capacities (2205 mAh g -1 for Mg and 2980 mAh g -1 for Al, Fig. 1) along with negative values of standard electrode potentials (-2.93 and -1.66 V relative to the standard hydrogen electrode) make Mg and Al very attractive elements [START_REF] Blurton | Metal/air batteries: Their status and potential -a review[END_REF][START_REF] Zhang | Magnesium-air batteries: from principle to application[END_REF]. anodes. Severe self-corrosion associated with negative difference effect (NDE) during the battery discharge are major drawbacks responsible for losses in capacity and anodic efficiency that have delayed the development of these systems with respect to the Zn-air technology [START_REF] Bender | A new theory for the negative difference effect in magnesium corrosion[END_REF][START_REF] Thomaz | The negative difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions[END_REF]. A lot of efforts have been devoted to the study of Zn-air [START_REF] Caramia | Materials science aspects of zinc-air batteries -A review[END_REF] and Al-air batteries [START_REF] Zhang | All-solid-state Al-air batteries with polymer alkaline gel electrolyte[END_REF], but works relative to Mg-air batteries are scarcer. In search of the improvement of electrochemical performances of metal-air batteries, several Mg-based materials have been tested as anodes; most of them are multiphase or composite materials [START_REF] Ma | Performance of Mg-14Li-1Al-0.1Ce as anode for Mg-air battery[END_REF][START_REF] Chen | Electrochemical behavior of AZ31 magnesium alloy in MgSO4 solution[END_REF][START_REF] Ma | Performance of Al-Mg-Zn 0.1Ga-0.1Sn as anode for Alair battery[END_REF][START_REF] Feng | Comparison of electrochemical behaviors of AZ31 and AZ61 magnesium alloys in MgSO4 solution[END_REF]. On the other hand, morphology effects (nano, meso...) of the anode materials have been reported [START_REF] Li | Magnesium microspheres and nanospheres: Morphology-controlled synthesis and application in Mg/MnO2 batteries[END_REF][START_REF] Li | Metallic magnesium nano/mesoscale structures: Their shape-controlled preparation and Mg/air battery applications[END_REF].

Rechargeable systems implying reversible reactions remain a facing challenge but the Mg-air primary batteries, mechanically "refuelable", can still be improved through investigation of new electrode materials able to replace Mg [START_REF] Zhang | Magnesium-air batteries: from principle to application[END_REF].

In this paper, we report on the synthesis and characterization of a bimetallic material Mg2Al3 containing both magnesium and aluminum. The aim of the study is to explore the capabilities of this single-phase intermetallic compound used as anode in a metal-air fuel cell. Its electrochemical performances are analyzed comparatively with those of Mg and Al pure metals.

EXPERIMENTAL SECTION

Materials synthesis

Mg2Al3 is prepared from elements in stoichiometric ratio of Mg (Alfa Aesar, 99.8%, 20-100 mesh) and Al (Alfa Aesar, 99.5%, 325 mesh). Reagents are kept and handled in a glove box filled with controlled argon atmosphere. Weighed Mg and Al powders are intimately mixed and inserted in a tantalum container weld-sealed under argon atmosphere, which was in turn held within an Ar-filled sealed stainless steel jacket to avoid oxidation in air at high temperatures. Samples are heated to 700 °C in a tubular furnace and kept at this temperature for 24 h, during which they are submitted to a continuous rotation in order to ensure a good homogenization of the melt. Afterwards, samples are quenched in cold water and, to favor formation of the Mg2Al3 cubic polymorph, are annealed at 400 °C for 10 days. Then product is recovered and finally ground, either manually in a mortar or mechanically with a planetary ball mill.

