Mohammad R Ghaani

Michele Catti

Angeloclaudio Nale

Thermodynamics of Dehydrogenation of the 2LiBH4-Mg2FeH6 Composite

Keywords: Hydride composite, hydrogen storage, dehydriding reaction, van't Hoff plot

Joint decomposition of hydrides may be energetically favored, if stable mixed compounds are formed. This 'hydride destabilization' improves the energetics of H2 release from hydrogen storage materials. The sequence of dehydrogenation reactions of the 2LiBH4-Mg2FeH6 composite was studied by PCI (Pressure-Composition-Isotherm) and TPD (Temperature-Programmed-Desorption) techniques in a Sievert apparatus. Produced phases were identified by ex-situ X-ray diffraction and FTIR spectroscopy. Three distinct plateaus are detected on each isotherm: A, B, and C on decreasing pressure. The A reaction, involving formation of FeB, MgH2 and LiH, occurs at higher pressure/lower temperature than dehydrogenation of either pure hydrides; these are then effectively destabilized thermodynamically. The B process is plain decomposition of MgH2, and in C the magnesium produced reacts with LiBH4 left forming MgB2 and LiH. The B+C sequence is fully reversible, and it corresponds to twostep dehydrogenation of the LiBH4/MgH2 system. Reaction enthalpies and entropies were obtained through van't Hoff plots of all processes, thus providing a full thermodynamic characterization of the system.

INTRODUCTION

Within compounds of interest for hydrogen storage applications, metal hydrides and light metal borohydrides have been covered by extensive research work. [1][2][3][4][5] The first class of materials are characterized by moderate weight percent hydrogen yields, but they usually show good reversibility of the hydrogen exchange reaction. Borohydrides, on the other hand, have a quite large hydrogen content; however, in most cases their dehydrogenation is hardly reversible. Further, a common problem shared by both classes of hydrides is that they are thermodynamically more stable than desired, mainly because of their large decomposition enthalpies: they thus release hydrogen at rather high temperatures, raising a serious technological difficulty for applications. Such an issue was addressed by attempts to stabilize the dehydrogenation products of the hydride, in order to reduce the overall reaction enthalpy. 6 This may be achieved when different hydrides react together, if particularly stable mixed compounds are formed by dehydrogenation. Theoretical predictions confirm that this should be a promising way of improving the performances of hydrogen storage materials. 7 This approach was applied in particular to LiBH4, an important member of the borohydride family. Lithium borohydride is a well known system, with a high gravimetric H2 density of 13.9 wt% (considering the LiH product as not decomposable), and with dehydrogenation occurring in the 400-600 °C thermal range at ambient pressure and without catalysts. 8 The reaction could be reversed only at high pressure and temperature. 9,10 Published work on LiBH4 destabilizing dehydrogenation reactions concerns mainly the LiBH4+MgH2 system, which was investigated quite thoroughly. [10][11][12][13] Also systems involving two borohydrides, like the LiBH4-Mg(BH4)2 assemblage, were considered in this respect. 14,15 The present research deals with the reaction of hydrogen release from lithium borohydride associated with Mg2FeH6. This metal hydride was considered because of its good performance for hydrogen storage, and aiming at the possible formation of stable iron borides which could favor the joint dehydrogenation process of both hydrides. Mg2FeH6 has an intermediate gravimetric (5.5 wt%) and a very high volumetric (150 kg m -3 ) hydrogen density, thus proving to be a quite interesting material for hydrogen storage. Yet the synthesis of this metal hydride is not so simple to perform because no stable Mg-Fe binary compounds are available, so that either pure Mg and Fe or MgH2 and Fe have to be employed as reagents, and some residual non-reacted Fe is often left with Mg2FeH6. [16][17][18][19][20][21][22][23] The work was undertaken with the main goals of (i) characterizing the phase composition of dehydrogenation products of the 2LiBH4-Mg2FeH6 composite along the whole reaction step sequence, (ii) determining the p(T) phase boundaries of the different hydride assemblages, and (iii) deriving the thermodynamic parameters of the reactions involved. By comparison with the known behavior of the pure components LiBH4 and Mg2FeH6, we expected possible energetic improvements due to formation of stable mixed compounds, so as to achieve lower T/higher p conditions for the dehydrogenation reaction.

EXPERIMENTAL

The Mg2FeH6 component was synthesized according to the route employing magnesium hydride and iron as reagents. 20,21 A 1.2 g sample of commercial MgH2 and -Fe (Sigma-Aldrich) in 2:1 molar ratio was loaded with ten 1 cm diameter stainless steel balls (ball-topowder weight ratio = 30) into a vial of the same material. The powder was ball milled in Ar atmosphere by a Retsch planetary mill for 30 h at 400 rounds min -1 . Then the ball milled sample was loaded in an automatic Sievert-type apparatus (Advanced Materials Corporation), brought to p(H2) =100 bar and then heated at 400 °C for various times. Several reaction times were employed: 3, 4, 6, 9, and 14 days, in each case analyzing the final product by X-ray diffraction.

Samples of the 2LiBH4-Mg2FeH6 composite were prepared by mixing synthesized Mg2FeH6 powder with commercial LiBH4 (Sigma-Aldrich) in a glove box filled with recirculated argon atmosphere. All subsequent handling of such samples took place in the glove box. Thermodynamic measurements of the hydrogenation/dehydrogenation reactions were done by the Sievert-type equipment. A 0.6 g sample of the composite was loaded into the 5 cm 3 reactor, located in a furnace with temperature control: after evacuation, hydrogen gas of high purity (99.9995%) was introduced at room temperature (RT) and high pressure (100 bar). Dehydrogenation experiments were performed in two different modes. Pressure-Composition-Isotherm (PCI) mode: the sample was heated to the target temperature, which was thereafter kept constant; then the pressure was progressively decreased from 100 to 0.1 bar by small steps of approximately 0.3 bar (desorption experiments). At each step, the sample was allowed to equilibrate until the rate of pressure change due to H2 release was less than 1 mbar min -1 ; in most cases, this condition was fulfilled within 2 h waiting time. Then the relative amount of hydrogen exchanged by the sample was computed by the system software. Temperature-programmed-desorption (TPD) dynamic mode: a temperature ramp from 25 to 550 °C with 1 °C min -1 rate, under H2 high pressure (3 to 80 bar at RT), was set up; the pressure change due to bare heating was corrected using a calibration curve; the residual pressure increase was converted into the amount of released hydrogen at each temperature. Hydrogenation experiments were carried out in PCI mode only.

A Bruker D8 Advance X-ray powder diffractometer (CuK radiation), equipped with secondary beam monochromator, was used for all diffraction measurements. All samples were always protected from air in a special holder covered with Kapton film; this was displaced from the diffraction plane, so as to avoid polymer contribution to the recorded pattern.

Infrared spectra were recorded by a JASCO FT/IR-4100A spectrophotometer in the 400 to 4000 cm -1 wavenumber range (2 cm -1 resolution). The powder sample was mixed with nujol and inserted between two KBr windows, sealed externally in glove box for air protection.

