Hydrogen Desorption Behavior of Nickel-Chloride-Catalyzed

May 29, 2009 - maximum hydrogen release and minimum NH3 cogeneration. Here we report the hydrogen and NH3 release characteristics of R-phase ...
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Hydrogen Desorption Behavior of Nickel-Chloride-Catalyzed Stoichiometric Li4BN3H10 F. E. Pinkerton* and M. S. Meyer Materials and Processes Laboratory, General Motors Research and DeVelopment Center, 30500 Mound Road, Warren, Michigan 48090-9055 ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: April 30, 2009

Quaternary Li-B-N-H hydrides with the R-phase crystal structure form over a range of compositions between Li3BN2H8 and Li4BN3H10 and have up to 11.9 wt % hydrogen capacity. Previous work has focused on the nonequilibrium Li3BN2H8 composition created by high energy ball milling because this composition shows maximum hydrogen release and minimum NH3 cogeneration. Here we report the hydrogen and NH3 release characteristics of R-phase material having the equilibrium Li4BN3H10 composition. In the absence of a dehydrogenation catalyst, this composition simultaneously releases H2 and NH3 in roughly equal quantities by weight (or about 10% NH3 by volume) at temperatures above 240 °C. When Ni in the form of NiCl2 is added as a dehydrogenation catalyst, the H2 release temperature is reduced by 122 °C. NH3 release, in contrast, still occurs only at the higher temperature. As a result, decomposition occurs in two steps separated in temperature, dominated by H2 gas at low temperature and NH3 release at high temperature. This result clearly demonstrates that the two gases are evolved in two distinct decomposition reactions that are coincident in uncatalyzed Li4BN3H10, but can be separated by a dehydrogenation catalyst. We expect that NH3 cogeneration could be completely eliminated at sufficiently low dehydrogenation temperatures. Introduction On-board hydrogen storage remains a significant challenge to the practical implementation of fuel cell vehicles, stimulating extensive worldwide research on new solid hydrides for hydrogen storage. Recently, we reported the discovery of a new quaternary hydride, R-Li-B-N-H, formed from mixtures of LiNH2 and LiBH4 either by ball-milling or by heating above about 95 °C.1 Similar results were subsequently reported by Aoki et al.2 The R-phase melts at ∼190 °C and releases up to 11 wt % hydrogen from the liquid on heating above 250 °C. A study of various LiNH2/LiBH4 mixtures3 determined that maximum hydrogen release by weight fraction was obtained near the R-phase composition Li3BN2H8, formed from a ball-milled mixture of LiNH2/LiBH4 ) 2:1, where complete hydrogen release was obtained through the decomposition reaction

Li3BN2H8 f Li3BN2 + 4H2

(1)

A small amount of NH3 comprising 2-3 mol % of the evolved gas was cogenerated during dehydrogenation. Subsequent crystal structure determination using samples of R-phase recrystallized from the melt revealed that the true equilibrium composition of the R-phase is Li4BN3H10,4-7 corresponding to LiNH2/LiBH4 ) 3:1. We infer that the Li3BN2H8 (2:1) composition is a nonequilibrium form of the R-phase resulting from energetic ball-milling. Heating of the R-phase at the Li4BN3H10 composition, however, produced a substantial increase in the amount of cogenerated NH3 to 10 mol % of the evolved gas, corresponding to a weight loss nearly equal to that due to the hydrogen,3 due to the presence of excess nitrogen compared to the final Li3BN2 dehydrogenated product. Hydrogen and NH3 appeared to be released essentially simultaneously above 250 °C. * Corresponding author. E-mail: [email protected].