Materials characterization

For identification and purity assessment purposes, powders are investigated by X-ray diffraction. The powder patterns are recorded on a Philips X'pert diffractometer equipped with a copper tube and a hybrid monochromator (parabolic multilayer mirror and two-crystal, Cu K radiation). A single crystal isolated from the reaction product has been used to record diffracted intensities within the complete diffraction sphere on Xcalibur CCD (Oxford Diffraction) four-circle diffractometer. The crystal structure has been solved and refined using SHELX programs [START_REF] Sheldrick | SHELXL97: A Program for Refining Crystal Structures[END_REF][START_REF] Sheldrick | [END_REF]. Details on experimental parameters for data collection and structural refinements have been deposited (CSD-427957) and are available on demand * . The final refined composition Mg37.3Al62.7 is found in very good agreement with the EDX analyses of the crystal. Semi quantitative analyses of the samples are performed using a scanning electron microscope fitted with an energy dispersive detector unit.

Self-corrosion measurements

The Mg2Al3 powdered material has been pressed into pellets to be used as active material for the electrochemical experiments. Self-corrosion measurements are performed in 0.6 mol L -1 NaCl and 0.6 mol L -1 KOH solutions. The Mg2Al3 pellet (Mg rod) is immersed in the solution for reaction times ranging from 16 to 96 hours. Samples weights are determined with accuracy before and after experiment. Immersed samples are washed in de-ionized water after removal of the corrosion products formed at their surface and dried prior weighing. The corrosion rate is calculated using the weight loss method and expressed as the weight loss per surface area and per immersion time.

Electrochemical measurements

The Teflon test cell designed for the electrochemical measurements includes a working electrode (sample pellet) of 13 mm in diameter, a platinum foil used as counter electrode and a silver chloride Ag/AgCl-KCl saturated reference electrode (Fig. 2). The sample is fixed onto a stainless steel holder using a Teflon mask then the area of working electrode is 0.785 cm².

Either NaCl or KOH aqueous solutions are used as electrolytes. The solutions are prepared from analytical grade reagents and de-ionized water (resistivity > 18 Mcm). Sodium chloride (Carlo Erba) or potassium hydroxide (Chem Lab) is added to water in adequate amount to make a solution at concentration 0.6 mol L -1 . Electrochemical measurements are ______________ * CIF file may be obtained from FIZ Karlsruhe, www.fiz-karlsruhe.de/request_for_deposited_data.html monitored using a Biologic VSP potentiostat controlled by the EC-Lab software. The potentiodynamic polarization curves have been recorded in a voltage domain that ranges from -1.8 to -0.5 V vs Ag/AgCl at a scan rate of 2 mV s -1 after OCP measurement.

Battery tests

Battery tests are performed in the same cell mounted in a two-electrode configuration and the battery then consists of anode, cathode and electrolyte (Fig. 2). The experimental device allows the anode testing in the real conditions of a metal-air battery. The discharge of the cell has been studied in galvanostatic mode at current densities of 5 and 10 mA cm -2 , the cell output potential is recorded for 16 hours. In order to evaluate the amount of anodic material that is consumed during the reaction, weight of the anode is determined before and after the discharge. The efficiency of the anode,  can be then calculated using the following equation:

  100 f i     W W M Q/nF η
In this expression Q is the electric charge (Coulomb) and n the number of electrons involved in the discharge reaction, F the Faraday constant (F = 96485 C mol -1 ) and M the atomic mass (g mol -1 ). Weights of the sample before and after discharge, Wi and Wf, are expressed in grams.

RESULTS AND DISCUSSION

Material characterization

Owing to their low density, the Mg-Al compounds are of great interest for technological applications. Few intermetallic compounds are known in the binary system and display quite complex structures. The compound commonly referred to as Mg2Al3 actually corresponds to a more complex stoichiometry and, according to the binary diagram, exists within a rather small composition domain [START_REF] Massalski | Binary alloy phase diagrams[END_REF]. Its structure is known from several decades [START_REF] Samson | -Mg2Al3[END_REF]. A recent comprehensive study has revealed that it undergoes a polymorphic transition →' at 214 °C from cubic (Fd 3 m, a = 28.24 Å) to rhombohedral (a = 19.97, c = 49.91 Å) symmetry [START_REF] Feuerbacher | The Samson phase, -Mg2Al3, revisited[END_REF].