RESULTS AND DISCUSSION

3.1. Synthesis and properties of the Mg2FeH6 hydride. X-ray measurements on the ball milled 2MgH2+-Fe powder showed the presence of Mg2FeH6 and alpha-iron only, without traces of either MgH2 or Mg. Thus, all magnesium hydride was consumed in the 6MgH2+3Fe  2Mg2FeH6+2Mg+Fe reaction; the residual Mg was not observed because it was possibly in nanocrystalline state. After the hydrogenation step in the Sievert apparatus, again the sole presence of Mg2FeH6 and of a variable quantity of -Fe (clearly decreasing with synthesis time) could be detected in the X-ray data (Figure 1). The amount of iron dropped from the 3 to the 4 days run, and then decreased very slowly to a constant value after 9 days. Thus, the direct synthesis reaction 2Mg+Fe+3H2  Mg2FeH6 was not complete in the Sievert equipment, leaving a fraction of not reacted Fe in the sample; this agrees with what reported in previous work on the magnesium iron hydride system. 21 On the other hand, the small quantity of Mg left did not react directly with hydrogen to form magnesium hydride, but it cannot be detected in the X-ray pattern because presumably in nanocrystalline or noncrystalline form.

The relative Fe/Mg2FeH6 amount was determined by means of two-phase Rietveld refinements (FULLPROF computer package [START_REF] Rodriguez-Carvajal | FULLPROF: a program for Rietveld refinement and pattern matching analysis[END_REF] ). For 9-days synthesized Mg2FeH6, a weight composition of 5.8(1)% Fe and 94.2(1)% Mg2FeH6 was obtained, corresponding to a Fe/Mg2FeH6 weight ratio of 0.062. The refined cubic unit-cell parameters were a = 6.4435 (3) and 2.8611(2) Å for Mg2FeH6 (space group Fm-3m) and -Fe (space group Im-3m), respectively. Final values of the agreement factors: Rp=0.127, wRp=0.160, reduced  2 =1.9.

A TPD run was performed on the Mg2FeH6 sample in the Sievert apparatus under 3 bar of hydrogen pressure. The inflexion point of the decomposition profile was located at 350 °C, and a total H2 release of 4.8 wt% up to 550 °C was observed. This compares well with the expected value of 4.9%, obtained from the ideal H2 yield of 5.47 wt% of pure Mg2FeH6 by correction for the iron and magnesium impurities content.

3.2.

Steps of 2LiBH4/Mg2FeH6 dehydrogenation. The 2:1 nominal molar ratio was selected for the LiBH4/Mg2FeH6 system, in order to study all multiple dehydrogenation steps of LiBH4 reacting with subsequent products of previous reactions. The theoretical hydrogen yield of Mg2FeH6-2LiBH4 is 7.85 wt%, considering that not decomposed LiH should be present in the final product. This can be an attractive value for many applications, in case it is coupled to a decreased dehydrogenation temperature with respect to that of pure LiBH4. Isothermal (PCI) H2 desorption experiments were performed at eight temperatures in the 325-550 °C range. In four of these cases, hydrogen absorption was carried out as well. The results of full desorption and absorption cycles for two isotherms are shown in Figure 2. We discuss hereafter desorption results; for absorption, cf. the next section. Three pressure plateaus can be observed in all desorption experiments, indicating distinct dehydrogenation reactions which are denoted as A, B, and C in the decreasing pressure order. The A process was not observed in the isotherms at T > 400 °C, because the maximum applied pressure of about 100 bar was not sufficient to prevent the reaction from occurring already during the heating step. The A dehydrogenation reaction is the most demanding one kinetically: over 2 h waiting time was necessary for the equilibration at each pressure step, whereas about 0.5 and 1.5 h was sufficient on the average for the B and C processes, respectively.

In order to understand the dehydrogenation chemistry of the three processes, the isothermal experiments at 345 and 375 °C were repeated in the following way. Hydrogen desorption was interrupted after the A and B reactions, the sample holder was quenched in water, and X-ray powder patterns were recorded. X-ray data were collected at the beginning (before A) and at the end (after C) of desorption, too. In another experiment at 345 °C FTIR spectra were collected on the samples obtained after A and B in desorption mode, and after the B+C reaction in absorption mode.

The X-ray results of the experiment at 375 °C are shown in Figure 3; those obtained at 345 °C are the same. In Figure 4 the FTIR spectra referring to the 345 °C experiment are reported. After reaction A, MgH2, FeB and LiH are formed as products. This proves that LiBH4 indeed reacts with Mg2FeH6 during this dehydrogenation process. A possible reaction of lithium borohydride with the residual iron present in the sample was investigated independently, by performing a PCI experiment on the LiBH4+Fe assemblage at the same temperature. The decomposition isotherm was identical to that of pure LiBH4, thus excluding any direct reaction with iron. Thus, the most probable reaction occurring during process A is as follows:

2Mg2FeH6 + 2LiBH4  2LiH +4MgH2 + 2FeB + 5H2 (1)
A small amount of residual Mg2FeH6 appears among the products, indicating that when the sample was taken out of the Sievert device (just at the end of the A horizontal plateau) the reaction was not fully completed yet.

The B process can be interpreted unambiguously as the dehydrogenation reaction of MgH2, as the Bragg peaks of magnesium hydride disappear after B and are replaced by the those of Mg:

MgH2  Mg + H2 (2)
No role should be played by LiBH4 in this process, because MgB2 is not formed.

In the final product of dehydrogenation, in addition to the previous H-free compounds also the new Bragg peaks of MgB2 are observed. This proves that in the C step the residual amount of LiBH4 has reacted with Mg according to:

2LiBH4 + Mg  2LiH + MgB2 + 3H2 (3)
A reaction of LiBH4 with MgH2 can be excluded, because magnesium hydride had disappeared after the B step.

At variance with the LiBH4+MgH2 system, the dehydrogenation thermodynamics of the LiBH4+Mg assemblage is not reported as such in the literature. Only the catalytic effect of Mg on the lithium borohydride decomposition was studied. [START_REF] Orimo | [END_REF] However, reaction (3) was reported to occur 12 as the second part of the two-step dehydrogenation of LiBH4+MgH2, after decomposition of MgH2 into Mg+H2. Indeed, the second step was shown to follow route (3) at higher pressure when sufficient equilibration time is provided, whereas when pressure is reduced quickly to a lower value Mg would not be involved in the reaction, but the alternative route 2LiBH4 + Mg  2LiH + Mg + 2B + 3H2 would be followed. 26 This is consistent with our result: reaction C occurs with slow pressure equilibration, so that Mg can react with LiBH4 according to route (3).

Surprisingly, no peaks of LiBH4 appear in the X-ray (a) and (b) patterns of Figure 3, although only half lithium borohydride was consumed in reaction A, and the rest of it must be involved in reaction C because of the observed H2 release with formation of MgB2 (cf. the (c) pattern). It should be noticed that LiBH4 melts at 280 °C; as PCI experiments were carried out at a higher temperature, re-solidification of unreacted lithium borohydride in nanocrystalline or amorphous form may have occurred. Indeed, results of FTIR measurements show clearly the presence of LiBH4 after the A and B reactions (spectrum (a) in Figure 4).