The decomposition of Li3BN2H8 has been successfully catalyzed by the addition of suitable transition metals such as Fe, Co, Ni, Pd, and Pt.8-10 In particular, the addition of Ni or Co in the form of NiCl2 or CoCl2 dramatically reduced the decomposition temperature by ∼100 °C.8,9 Furthermore, adding NiCl2 as a catalyst decreased the amount of coevolved NH3 by about an order of magnitude.8 Reduction, and preferably elimination, of NH3 release is critical from an application point of view both because it represents degradation of the storage material and because ammonia is detrimental to the fuel cell system. Investigations of other amide-containing hydrogen storage materials based on the LiNH2-LiH system11 and the Mg(NH2)2-2LiH system12-14 have reported similar improvements in NH3 release. In both systems, the NH3 concentration in the desorbed H2 gas decreased with decreasing desorption temperature,15-18 and lowering the H2 desorption temperature by adding a suitable catalyst19 or by product seeding20 substantially reduced the amount of NH3 liberated. These observations provide motivation to look again at hydrogen and NH3 release from the Li4BN3H10 composition in the presence of a catalyst, specifically to determine whether H2 and NH3 are cogenerated products from a single decomposition reaction or whether they are produced by independent reactions that occur at about the same temperature. We will indeed show that NiCl2 added to Li4BN3H10 reduces the hydrogen release temperature to below 235 °C, such that essentially all of the available hydrogen can be released with very little cogenerated NH3; the NH3 is evolved in a separate decomposition at higher temperature. Experimental Methods Li4BN3H10 was synthesized by mixing LiNH2 (Aldrich 95%) and LiBH4 (Aldrich 95%) in a 3:1 stoichiometry totaling 1 g and ball-milling for 5 h in a SPEX 8000 Mixer/Mill. Catalyzed Li4BN3H10 + 11 wt % NiCl2 was similarly prepared by adding 0.110 g of anhydrous NiCl2 (Strem 98%) prior to ball-milling.

10.1021/jp810208k CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

Nickel-Chloride-Catalyzed Stoichiometric Li4BN3H10

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Loading and unloading of the ball-mill jar were done in an Ar inert gas glovebox for protection against atmospheric exposure. Desorption properties were measured using a Cahn 2151 thermogravimetric analyzer (TGA) under 0.13 kPa of flowing He gas, as previously described.10 The composition of the exhaust gas flow was sampled using a Stanford Research Systems (SRS) model CIS 100 quadrupole mass spectrometer operated in residual gas analysis (RGA) mode. Each sample was pretreated by heating to an intermediate temperature (220 °C for the uncatalyzed sample and 140 °C for the NiCl2 catalyzed sample) followed by cooling to room temperature. The intent of this heating was to remove a separate transient low-temperature NH3 release event, as described in more detail below. After pretreatment, each sample was heated from room temperature to 400 °C and held at that temperature until desorption was complete. An additional sample of Li4BN3H10 + 11 wt % NiCl2 was heated just to 235 °C and held for 470 min. Phase composition was established by X-ray diffraction (XRD) using a Bruker AXS general area detector diffraction system (GADDS) with Cu KR radiation. Powders were loaded in the glovebox into 1 mm diameter quartz capillaries and sealed with clay for transport to the diffractometer. Temperaturedependent in situ XRD was performed in the same instrument using quartz capillaries loaded under Ar and mounted within a furnace, as previously described.21 For in situ experiments, the quartz capillary was connected to a large pressure manifold filled with inert gas to prevent the accumulation of evolved gases within the capillary during heating. Results and Discussion Both as-milled samples consisted predominantly of R-Li4BN3H10, together with a small amount of LiNH2 minor phase, as shown by the XRD pattern of the catalyzed sample in Figure 1a. The XRD of the uncatalyzed sample was nearly identical; see Supporting Information Figure S1a. For the uncatalyzed sample, the main LiNH2 peak at 2θ ) 30.7° was unshifted from reference values.22 In contrast, the LiNH2 peak in the catalyzed sample (labeled “d” in Figure 1a) was shifted toward higher lattice constant (lower angle) by roughly 1%. Because LiNH2 and LiCl have very similar XRD patterns, this peak could also have a contribution from LiCl formed from NiCl2 during ball milling, although the line intensities are not consistent with a strong LiCl component. Both the catalyzed and uncatalyzed Li4BN3H10 samples exhibited a small NH3 release at low temperatures unrelated to the main decomposition events of interest here. To further elucidate the nature of the low temperature NH3 release, and to remove its effect from the subsequent experiments, both samples were given a preliminary heat treatment. Figure 2 summarizes the effect of the pretreatment step. For ease in comparison of later results, the weights are normalized to 100% at the end of the heat treatments. Both samples lost just over 1 wt % during the pretreatment, clearly indicating that its occurrence was unrelated to the addition of NiCl2. The details of the weight loss showed significant differences between the uncatalyzed and the NiCl2-catalyzed samples. The uncatalyzed Li4BN3H10 sample lost weight relatively slowly up to its soak temperature of 220 °C, and only NH3 was observed in the RGA (Figure 2b). The H2 signal rose very slightly over time but was uncorrelated with the temperature and thus is attributable to a small drift of the mass 2 amu baseline in the RGA. In contrast, the catalyzed sample lost weight more rapidly at low temperatures, and the RGA (Figure 2c) showed unambiguously that H2 was also being