In present work, alloys are prepared from high temperature synthesis at composition 60% Al and 40% Mg and then submitted to annealing treatment to obtain the pure compound in its cubic form. X-ray diffraction is used for purity evaluation and complete identification of the product. This structure is characterized by a high level of disorder with a large number of partially filled atomic sites, atomic mixing and split positions. Our refined model perfectly fits the structures and stoichiometries established earlier for this specially complex structure described with 672 icosahedral clusters, 252 Friauf polyhedra and 244 other polyhedra [START_REF] Samson | -Mg2Al3[END_REF][START_REF] Feuerbacher | The Samson phase, -Mg2Al3, revisited[END_REF]. Note that the refined composition (Al/Mg ratio of 1.68) markedly deviates from the Mg2Al3 formula commonly used for this compound. The disordered nature of the compound probably result for packing and/or electronic constraints and would account for specific properties. The behavior of Mg2Al3 compound as anode material in a metal-air battery is expected to be different from that of classical magnesium alloys as AZ31 etc… In the former case, the material is composed of a homogeneous distribution of a bimetallic compound while in the latter case the material rather consists of a distribution more or less uniform of Mg and Al within a composite material. Consequently these two situations are significantly different, particularly from the point of view of the atomic interactions and bonding types, which could result in discrepancies in electrochemical behavior.

Corrosion potentials of pure metals

Electrochemical experiments have been carried out in 3-electrode cells filled either with a 0.6 mol L -1 NaCl or a 0.6 mol L -1 KOH solution used as electrolyte. The corrosion potential determined for a pure Mg anode in these conditions with a salt electrolyte is -1.59 V vs Ag/AgCl (-1.63 V vs. SCE) and compares well with values given in literature, as for example -1.57 V vs. SCE measured in a NaCl solution at a very slightly higher concentration (0.615 mol L -1 or 3.5 wt.%) [START_REF] Li | Metallic magnesium nano/mesoscale structures: Their shape-controlled preparation and Mg/air battery applications[END_REF]. On the other hand, when using the alkaline electrolyte, the measured potential of -1.55 V vs Ag/AgCl (-1.59 V vs. SCE) indicating a slightly lower reactivity (and a lower current density). It appears that the corrosion potential of the pure magnesium anode is not strongly affected when saline electrolyte is changed for alkaline electrolyte, at least at compositions and concentrations used in this work (table 1). Owing to chemical composition (~63 at.% Al) of the material investigated in this work, Mg2Al3, similar electrochemical experiments are also performed using a pure Al anode. The corrosion potential of metallic Al determined in aqueous NaCl electrolyte is -0.71 V vs Ag/AgCl while it is strongly shifted to -1.67 V vs Ag/AgCl when measured in aqueous KOH electrolyte. In contrast to results obtained for magnesium, the aluminum anode behavior is very different in saline or alkaline electrolytes with an important shift in the corrosion potential. It is well known that Al easily forms an Al2O3 protective layer on its surface which is responsible for the high potential in NaCl solution. Instead the alkaline solution is able to attack this layer and clean the electrode from oxide, thus pure Al is available to be involved in a corrosion reaction. This well illustrates the pH effects on the corrosion potential of an aluminum electrode which passivates in saline media while it is relatively rapidly consumed in an alkaline electrolyte. A recent study has evidenced pH effects on the corrosion potential of an Al-based, Mg-containing, ternary alloy and, as could be expected, corrosion rate is lower in neutral solutions [START_REF] Ma | Effects of acidity and alkalinity on corrosion behaviour of Al-Zn-Mg based anode alloy[END_REF].

Corrosion potential in alkaline electrolyte

The corrosion potential of the intermetallic compound Mg2Al3 has been measured in alkaline electrolyte. Tafel representation of the voltametric curves I = f (V) is given in Fig. 5 comparatively with magnesium and aluminum references. Experiments have been performed

for different sample conditioning, starting from Mg2Al3 powder. As shown in scanning electron microscopy images, the original ground Mg2Al3 powder is homogeneous and formed of micron-sized particles (Fig. 6a).