The three IR absorption peaks at 2225, 2291 and 2380 cm -1 correspond to active B-H stretching modes, and the peak at 1125 cm -1 indicates a B-H bending. [27][28][START_REF] Brampton | Destabilization of lithium borohydride with additions of magnesium hydride[END_REF] Further, after re-hydrogenation the fingerprints of LiBH4 appear with even greater evidence (spectrum (b) in Figure 4), but the corresponding X-ray pattern shows only the peaks of MgH2 and FeB. Our results are consistent with what reported by other authors, who found the X-ray peaks of LiBH4 to be hardly visible after re-solidification from melt, [START_REF] Orimo | [END_REF] and who could detect by FTIR but not by X-ray measurements lithium borohydride formed by hydrogenation of the 2LiH+MgB2 assemblage. [START_REF] Shaw | [END_REF] In summary, the global desorption reaction can be written as A+4B+C:

2Mg2FeH6+4LiBH4  4LiH+3Mg+2FeB+MgB2+12H2 (4)
corresponding to a theoretical hydrogen yield of 7.85 wt%. Taking into account that the actual molar ratio of LiBH4 to Mg2FeH6 is slightly larger than the nominal value of 2, because of the presence of little residual iron and magnesium in the as-synthesized Mg2FeH6 sample, the small excess of LiBH4 is presumed to have reacted with excess Mg in reaction C.

In a previous dehydrogenation study of the 5LiBH4/Mg2FeH6 mixture 31 no clear diffraction peaks could be detected in the X-ray pattern of the decomposed sample, possibly because of the small content of magnesium iron hydride and/or the absorption of the parafilm used. Therefore, the phases obtained could not be identified. The Differential Scanning Calorimeter (DSC) pattern in flowing N2 showed two peaks at 367 and 410 °C, which were tentatively interpreted as due to decomposition of pure Mg2FeH6, and to dehydrogenation of LiBH4 reacting with Fe and Mg, respectively. On the basis of our phase identifications, we suggest to relate such DSC peaks to the A and B processes considered above (cf. reactions

(1) and ( 2)).

Reaction reversibility and thermodynamic properties.

The final samples obtained after full decomposition (4) could always be rehydrogenated successfully in the Sievert apparatus. However, only the equivalent of the H2 amount released in the C and B steps was reabsorbed, whereas the A reaction could not be reversed (Figure 2). In the absorption cycles at lower temperatures, the C and B plateaus appear to be distinct; then the reversible B+C process can be clearly identified with the two-step reaction 2LiBH4 + MgH2  2LiBH4 + Mg + H2  2LiH + MgB2 + 4H2 previously proposed for the 2LiBH4 + MgH2 composite. 12 Above 350 °C the two absorption steps merge into a single one, according to the one-step 2LiH + MgB2 + 4H2  2LiBH4 + MgH2 hydrogenation reaction. [START_REF] Shaw | [END_REF] This reaction was found to occur also in rehydrogenation experiments carried out at 445 °C on products of the decomposition of the 5LiBH4/Mg2FeH6 mixture. 31 Comparable amounts of exchanged H2 are observed in absorption and desorption modes values, fitted by linear regression and normalized to 1 mole of H2, are reported in Table 1.

Also the plots for decomposition of the pure LiBH4, Mg2FeH6 and MgH2 hydrides from the literature 9,16,32 are shown in Figure 5 for the sake of comparison; in Table 2 the corresponding reaction enthalpies and entropies are given. For Mg2FeH6, the entropy for data from Ref. 16 was derived by the van't Hoff fitting of original points; results by other authors [18][19][20] are also reported for comparison.

By inspection of Figure 5, the van't Hoff plot of process A lies well above the decomposition lines of Mg2FeH6 and LiBH4 (and also of MgH2). This proves definitely that both components of the composite are strongly destabilized thermodynamically by reacting together with formation of iron boride FeB, according to reaction (1). For the sake of comparison it can be useful to extrapolate the ideal dehydrogenation temperature at standard pressure by the rH/rS approximation, on neglecting the thermal dependence of enthalpy and entropy. We get 217 °C, to be compared with 302-319 °C for Mg2FeH6 and 370 °C for LiBH4, on the basis of data in Table 2. Thus, the first H2 desorption step of the 2LiBH4-Mg2FeH6 composite is confirmed to be thermally more favourable than dehydrogenation of either pure hydride component. Interestingly, this result is due not to the reaction enthalpy, which is comparable for the three reactions (cf. Tables 1 and2, and the slopes of van't Hoff plots in Figure 5), but to the entropy, which is substantially larger for reaction (1) than for decomposition of either simple hydride. Therefore in this case the thermodynamic destabilization (i.e., the decrease of rG = rH-TrS) is due to the entropic rather than to the enthalpic effect.

According to plots of Figure 5, reaction B is clearly confirmed to be the dehydrogenation of magnesium hydride; the numerical values of van't Hoff parameters in Table 1 match well with literature data (Table 2). Reaction C shows a remarkably smaller enthalpy than all other processes (including those of Table 2). This enthalpy is comparable to that given in the literature for decomposition of the LiBH4+MgH2 assemblage in a single step; 10 we are not aware of previous results for the individual second step C. Indeed, it is interesting to notice how the plots of reactions B and C are well separated in Figure 5, fully consistent with the two-step model proposed for the LiBH4+MgH2 decomposition. 12 Thus, LiBH4 is destabilized by reaction (3) because of its smaller enthalpy, which overcompensates the slight increase of entropy, with respect to dehydrogenation of pure LiBH4 (cf. also Figure 5).

The results of TPD measurements are shown in Figure 6. Three curves were recorded at 3, 20 and 80 bar of hydrogen pressure. In the 3 bar data the A and B processes are not resolved but merged in a single inflexion point at 345 °C. At higher pressures, on the other hand, the inflexion point splits clearly into two ones corresponding to A and B reactions (380 and 401 °C at 20 bar, 414 and 472 °C at 80 bar). The C reaction appears only in the 3 bar experiment (at 460 °C), whereas it moves above the upper T limit of the experimental setup (550 °C) in the high pressure runs. On comparing the TPD pairs of p,T values for each reaction with the corresponding PCI results (cf. the van't Hoff plot of Figure 5), lower p/higher T values are observed as expected, due to the large hysteresis effects of the dynamic thermal ramp mode even with low heating rate. The qualitative agreement is however good, confirming the sequence of reactions determined by the more accurate isothermal mode.

The scheme of dehydrogenation reactions should be discussed also in connection with the amount of hydrogen released in the three different identified steps. In Table 3, the average values of desorbed H2 wt% (PCI and TPD results) are given for the A, B, C processes and for the overall decomposition reaction. Significant deviations from the PCI averages can be observed for individual isotherms (cf. also Figure 2). For comparison, the theoretical ideal wt% values are reported, too; these quantities are always referred to the initial weight of the 2LiBH4-Mg2FeH6 composite. As a small fraction of unreacted Fe and Mg is present in the sample, as discussed in the previous section, the ideal overall H2 wt% should be corrected accordingly. On using the Fe/Mg2FeH6 weight ratio obtained by Rietveld refinement of the Xray pattern, an expected overall wt% hydrogen yield of 7.45% is obtained for the system (cf.

the ideal value of 7.85%). The other corrected values for individual reaction steps are also reported in the last column of Table 3. Average PCI and TPD results are slightly lower than expected for the B and C reactions; the deficit is somehow larger for the A step (from PCI rather than TPD data). On the whole, therefore, the observed relative amounts of desorbed hydrogen are consistent with the interpretation of reactions A, B and C according to (1), (2), and

.