Figure 1. Room temperature XRD patterns of Li4BN3H10 + 11 wt % NiCl2 (a) as milled, (b) after pretreatment at 130 °C, (c) after heating to 235 °C and holding for 470 min, and (d) after full desorption to 400 °C. The diffraction peaks are indexed to the following phases: a ) R-Li4BN3H10; b ) bct Li3BN2; c* ) cubic LiCl, Li-N-Cl-(H?), and/ or lattice-shifted Li2NH (see text); d ) LiNH2; e ) hydrogen-poor intermediate phase; f ) monoclinic Li3BN2; n ) Ni, and o ) Li2O. A smooth background has been subtracted from all scans.

produced. An earlier experiment on an NiCl2-added sample had shown that temperatures above 140 °C do not result in further NH3 loss but that the higher temperatures do produce substantially more H2. For these reasons, the catalyzed sample was limited to a maximum temperature of 140 °C and a soak time of 20 min, sufficient to achieve full NH3 loss while limiting the amount of H2 also lost. The NH3 loss during the pretreatment was a one-time event, as demonstrated in Figure 3. Starting from the upper left, NH3 loss during heating began at about 75 °C. After the soak, cooling back to room temperature and reheating produced no further weight loss in the pretreatment temperature range, showing that NH3 loss was complete at the end of the soak. On heating above the soak temperature, renewed weight loss began at about 240 °C for the uncatalyzed Li4BN3H10, consistent with the onset temperature observed previously for the main desorption event in Li3BN2H8,1 or at about 160 °C for the sample with NiCl2. In situ XRD showed that the pretreatment was accompanied by disappearance of the residual LiNH2 peak, as shown by the XRD pattern in Figure 1b taken after heating to 130 °C and cooling back to room temperature. At the same time, the Li4BN3H10 peaks sharpened significantly (the full-width-at-half-maximum decreased from 0.53° to 0.40°) and shifted toward a smaller lattice constant (higher angle) by about 0.2%. Similar behavior was observed for the uncatalyzed Li4BN3H10 sample, as shown

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Figure 2. Weight loss and H2 and NH3 RGA signals during pretreatment of Li4BN3H10 and Li4BN3H10 + 11 wt % NiCl2. The temperature profiles for the two samples are included as the dash-dotted lines in panel a. For ease of comparison the RGA curves have been displaced vertically to set the background level at 2 × 10-9 Torr for both gas species.

Figure 3. Weight loss as a function of temperature for Li4BN3H10 and Li4BN3H10 + 11 wt % NiCl2 during the initial pretreatment, cooling after pretreatment, and heating through the onset of the main desorption event.

in the Supporting Information Figure S1b. The low temperature NH3 event was not associated with direct decomposition of the slight excess of Li amide in the sample, because the decomposition rate of LiNH2 by itself is very low at these temperatures (at 220 °C, only about 2% of the total LiNH2 will be decomposed after 40 min).23 Furthermore, when tested separately, neither ball-milled LiNH2 nor ball-milled LiBH4 showed evidence of such a low temperature weight loss event (see Supporting Information Figure S3). Rather, it arose from accommodation of the excess LiNH2 into the R-phase during

Pinkerton and Meyer

Figure 4. Weight loss and H2, NH3, and N2 RGA signals during the main desorption from Li4BN3H10 and Li4BN3H10 + 11 wt % NiCl2. The temperature profile for both samples is included as the dash-dotted line in panel a. For ease of comparison, the RGA curves have been displaced vertically to set the background level at 2 × 10-9 Torr for all gas species.