For a sample prepared from Mg2Al3 powder pressed at room temperature, the corrosion potential measured in KOH electrolyte is -1.35 V vs Ag/AgCl. Nevertheless, pellets prepared by this technique are very fragile and difficult to handle, so various routes have been investigated to improve their mechanical solidity. When 10 wt.% of polyethylene powder is added to Mg2Al3 as a binder, the corrosion potential increases to -1.26 V vs Ag/AgCl. The material is globally more corrosion resistant, the reason for that would be the presence of an inert component resulting in no reactive domains (polyethylene grains) at the sample surface.

Powdered samples have also been included in a poly(vinylidene fluoride) slurry and a similar effect is observed for the corrosion potential that shifts towards higher values beyond -1 V vs Ag/AgCl. It should be noted that in the latter two experiments the anode is actually a composite material in which Mg2Al3 inter-grain contacts are greatly reduced because of the binder.

Fig. 5 Potentiodynamic polarization curves in KOH electrolyte

Mg2Al3: pressed at room temperature, d-Mg2Al3: pressed at 275 °C, p-Mg2Al3: mixed with polyethylene binder

As one can see by looking at SEM images, the samples prepared from Mg2Al3 powder pressed at room temperature are composed of grains compacted together but still display some porosity (Fig. 6b). This is in agreement with the calculated density which is found about 60%.

Definitely, sample surface displays roughness which could support some heterogeneity in the corrosion phenomena (preferential domains, inter granular attack, …) and have some influence on the anode performances as reported for Mg samples of various controlled morphologies [START_REF] Li | Magnesium microspheres and nanospheres: Morphology-controlled synthesis and application in Mg/MnO2 batteries[END_REF][START_REF] Li | Metallic magnesium nano/mesoscale structures: Their shape-controlled preparation and Mg/air battery applications[END_REF].

For the Mg2Al3 powder pressed at 275 °C, the sample density is significantly increased and calculated values are higher than 96%. This result is well illustrated by the SEM images of samples pressed at this temperature that show a very dense material in which no individualized grains can be observed neither on sample surface (Fig. 6c) nor on the section of a cut pellet (Fig. 6d). When measured using such dense samples, the corrosion potential reaches a more negative value of -1.46 V vs Ag/AgCl. In the mean time, the current density is found fairly lower than for samples pressed at room temperature indicating a smaller electrochemical activity.

The corrosion potential of Mg2Al3 moves from weakly negative values when powder is dispersed into a binder to clearly more negative values when it is well compacted into dense pellets. Such displacement associated with the sample conditioning would originate in some particle coalescence and densification phenomena that may have an effect on the material properties and its electrochemical behavior. With a corrosion potential of -1.46 V, the dense Mg2Al3 sample is found slightly more corrosion resistant than pure Mg (-1.57 V).

For Mg2Al3 samples conditioned differently, the corrosion potential is found to decrease towards more negative values as the sample compacity increases. Surprisingly, the Mg2Al3 denser samples are found the more reactive in alkaline electrolyte. In the mean time, the current density globally decreases indicating a better resistance to corrosion for the material;

the weaker the current density, the slower the kinetics of the reaction.

Corrosion potentials in saline electrolyte

Analogous electrochemical potentiodynamic experiments are carried out using a 0.6 mol L -1 salt water electrolyte, the corrosion potentials measured for Mg2Al3 powders pressed at room temperature with and without polyethylene binder are very close (-1.37 and -1.39 V vs Ag/AgCl) as shown in Fig. 7.