CONCLUSIONS

The 2LiBH4-Mg2FeH6 composite has proved to undergo dehydrogenation in three distinct steps A, B, and C. Reaction A occurs well within the p(T) stability regions of both pure components, with formation of the mixed compound FeB. Each of the two hydrides is found to be really destabilized thermodynamically by the presence of the other one in desorption conditions; the effect was shown to be of entropic rather than enthalpic nature. As an example, at 30 bar of H2 pressure the composite should decompose at 330 °C, whereas at the same pressure Mg2FeH6 would release H2 only at 455 °C, and LiBH4 even at 580 °C. For comparison, also MgH2 would require heating to 425 °C to decompose at 30 bar. This confirms that the destabilization method 6 may be quite effective in a broad chemical context.

Of course, however, the wt% hydrogen yield of the composite from reaction A is smaller than the full decomposition yields of the two single hydrides. Further, below p(H2) = 100 bar the reaction A could not be reversed, so that higher pressures would be possibly necessary to rehydrogenate FeB.

The B and C steps are well reversible, and they correspond to two distinct decomposition stages of the well known LiBH4-MgH2 assemblage, according to what previously found by different experimental methods. 12 In particular, the present study provides a full thermodynamic characterization of the poorly defined second stage C (reaction of LiBH4 with Mg). We thus believe that fruitful results will be obtained in the future by extending investigations on the dehydrogenation properties of complex hydride systems. composite from PCI desorption results (full symbols including best fit lines). Open symbols denote literature data for the LiBH4 (circles), 9 and Mg2FeH6 (squares) 16 decompositions. The dotted line without symbols corresponds to MgH2 dehydrogenation. 32

  for both the B and C steps. Re-desorption experiments were also performed after absorption, showing a very good reproduction of the B and C dehydrogenation steps as in the first desorption cycle. The p(T) equilibrium relationships (desorption mode) were determined for each of the three A, B, and C reactions, by measuring the pressure values at inflexion points of all isothermal plateaus. On this basis, the corresponding van't Hoff plots ln(p/p0) = rS/R -(rH/R)(1/T) were obtained, and they are shown in Figure 5. The corresponding rH and rS

Figure 1 .

 1 Figure 1. X-ray diffraction pattern (CuK radiation) of the as-synthesized Mg2FeH6 hydride.

Figure 2 .

 2 Figure 2. PCI (Pressure-Composition-Isotherm) results on the 2LiBH4-Mg2FeH6 system for the 345

Figure 3 .

 3 Figure 3. X-ray diffraction patterns (CuK radiation) of the subsequent products of

Figure 4 .

 4 Figure 4. FTIR spectra (a) of H2 desorption products of 2LiBH4-Mg2FeH6 after reaction B, and (b)

Figure 5 .

 5 Figure 5. Van't Hoff plots of the three dehydrogenation steps A, B, C of the 2LiBH4-Mg2FeH6

Figure 6 .

 6 Figure 6. Dynamic thermal decomposition of 2LiBH4-Mg2FeH6 under variable hydrogen pressure

Figure 3 25

 3 Figure 3

Figure 4 26 Figure 5 27Figure 6

 2656 Figure 4

Table 1 -

 1 Thermodynamic parameters (PCI measurements, Figure5) for the H2 desorption steps of the 2LiBH4-Mg2FeH6 composite, referred to 1 mole of H2. The e.s.d.'s are given in parentheses.

	____________________________________________________________________________________
		Chemical reaction	rH	rS	T range
			(kJ mol -1 )	(J K -1 mol -1 )	(°C)
	____________________________________________________________________________________
	A	2Mg2FeH6 + 2LiBH4  2LiH +4MgH2 + 2FeB + 5H2	72(4)	147(7)	310-400
	____________________________________________________________________________________
	B	MgH2  Mg + H2	74.6(7)	138.0(8) 310-440
	____________________________________________________________________________________
	C	2LiBH4 + Mg  2LiH + MgB2 + 3H2	48(2)	87(2)	350-530
	____________________________________________________________________________________

Table 2 -

 2 Literature thermodynamic parameters for the dehydrogenation reactions of Mg2FeH6,

	MgH2 and LiBH4, referred to 1 mole of H2.
	_____________________________________________________________________
	Chemical reaction	rH (kJ mol -1 ) rS (J K -1 mol -1 ) Ref.
	_____________________________________________________________________
	2Mg2FeH6  2Mg + Fe + 3H2	77	134	16
		87(3)	147(15)	18
		80(7)	137(13)	19
		79	135	20
	______________________________________________________________________
	MgH2  Mg + H2	74.5	135	32
	______________________________________________________________________
	2LiBH4  2LiH + 3H2	74	115	9
	_______________________________________________________________________

Table 3 -

 3 Average hydrogen yields of the decomposition steps of the 2LiBH4-Mg2FeH6 composite from isothermal (PCI) and dynamic (TPD) measurements, compared with ideal and corrected values (cf. the text).

	_____________________________________________________________
		PCI H wt% TPD H wt% ideal H wt% corrected H wt%
	_____________________________________________________________
	A	1.9	2.6	3.27	2.93
	B	2.2	2.0	2.62	2.34
	C	2.0	1.9	1.96	2.18
	Total	6.1	6.5	7.85	7.45
	_____________________________________________________________

AUTHOR INFORMATION

Corresponding Author * E-mail: catti@mater.unimib.it


Michele Catti

Mohammad R Ghaani

On the lithiation reaction of niobium oxide: structural and electronic properties of Li1.714Nb2O5

Monoclinic -Nb2O5 was chemically lithiated by reaction with n-butyllithium, mimicing the product of electrochemical discharge of a niobium oxide cathode vs. a Li anode. The compound was investigated by neutron powder diffraction (D2B equipment at ILL, France) and its structure was Rietveld refined in space group P2 to wRp=0.045, locating the Li atoms inserted in the -Nb2O5 framework. The ensuing chemical formula is Li12/7Nb2O5. A part of Li atoms are more strongly bonded (five coordinated O atoms), a part are less (coordination number = 4). Starting from the experimental structure, first-principles periodic calculations based on the hybrid B3LYP functional were performed. The electrochemical voltage of Li insertion was computed to be 1.67 V, fully consistent with the experimental 1.60 V plateau vs. capacity. An analysis of the electron band structure shows that lithiation changes the insulating oxide into a semi-metal; some of the extra electrons inserted with lithium become spin-polarized and give the material weak ferromagnetic properties.

Introduction

In the research field on rechargeable lithium batteries, an important role is played by the search for electrode materials with a medium to low discharge voltage vs. a Li + /Li anode (2.5 to 1.0 V range).

This contrasts with the well known trend towards higher and higher cathode voltages, aiming to attain the maximum energy density (cf. the 4.8 V of the LiNi0.5Mn1.5O4 cathode 1 ), but it is motivated by two specific reasons. The first one concerns the use as anode rather than cathode, replacing the conventional LiC6 anode vs. a high voltage cathode like the one just mentioned above. [2][3][4][5] Indeed, the electrochemical potential of conventional organic liquid-carbonate electrolytes (1.0 V) is slightly larger than that of the lithium or lithium/carbon anode, so that the carbonate would be reduced if no layer of passivating solid-electrolyte interface were formed, preventing the direct electrolyte-anode reaction. However, lithium dendrites tend to grow on that layer, leading to danger of short-circuits which may fire the flammable electrolyte. 6,7 For this reason anodes with no passivating layer and with potentials in the 1.0 to 1.5 V range, so as to be exempt from possible interactions with the electrolyte, would be highly desirable. [2][3][4][5] An already reported stable anode of this kind is Li4Ti5O12, with spinel structure, and working at a 1.5 V voltage on the basis of the Ti 4+ /Ti 3+ redox couple. 8,9 The second application (1.5 to 2.5 V range) is for cathode use in batteries powering the memory backup in portable electronic devices. 10,11 The microfabrication of integrated circuits brings about a reduction of power supply voltage to about 2 V or less, and the mismatch with a high voltage battery should be possibly avoided. That can be achieved by employing a battery cathode with electrochemical potential in the required range. With this purpose the lithium insertion reaction into Nb2O5 was studied electrochemically, in order to exploit the convenient potential of the Nb 5+ /Nb 4+ couple. 10,11 All three main polymorphs of niobium oxide were tested, finding a maximum capacity of 200 mAh (g-oxide) -1 (i.e. 1.98 Li atoms inserted on discharge) in the voltage range 2.2 to 1.5 V vs. Li anode. The behaviour of the monoclinic phase is claimed to differ slightly from that of the other modifications, because a flat portion of the potential vs. capacity curve would suggest a possible two-phase character of the reaction. However, the in-situ X-ray diffraction results are not exhaustive in this respect. Significant improvements of cycling stability and high rate capability were reported on using Nb2O5 nanobelts as electrode material. 12 We have thus undertaken a research programme to clarify the structural and electronic properties of lithiated niobium oxides, with a particular focus on the high-temperature monoclinic phase often reported in the literature as -Nb2O5 or H-Nb2O5; 13,14 the latter notation may cause possible confusion with another polymorph with hexagonal symmetry. In order to solve the challenging structural problem (14 f.u. are contained in the unit-cell of -Nb2O5), 15 an accurate neutron diffraction study of the lithiated oxide is necessary; we thus decided to perform the lithium insertion reaction chemically to obtain a sample in the required form and amount. By locating the lithium atoms in the niobium oxide framework an important insight into the mechanism of the insertion reaction was expected; further, this would provide the prerequisite knowledge for the subsequent theoretical analysis of the electronic and magnetic properties of the material.

In the second part of the work a detailed first-principles study of the band structure of lithiated niobium oxide is presented, using a hybrid DFT (Density-Functional-Theory)-HF (Hartree-Fock) functional which performs very well for simulating the band gap region and Fermi energy surrounding. 16 The aim of this section is to provide a full understanding of how the extra electrons added to niobium oxide by the insertion of Li atoms modify its electronic properties from insulating to conducting ones, with possible emergence of magnetic behaviour. Several quantum-mechanical studies of lithiated compounds have shown that the theoretical approach is extremely enlightening and useful for a deep comprehension of the reversible electrode functionality in lithium batteries. 17- 20

Experimental

Monoclinic niobium oxide was prepared by thermal annealing of commercial (Sigma-Aldrich) orthorhombic Nb2O5 at 1200 °C for 24 h, 10 followed by quenching in liquid nitrogen. The product was confirmed to be pure -Nb2O5 by X-ray powder diffraction (Bruker D8 Advance diffractometer, CuK radiation). Lithiation experiments were carried out by making this compound react with n-butyllithium in hexane, under N2 flow. The reaction at 50 °C for 60 h produced a black powder, which was filtered, washed repeatedly with hexane and dried in a glove-box with recirculated Ar atmosphere. By X-ray diffractometry (with a protected sample holder) a single-phase pattern was obtained which could be indexed according to the monoclinic unit-cell a=20.800, b=4.134, c=19.116 Å, =119.54°, similar to that reported previously for Li1.9Nb2O5. 21 Chemical analysis by atomic emission flame photometry gave a Li content of 1.8 Li per f.u., but the Li1.714Nb2O5 chemical formula was retained because consistent with the results of the structure determination by neutron diffraction (cf. the corresponding section below).

A sample of 3 g of Li1.714Nb2O5 was loaded into a vanadium can which was sealed with indium wire in Ar atmosphere. Data were collected in the 5-160° 2 range on the high-resolution (Δd/d = 5x10 -4 ) D2B powder diffractometer at the neutron source (nuclear reactor) of the Institut-Laue-Langevin (Grenoble, France). A primary beam monochromator of 28 Ge(115) crystals, set at  = 1.595 Å, was employed.

All calculations for determining and analysing the crystal structure were carried out by the GSAS computer package. 22 The intensity background was modeled by six Chebyshev polynomials, and the peak shape was represented by a pseudo-Voigt function (linear combination of Gaussian and Lorentzian components, with  and  half-widths, respectively). The σ and  parameters vary with  as σ = (U tan 2  + V tan + W + P/cos 2  ) 1/2 and  = X/cos + Y tan ; the U, V and W coefficients are mainly related to instrumental resolution, 23 P and X are the Scherrer coefficients for Gaussian and Lorentzian particle size broadening, respectively, and Y is related to particle strain broadening. The mixing coefficient and the full width of the pseudo-Voigt function depend on  and  according to equations given in the literature. 24

Computational

Quantum-mechanical calculations of the ground-state total crystal energy were performed by the computer code CRYSTAL09, [START_REF] Dovesi | CRYSTAL09 User's Manual[END_REF] based on the periodic LCAO (Linear Combination of Atomic Orbitals) approach. A hybrid B3LYP functional, i.e. a balanced mixture of the DFT (Density-Functional-Theory) LYP non-local correlation [START_REF] Lee | [END_REF] with the DFT Becke's 27 and the Hartree-Fock exchange, was employed. All-electron basis sets of Gaussian-type functions were adopted for the radial parts of atomic orbitals for O and Li atoms, according to the schemes 8(s)411(sp)G and 5(s)11(sp), respectively. The basis set of Nb was based on a large core Hay-Wadt-type Effective Core Potential for inner electrons and a 31(sp)31(d)G Gaussian scheme for valence electrons.

The five tolerances related to cut-off limits for Coulomb and exchange series summation were set to 10 -8 , 10 -8 , 10 -8 , 10 -8 , and 10 -16 , and DFT functionals were integrated by use of the XLGRID high accuracy conditions. The reciprocal space was sampled according to a regular sublattice of volume about 4 x 10 -5 Å -3 per point, requiring a Monkhorst grid defined by 2, 10, and 2 points along the three reciprocal axes. One-electron wavefunctions were allowed to be spin-polarized, according to majority () and minority () electron spins. Convergence of the SCF cycles, controlled by a E threshold of 10 -8 hartree per primitive unit cell, turned out to be delicate; a mixing factor of 90% with the density matrix of the previous cycle was necessary to achieve a stable iteration process.