Instead, values -1.18 and -1.17 V vs Ag/AgCl are measured for the corrosion potential of dense samples, the first is obtained for a hot pressed pellet and the second for a polished cast sample (bulk material). Note that a value of -1.33 V vs Ag/AgCl has been measured for corrosion potential of highly divided Mg2Al3 powder prepared by mechanical alloying and pressed at room temperature. This value is slightly higher than that of -1.39 V obtained starting from powder pressed under the same conditions, probably because of a higher density for the sample resulting from highly divided powder.

Contrary to what is observed in alkaline electrolyte, in saline electrolyte the denser samples provide the higher corrosion potentials (less negative values). Nevertheless lower current densities indicate that dense samples are characterized by a better resistance to corrosion and slow reaction rates. The Mg2Al3 denser samples display the lower reactivity in saline electrolyte.

Self-corrosion rates

Before evaluating performances in electrochemical metal-air batteries, the self-corrosion behavior of the samples has been studied in alkaline and saline electrolytes. Actually, selfcorrosion corresponds to chemical corrosion of the electrode, a side-reaction which should be as limited as possible.

The corrosion rates are measured by the weight loss method. As reported in table 2, pure Mg samples show a corrosion rate of 0.11 mg cm -2 h -1 in NaCl solutions, a value which is fairly lower than results reported in literature [START_REF] Ma | Performance of Mg-14Li-1Al-0.1Ce as anode for Mg-air battery[END_REF]. In contrast, dense Mg2Al3 samples (hot pressed or bulk) are found to display no measurable corrosion since no weight loss could be measured even after two weeks immersion. However, for long immersion times, samples are covered with a very slight white film indicating their surface could have been passivated.

Considering the Mg2Al3 good resistance to corrosion towards the electrolytes, it is reasonable to expect low self-discharge phenomena in assembled batteries. During the discharge process of a battery containing Mg2Al3 as anode material, the oxygen reduction still occurs at the cathode but the oxidation reaction expected at anode could be more complex because it could involve the participation of the two elements, Mg and Al.

Trying to elucidate what are the reactions, the product which is formed as a white precipitate at the bottom of the cell has been recovered for analysis. The XRD powder pattern of the raw precipitate indicates the presence of an amorphous component which cannot be thus identified and contains a few peaks that are characteristic of the aluminum hydroxide. At this stage the participation of aluminum in the reaction mechanism has been evidenced but no proof of the magnesium contribution is obtained. In a thermal analysis experiment, the collected precipitate is heated up to 1000 °C under N2 atmosphere. The two endothermic events that are After a long discharge of 16 hours operated in a battery that uses NaCl as electrolyte, the anode pellet of Mg2Al3 has been analyzed. It has been reported in literature that, for Mg alloys used as anodes, a part of the discharge product remains on the electrode surface and forms a layer with pore and crack that prevent passivation [START_REF] Cao | Electrochemical behavior of Mg-Li, Mg-Li-Al and Mg-Li-Al-Ce in sodium chloride solution[END_REF][START_REF] Ma | Performance of Mg-14Li-1Al-0.1Ce as anode for Mg-air battery[END_REF]. At a macroscopic scale, no thick corrosion layer covering the Mg2Al3 anode has been observed, the product which forms during discharge mainly deposits in the electrolyte at bottom of the cell. The surface of the anode still displays a great homogeneity with no breaks or deep holes (Fig. 9). It is widely reported that pitting corrosion can be initiated at sample surface of alloys in which coexist different metallic micro-domains that behaves as micro-cells. In the case of dense intermetallic Mg2Al3 anodes, it would seem that such localized galvanic corrosion phenomena do not take place at the surface sample. The corrosion products mainly deposit and form an extremely thin layer which does not passivate the electrode, allowing a homogeneous corrosion process. As indicated by the formula given above, anodic efficiency can be calculated from the weight loss during discharge. Its expression depends on the number of electrons involved in the reaction. For the Mg anode, the reaction that occurs at anode is well known and evaluation of this number is obvious. In case of Mg2Al3, the calculated efficiency would depend on the reactions that take place at the electrode. Since proofs are provided that both Mg and Al are involved in oxidation reactions at anode, their both contributions will be considered in calculation. Most likely, the oxidation reaction of Mg2Al3 will provide Mg and Al at the respective oxidation states +2 and +3. Then, assuming the +2 oxidation state for Mg and +3

for Al, the number of electrons and the atomic mass should be replaced by average values calculated as weighted sums. The efficiency is then calculated according to the formula

  100 f i a a     W W M F Q/n η with Al Al Mg Mg a z x z x n   and Al Al Mg Mg a M x M x M  
where x represents the mole fraction, z the oxidation state and M the atomic elemental mass for each component.