Atomic coordinates and unit-cell constants were optimized together by calculation of analytical gradients and subsequent conjugate gradients algorithm.

Results and discussion

Structural properties

A Rietveld refinement of the crystal structure was started from the atomic coordinates of Nb and O atoms of -Nb2O5 in space group P2/m, 15 with isotropic displacement factors. This structural model implies that all atoms are in special positions on mirror planes (y coordinates either 0 or ½), except for the Nb1 atom in tetrahedral coordination which is disordered with occupancy 0.5. After reaching convergence a difference Fourier map was computed, revealing the positions of 12 negative peaks in the asymmetric unit in special positions on mirror planes. These could be interpreted as Li atoms on the basis of their distances to first neighbours, and their coordinates were included in the refinement successfully. Restraints on the Nb-O and Li-O distances had to be introduced, in order to prevent some of these bonds to become too short. A subsequent difference Fourier map did not show any negative peaks with crystal-chemical surroundings suitable for additional Li atoms. Therefore, the Li1.714Nb2O5 chemical composition consistent with 24 Li atoms per unit-cell (with 14 formula units) was accepted.

At this stage we decided to test the lower symmetry P2 space group, in order to remove the severe geometric constraint of mirror symmetry on the Li atoms, which may prevent them from attaining the most favourable bonding environment. The origin on the twofold polar axis was fixed at y(Nb2) = 0, and the Nb1 atom became ordered with full occupancy. Improved refinement results were obtained with respect to P2/m symmetry, and eventually convergence was attained with the agreement factors and unit-cell constants given in Table 1. Another attempt to find additional Li atoms in a difference Fourier map failed. Observed, calculated and difference intensity profiles obtained after the final cycle are shown in Fig. 1. The list of refined atomic fractional coordinates and displacement factors is reported in Table 2.

One can see by the agreement factors in Table 1 and by Fig. 1 that the fit of data is quite good; however, the e.s.d.'s of atomic coordinates are larger than desirable, particularly for the y values.

This is yet not unexpected, despite the high quality of the experimental measurements and equipment, taking into account the large number of refined parameters due to the structure complexity (number of observations to number of variables ratio as low as 13.5). The particularly large e.s.d.'s of the y coordinates are related to the strong P2/m pseudo-symmetry of the structure, which introduces significant atomic positional correlations along [010] normal to the pseudo-plane.

Other features of the results which are not fully satisfactory concern some of the displacement factors. Those of Nb1 and Nb4 had to be fixed at quite large values; as Nb1 lies on the twofold symmetry axis, its large apparent thermal motion suggests that the symmetry should be possibly lowered to P1. Other values were fixed to 0.1 to prevent them from becoming negative.

On comparing the unit-cell geometry of lithiated to unlithiated niobium oxide (Table 1), a significant volume expansion (+5.3%) is observed, as expected. However, the effect is highly anisotropic, as all the expansion is concentrated along the [010] direction (+8.1%), whereas the a and c cell edges even contract by -1.7 and -1.2%, respectively, on lithium insertion. This is related to the peculiar alignment of Li atoms parallel to the b vector, as explained below.

Indeed, the most interesting feature of the refined crystal structure of Li1.714Nb2O5 is the distribution of Li atoms in the framework of niobium oxide (Fig. 2). In perovskite-related structures, the preferred lithium sites are known to lie close to the square windows delimiting the cube-octahedral cages typical of these arrangements. In such cases, Li lies close to the window centre with a flat square pyramidal coordination: cf. the example of lithium-lanthanum-titanate (LLTO). 15 However, Nb2O5 phases show an additional feature not present in the perovskite-type, but rather in the rutile-type arrangement, i.e. double planes parallel to [010] of NbO6 octahedra sharing edges (Fig. 2). Thus, Li atoms near windows in such planes acquire an extra contact with an O atom belonging to the neighbouring plane, so as to raise their C.N. from 4 to 5 (Fig. 3). In the structure of lithiated Nb2O5 there are, therefore, two kinds of possible Li sites: the tetracoordinated perovskite-type ones, and those which are pentacoordinated. These sites can be counted: there are 16 of the first and 11 of the second type per asymmetric unit.

In the structure of Li1.714Nb2O5 there are 6 Li atoms with C.N.=5 and 6 with C.N.=4; they are denoted by circles and squares, respectively, in Fig. 2. The higher ratio of the number of fivefold-to fourfold-coordinated Li atoms (1:1) with respect to the average value (1:1.5) is consistent with the lower energy of the former class of Li sites, as suggested by obvious bonding considerations.

However, entropic factors and non-equilibrium kinetic conditions of the lithiation reaction can also account for the significant observed fraction of Li atoms with C.N.=4.

The statistical distribution of Li-O bond distances (Fig. 4) appears to be quite asymmetric, with a fall above the most populated range (2.1 to 2.2 Å). Shorter bonds are more populated than longer bonds, so as to depress the mean value (2.064 Å) below that range. This feature is common to both fourfold and fivefold coordinated Li atoms, whose partial distributions are not very dissimilar from the overall one. A comparable situation is observed also for the distribution of Nb-O bond lengths, which is plotted in Fig. 5 with exclusion of the shorter bonds of tetrahedrally coordinated Nb1 (1.81 Å x2 and 1.84 Å x2). Again the observed mean and median (2.056 Å) is smaller than the abscissa of the maximum (2.15 Å), giving rise to a skewed distribution.

First-principles simulations

Starting from the experimental P2 crystal structure of Li1.714Nb2O5, both atomic coordinates and unit-cell geometry were relaxed searching for least energy. The same procedure was applied to monoclinic -Nb2O5 as well, using again the P2 space group with ordered Nb1 atom instead of the experimental P2/m with split Nb1. The optimized unit-cell parameters of both phases are reported in Table 1, and the corresponding fractional coordinates can be obtained on request from the corresponding author. Theoretical lattice constants are moderately overestimated with respect to experimental values, as expected from the well known behaviour of the B3LYP functional. The Li-O and Nb-O bond length distributions were calculated and plotted in Figs. 4 and5, below the corresponding results from neutron diffraction measurements.

In the Li-O case, the predicted mean bond distance is slightly smaller than the experimental one (2.024 against 2.064 Å), at variance with what would be expected on the basis of the theoretical overestimate of the unit-cell constants. Further, the calculated Li-O distribution is more peaked than that from neutron diffraction, with a smaller range and a smaller variance; yet a similar asymmetry is shown in both cases. It can thus be inferred that the B3LYP functional slightly overemphasizes the strength of the ionic interaction of Li with the O nearest neighbours. On the other hand, the expected behaviour is observed for Nb-O distances, with a theoretical mean value a bit larger than the experimental one (2.087 vs. 2.056 Å). Accordingly, the distribution range is shifted to longer values, but the asymmetry is reversed.

The total energies calculated for the optimized structures of Li1.714Nb2O5 and -Nb2O5 (Table 1) were used, together with the value (-7.528014 hartree) obtained for BCC metal lithium, to compute the electrochemical potential of the Li insertion reaction (1/x)Nb2O5 + Li  (1/x)LixNb2O5 (with x=12/7). On changing the energy units into eV, one obtains the value -(1/x)[E(LixNb2O5) -E(Nb2O5)] + E(Li) = 1.667 V. This is our predicted discharge voltage for the monoclinic niobium oxide cathode against a Li anode, which compares remarkably well with the experimental plateau at about 1.6 V. 10,11 The wave functions of the least-energy structures of Li1.714Nb2O5 and -Nb2O5 can now be analyzed and compared, to derive the electronic and magnetic properties ensuing from the lithiation reaction. For this purpose, the well known performance of the B3LYP hybrid functional in reproducing experimental band gaps reliably will be very useful. 16 At variance with the insulating character of the pure oxide, the lithiated material is predicted to be an electronic conductor with semi-metallic character (low density of states at the Fermi energy) and weak ferromagnetic behaviour, with a spin magnetic dipole moment of 0.55 B/f.u. However, the crucial role is played by the small peak at the bottom of the conduction band of Nb2O5: on lithium insertion and addition of extra electrons the Fermi energy moves upwards beyond that peak, which becomes a tiny occupied band, with a surprisingly modest relaxation of the density of states. The small band, magnified in Fig. 7, proves to have a pure Nb character and to be completely spin-polarized; further, it is connected to the main part of the valence band at lower energy, and to the conduction band at higher energy, by thin low-density regions which close the 3.2 eV gap of Nb2O5 and explain the semi-metallic nature of Li1.714Nb2O5. A finer analysis of the DOS projections of the small band onto single Nb atoms shows that only four of them, Nb4, Nb5, Nb11 and Nb15 contribute substantially to it with their d-type levels. This point can be better understood by analyzing the atomic spin density distribution.

The electron density of states (DOS) is shown in

Mulliken charges for the Nb atoms of Li1.714Nb2O5 are reported in Table 3, in form of d contributions for total values (sum of charges from electrons with majority  and minority  spins), and of overall contributions for spin polarization density (difference of - charges). In facts, the Nb spin density is nearly completely due to contribution from d electrons. As the partitioning of Mulliken electron charge between Nb and O atoms suffers of some arbitrariness, the relative rather than absolute + values in Table 3 are fully physically reliable. Absolute - spin densities, on the other hand, are free from any bias. The maximum change of Nb total charge is 0.6 e, indicating a not negligible tendency to charge ordering in favour of Nb12, Nb8 and Nb6 as more reduced metal atoms. Such atoms with their centrosymmetrical ones form the most condensed block of six octahedra sharing three (Nb6) or four (Nb8, Nb12) edges with one another and with those translated by b (rectangular encirclement in Fig. 2). The octahedra of all other Nb atoms share either two or no edges with their neighbouring ones. As Nb-Nb distances become shorter with increasing condensation of octahedra, extra electrons can be better delocalized in the area between adjacent metal atoms at octahedra centres.

Charges on Li atoms range from 0.93 to 0.95 e, indicating a practically full ionization to the Li + state. This confirms that all of lithium valence electrons move to either d-type Nb bands or Nb-O hybridized levels, with no Li-O delocalized bonding contribution. On the other hand, negative charges on O atoms show significant variations in the -0.89 to -1.53 e range, with the expected trend of larger charges (in absolute value) for larger numbers of coordinated Nb and Li atoms.

A clear, strong ordering of the spin density on the Nb4, Nb5, Nb11, Nb15 atoms appears from Table 3. These atoms (included in the zigzag encirclement in Fig. 2) are exactly those contributing to the spin-polarized band shown in Fig. 7, as discussed above, and their total spin density (7.97 e/unit-cell) is similar to the overall value of 7.71 e/unit-cell. We can summarize our results by stating that, out of the 24 electrons/unit-cell added to niobium oxide by lithiation to form Li1.714Nb2O5, about one third polarize their spins concentrating in the d levels of eight Nb atoms and giving the material a weak ferromagnetic behaviour. The rest distribute equally between  and  spins, and fill valence states with hybridized Nb-O character preferentially in the condensed blocks of six NbO6 octahedra.

Further insight into the electronic character of Li1.714Nb2O5 can be obtained by analyzing the dispersion in reciprocal space of the energy band structure. This was computed for the optimized least-energy configuration as in the DOS and Mulliken charge cases. In Fig. 8 It is now possible to interpret the DOS of Fig. 7 in more detail. The spin-polarized ( electrons only) peak in the 1.5 eV range below EF is mainly contributed by the flat  bands at ky=0 and ½ in the same energy interval; the low-density region connecting valence to conduction states, which is responsible for electronic conductivity, is due to a few strongly dispersed  and  bands crossing the Fermi energy along the ky (b * ) direction.

Conclusions

Chemical lithiation of monoclinic -Nb2O5 has been performed, so as to mimic the corresponding electrochemical reaction during discharge vs. a lithium anode, or charge vs. a high voltage cathode.

The lithium atom distribution in the Li1.714Nb2O5 product of Li insertion into niobium oxide has been determined by neutron diffraction. Lower energy fivefold-and a higher energy fourfoldcoordinated sites are occupied by lithium in the reaction course; the former ones are more but not exclusively populated, indicating that entropic in addition to energetic factors play a substantial role in the insertion process.

By first-principles calculations on the least-energy relaxed structure of Li1.714Nb2O5, the cathode voltage of the material vs. a Li anode has been computed to be 1.67 V, in excellent agreement with the experimental plateau at 1.6 V. This proves the ability of the B3LYP hybrid functional to predict the electrochemical potential of Li insertion electrodes correctly. Further, the effect of electrons brought by inserted Li atoms into the band structure of the solid has been demonstrated, explaining the electronic conduction properties induced in the material and predicting a weak ferromagnetism close to 0 K. About one third of the added electrons polarize their spins and occupy a peculiar energy band just below EF, which is due to pure d states localized on 8 Nb atoms out of 24 per unit-cell. The other electrons fill lower energy levels with balanced spin populations and hybridized Nb-O character. Fig. 9 Computed electron band structure of Li1.714Nb2O5. Energy is plotted vs. the wave vector fractional coordinate k, along the  (0,0,0) to B (0,0,½), D (0, ½, ½), and Z (0,½,0) pathway in reciprocal space. Full and dotted lines represent energies of majority  and minority  spin states, respectively.

Fig. 6

 6 for both unlithiated (top) and lithiated (bottom) oxides, in the energy range comprising valence band and a part of the conduction band. In addition to the total DOS (full line), also the contribution projected onto Nb atoms is shown as dashed line. For Li1.714Nb2O5, positive and negative density values correspond to  (majority spin) and  (minority spin) polarized electrons, respectively; the energy zero was set at the Fermi energy value. The conduction bands of both phases are contributed by nearly pure Nb d states, whereas the valence bands are made of a main portion with prevailing O character, with some contribution from Nb-O hybridization.

  the first Brillouin zone of the monoclinic primitive lattice is shown, and results of computations along the -B-D-Z pathway in the energy range close to Fermi energy are plotted in Fig. 9. These three sections actually represent the behaviour along all edges of the irreducible part of the B.Z., as the plots along -Y, Y-A and Y-B are very similar to the -B one, those along Z-C, C-E and E-D correspond to the Z-D one, and the B-D plot represents also those along -Z, Y-C and A-E. One can see that bands are very flat, practically dispersionless, in the -B and D-Z ranges corresponding to the short c * and a * reciprocal vectors; on the other hand, the band dispersion is large along the long b * vector (-B).