The calculated anodic efficiency for Mg and Mg2Al3 anodes experiments are given in Table 3.

They are associated with 16 hour discharges of metal-air batteries using saline electrolyte.

Compared to pure Mg, the Mg2Al3 anode displays an increase in the anodic efficiency of about 15%. In fact, the anodic efficiency calculated for an Mg anode is in the range 52-56%, either for current densities of 5 or 10 mA cm -2 . In the same conditions the Mg2Al3 anode efficiency is fairly higher and reaches a value comprised between 70 and 73%. As attested by its corrosion potential shifted towards positive region, Mg2Al3 is more corrosion resistant than pure Al and Mg in alkaline electrolyte. On the contrary, Mg2Al3

CONCLUSION

behavior in NaCl environment is intermediate between those of pure metals. Battery tests with an Mg2Al3 anode and an oxygen-air cathode show a rapid decrease in potential using KOH electrolyte but an extremely stable potential using NaCl, a behavior which is similar to that observed for a pure Mg anode. The participation of both Mg and Al has been evidenced in the corrosion process that appears to be homogeneous. This may be related to the single-phase nature of the compound which protects the electrode from localized corrosion phenomena (galvanic, pitting…) commonly observed for alloys. In spite of a lower electrochemical activity, Mg2Al3 displays a higher anodic efficiency than pure Mg. Anodic polarization effects are still observed which can be overcome by increasing electroactive surface, using highly divided materials or by adding stabilizing additives in the electrolyte [START_REF] Chen | Combination of Lightweight Elements and Nanostructured Materials for Batteries[END_REF]. Many routes can be explored to gain performance improvements.

Fig. 1

 1 Fig. 1 Theoretical energy densities

Fig. 2

 2 Fig. 2 Schematic representation of the experimental cell: 3-electrode configuration for electrochemical measurements (left) and metal-air battery configuration (right)

Fig. 3

 3 schematically shows how operates a full magnesium-air battery. In these experiments, anode is an Mg or Mg2Al3 sample, cathode is an air-electrode with nanostructured Pd/C catalyst and electrolyte is a 0.6 mol L -1 NaCl or KOH aqueous solution. The cathode is made of C-supported nano-Pd catalyst deposited onto a Ni-foam to form a diffusive layer allowing oxygen flow. Palladium nanoparticles are obtained by a solution route from palladium dichloride and acidified ethylene-glycol using the polyol process. The cathode conditioning consists in making a deposit onto nickel foam from a mixture of Pd nanoparticles, carbon black and aqueous polytetrafluoroethylene suspension.

Fig. 3

 3 Fig.3Operating mechanism of a magnesium-air battery.

Fig. 4

 4 Fig. 4 Mg2Al3 experimental XRD powder pattern ( = 1.541 Å) given comparatively with the calculated and difference powder diffraction profiles

Fig. 6

 6 Fig. 6 SEM micrographs of Mg2Al3: up (a)-powder, (b)-pressed at room temperature; down (c) surface of a dense pellet pressed at 275 °C and (d)-section

Fig. 7

 7 Fig. 7 Potentiodynamic polarization curves in NaCl electrolyte Mg2Al3: pressed at room temperature, d-Mg2Al3: pressed at 275 °C, p-Mg2Al3: mixed with polyethylene binder

2Fig. 8

 8 Fig. 8 Discharge of Mg and Mg2Al3 anodes in metal-air batteries using NaCl electrolyte and operating at 5 mA cm -2 (left) and 10 mA cm -2 (right)

  detected have been attributed to the dehydration and transformation of the product while a 45% total mass loss is measured. The XRD pattern of resulting product reveals the presence of a mixed Mg and Al oxide, MgAl2O4 and a small amount of MgO. This brings a confirmation of both participation of magnesium and aluminum in the discharge reaction at the anode. Such finding corroborates the assumption that bonding nature around Mg plays a part in the oxidation phenomena occurring at the Mg2Al3 electrode.