Fig. 1

 1 Fig. 1 Neutron powder diffraction pattern (ILL-D2B, =1.5951 Å) of Li1.714Nb2O5; observed, Rietveld calculated and difference intensity profiles are plotted vs. the 2 angle (°).

Fig. 2

 2 Fig. 2 Projection onto (010) of the P2 structure of Li1.714Nb2O5, from Rietveld refinement of neutron diffraction data. NbO6 coordination octahedra are shown; circles and squares emphasize fivefold-and fourfold-coordinated Li atoms, respectively. For the two large encirclements, see the text.

Fig. 3

 3 Fig.3Crystal-chemical environment of fivefold-coordinated Li atoms in the structure of Li1.714Nb2O5.

Fig. 4

 4 Fig. 4 Statistical distribution of the Li-O bond lengths from fivefold-and fourfold-coordinated Li atoms in the structure of Li1.714Nb2O5. Top: neutron diffraction results; bottom: DFT calculations.

Fig. 5

 5 Fig. 5 Statistical distribution of the Nb-O bond lengths from NbO6 coordination octahedra in the structure of Li1.714Nb2O5. Top: neutron diffraction results; bottom: DFT calculations.

Fig. 6

 6 Fig.6 Electron density of states (DOS) of monoclinic Nb2O5 and Li1.714Nb2O5, computed by the hybrid DFT-HF B3LYP functional. Full and dashed lines show the total DOS and the contribution due to Nb atoms, respectively. For Li1.714Nb2O5, spin majority  and minority  states correspond to the positive and the negative vertical directions.

Fig. 7

 7 Fig. 7 Magnification of the DOS of Li1.714Nb2O5 (Fig. 6), showing the spin-polarized occupied band due to d states of Nb4, Nb5, Nb11 and Nb15 atoms.

Fig. 8

 8 Fig.8 Brillouin zone of the monoclinic P lattice (Bilbao Crystallographic Server, http://www.cryst.ehu.es).

Fig. 1 Fig. 3 22Fig. 4

 134 Fig.1

Fig. 5

 5 Fig. 5

Fig. 6 25Fig.. 7 Fig. 8

 678 Fig. 6

Table 1

 1 Rietveld refinement results of neutron diffraction data for Li1.714Nb2O5 (space group P2, Z=14); the e.s.d.'s are reported in parentheses. Least-energy unit-cells and total energies from DFT calculations for Li1.714Nb2O5 and monoclinic Nb2O5.

		Li1.714Nb2O5	-Nb2O5
		Experimental	Theoretical	Experimental 15 Theoretical
	a (Å)	20.7914(9)	21.526(+3.5%)	21.153(7)	21.634(+2.3%)
	b (Å)	4.1346(2)	4.149(+0.3%)	3.8233(5)	3.869(+1.2%)
	c (Å)	19.1318(9)	19.469(+1.8%)	19.356(5)	19.710(+1.8%)
	 (°)	119.571(3)	119.59	119.80(2)	119.80
	V (Å 3 )	1430.42	1512.03(+5.7%)	1358.30	1431.61(+5.4%)
	E (hartree/f.u.)		-501.89101		-488.88085
	Data	2969
	Variables	220
	wRp	0.0447
	Rp	0.0347
	R(F 2 )	0.0285
	Red. 2	3.922
	___________________________________________________________________________

Table 2

 2 Refined atomic fractional coordinates and displacement factors of Li1.714Nb2O5, with e.s.d.'s in parentheses.

	__________________________________________________________
		x	y	z	U(10 2 Å 2 )
	Nb1	0	0.265(30)	0	80
	Nb2	0.5	0.0000	0.5	0(1)
	Nb3	0.1506(31)	0.008(26)	0.9765(34)	8(2)
	Nb4	0.240(4)	0.950(27)	0.237(4)	40
	Nb5	0.2956(22)	0.014(20)	0.4574(22)	1(1)
	Nb6	0.3569(34)	0.973(26)	0.026(4)	14(4)
	Nb7	0.4217(28)	0.955(19)	0.2498(28)	5(2)
	Nb8	0.5738(16)	0.983(19)	0.0717(17)	0.1
	Nb9	0.6349(17)	0.977(19)	0.3152(18)	0.1
	Nb10	0.1044(18)	0.471(19)	0.1978(20)	0.1
	Nb11	0.167(4)	0.491(30)	0.427(4)	18(4)
	Nb12	0.6829(17)	0.474(18)	0.0981(18)	0.1
	Nb13	0.7574(19)	0.448(18)	0.3383(20)	0.1
	Nb14	0.8927(24)	0.491(18)	0.1446(26)	3(1)
	Nb15	0.9655(22)	0.435(17)	0.3888(21)	2(1)
	O1	0	0.456(35)	0.5	5(3)
	O2	0.5	0.560(15)	0.5	0.1
	O3	0.1237(27)	0.978(23)	0.209(4)	12(4)
	O4	0.1807(29)	0.011(24)	0.4323(30)	3(2)
	O5	0.1902(23)	0.929(16)	0.1104(25)	0.1
	O6	0.2600(18)	0.987(17)	0.3353(19)	0.1
	O7	0.2521(18)	0.951(19)	0.0057(20)	0.1
	O8	0.3210(23)	0.041(18)	0.2366(26)	2(1)
	O9	0.4006(26)	0.116(19)	0.4796(32)	4(2)
	O10	0.3922(20)	0.025(18)	0.1462(21)	0.1
	O11	0.472(4)	0.989(25)	0.3828(31)	11(3)
	O12	0.4628(30)	0.072(23)	0.049(4)	11(4)
	O13	0.5292(17)	0.047(17)	0.2742(22)	0.1
	O14	0.6106(27)	0.031(21)	0.1905(25)	2(1)
	O15	0.6632(19)	0.006(18)	0.4278(20)	0.1
	O16	0.6870(28)	0.987(21)	0.1041(29)	3(1)
	O17	0.7575(19)	0.011(18)	0.3455(23)	0.1
	O18	0.895(4)	0.999(19)	0.1574(30)	8(3)
	O19	0.9572(19)	0.009(18)	0.0425(22)	0.1
	O20	0.9514(32)	0.957(18)	0.3693(29)	6(2)
	O21	0.0715(18)	0.510(19)	0.0816(18)	12(4)
	O22	0.0695(18)	0.480(20)	0.4046(20)	0.1
	O23	0.1371(19)	0.506(19)	0.3143(20)	0.1
	O24	0.2176(21)	0.450(17)	0.2266(22)	0.1
	O25	0.2829(21)	0.526(17)	0.4553(19)	0.1

Acknowledgements

We thank Emmanuelle Suard (ILL, Grenoble, France) for help with the neutron diffraction data collection on the D2B equipment.