Fig. 9

 9 Fig. 9 Scanning electron microscopy images of sample surface of Mg2Al3 anode after 16 h of discharge in NaCl electrolyte

  This study has been motivated by the investigation of Mg2Al3 electrochemical properties towards the corrosion. This intermetallic compound, known to undergo a polymorphic transformation has been prepared in its -Mg2Al3 cubic low temperature form. Its very complex structure displays both positional and atomic disorder. Despite the important number of disordered positions (split and partially filled sites, atomic mixing) our refinement from single crystal data is in very good agreement with previously published works. The very close stoichiometries would indicate a very narrow composition domain for this intermetallic compound.The intermetallic compound performances have been evaluated in metal-air devices. Resultsshow a very good resistance to self-corrosion for Mg2Al3, highly improved compared with pure Mg. Side reactions between electrode and electrolyte are almost suppressed either in alkaline or saline aqueous solutions.

Table 1

 1 Measured corrosion potential and OCP open circuit voltage (V vs Ag/AgCl) in 0.6 mol L -1 KOH and 0.6 mol L -1 NaCl aqueous electrolytes

		Corrosion potential	Open circuit voltage
	Material	NaCl	KOH	NaCl	KOH
	Al	-0.71	-1.67	-0.71	-1.72
	Mg	-1.59	-1.55	-1.62	-1.59
	Mg2Al3	-1.39	-1.35	-1.46	-1.41
	Dense Mg2Al3	-1.18	-1.46	-1.46	-1.52

Table 3

 3 Anode efficiency for a metal-air battery discharge (0.6 mol L -1 NaCl electrolyte)

		Current (mA cm -2 )	Anode efficiency (%)
	Mg	5	56.4
		10	52.2
	Mg2Al3	5	69.4
		10	72.5

Battery tests

The compound Mg2Al3 has been employed as anode material in experimental metal-air batteries assembled in the laboratory with a Pd catalyst air-cathode. The discharge has been processed at constant current densities of 5 and 10 mA cm -2 . The starting potential of a battery using an alkaline electrolyte is found relatively high at ~1.2 V but continuously decreases and rapidly falls down, reaching nought after less than one hour. Therefore, no further investigation has been carried out using alkaline electrolytes.

Performances of Mg2Al3 anodes are more attractive in a battery containing a salt water solution used as electrolyte. The open circuit voltage OCV measured before discharge is found rather high at 1.4 V. Starting the discharge causes a rapid drop in the potential which stabilizes after few seconds to a value close to 0.7 V, the battery then operates at a constant voltage during several hours. This working potential value should be compared to 1.3 V, the operating potential of a pure magnesium anode in same experimental conditions (Fig. 8). Such a discrepancy in the working potentials may be correlated to differences in interatomic interactions within materials. Actually, Mg is involved in metallic bonding in the elemental Mg while it is mainly surrounded by Al in the intermetallic Mg2Al3 compound where some covalent bonding occurs.

Typical experiments have been carried out up to 24 h for different sample conditionings. After discharge experiment, anodes made of room temperature pressed pellets are seriously dismantled while anodes made of dense pellets are only partially corroded. For a higher current density of 10 mA cm -2 , the operating potential of the battery still remains stable for several hours but is reduced to 1.0 V and ~0.4 V for Mg and Mg2Al3 anodes, respectively.

In the battery composed of a pure magnesium anode, the reaction that takes place at the anode is the oxidation of magnesium following