heating. The nonequilibrium ball milling process created a slightly off-stoichiometric borohydride-rich R-phase composition along with a little residual LiNH2, which annealed to the equilibrium Li4BN3H10 composition with the associated release of a small quantity of excess N as NH3. Figure 4 shows the main desorption event (after pretreatment) for Li4BN3H10 with and without NiCl2 addition. Uncatalyzed Li4BN3H10 lost a total of 17.0 wt % above 250 °C comprised of ∼53% NH3 and ∼47% H2 by weight, in accord with previous results.3 A slight increase was observed in the N2 signal after the temperature reached 400 °C; however, the presence of substantial quantities of H2 and/or NH3 in the RGA gas stream can produce small shifts in the baseline levels of other gases, and we infer that the small change in N2 level is an experimental artifact. The fully desorbed material consisted of a mixture of monoclinic Li3BN2, Li2NH, and a small quantity of Li2O as determined by XRD (Supporting Information Figure S2). The overall reaction for the uncatalyzed material can thus be represented as

Li4BN3H10 f Li3BN2 + 1/2Li2NH + 4H2 + 1/2NH3

(2) The predicted weight loss for this reaction is 18.3 wt %, comprised of 51.4% NH3 and 48.6% H2 by weight, in excellent agreement with the observed values. Using a similar gravimetric technique, Chater et al. have previously reported on NH3 and H2 release from uncatalyzed Li4BN3H10.24 Although they reported about the same total weight loss (17.3 wt %), they found a substantially different partitioning

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between the low temperature and high temperature events, with much more NH3 (5.8 wt %) released at temperatures between 60 and 160 °C and only about 5% NH3 cogenerated with the H2 above 260 °C. They suggested that NH3 may be an essential intermediate species in the hydrogen-generating reaction. We have measured a number of Li4BN3H10 samples made either by ball-milling or by heating mixtures of 3LiNH2 + LiBH4 powders using two different flowing gas gravimetric apparatus (Cahn high-pressure TGA and Hiden IGA-3) and routinely find 1.1-1.6 wt % NH3 loss in the low temperature release and 10-12% NH3 cogeneration with the H2 release at higher temperature. Our results are more difficult to reconcile with an NH3-controlled reaction model. The origin of the different behaviors is unclear at the present time. Further study of the differences and similarities of sample syntheses and experimental procedures could elucidate the details of the lowtemperature NH3 mechanisms in the two cases. In contrast to the case for uncatalyzed Li4BN3H10, the main weight loss for Li4BN3H10 + 11 wt % NiCl2 occurred in two distinct steps. The first step, between 160 and 260 °C, produced a weight loss of 7.5 wt % that was comprised virtually entirely of H2 to within the RGA detection limits. The midpoint of the weight loss was reduced by 122 °C compared to the midpoint for uncatalyzed Li4BN3H10, about the same as the 112 °C improvement observed in NiCl2-added Li3BN2H8 (although the midpoint value, 244 °C, is higher than the midpoint value of 217 °C for Li3BN2H8 + 11 wt % NiCl2).8 A slight increase of the NH3 signal appeared at about the time that the H2 signal peaked (∼250 °C); this may again be a baseline shift, but even if real, the amount of NH3 in the evolved gas below 260 °C did not exceed 0.2 mol %. This is consistent with the reduced NH3 cogeneration in NiCl2-added Li3BN2H8.8 The composition of Li4BN3H10 + 11 wt % NiCl2 after the H2 desorption step was probed by performing XRD on another sample that had been heated to 235 °C and held at that temperature for 470 min, shown in Figure 1c. The R-phase decomposed to form an intermediate compound (peaks labeled “e” in Figure 1c). This intermediate phase (IP) is the same as that observed during in situ XRD on Li3BN2H8 + 11 wt % NiCl2.8,10 Indeed, observation that the NiCl2-catalyzed hydrogen desorption step produced the same IP in both Li3BN2H8 and Li4BN3H10 provides direct evidence that the same hydrogen desorption mechanism occurred in both materials, independent of R-phase composition. The composition and structure of the IP are unknown, but we infer that it must be a hydrogen-poor Li-B-N-H phase. TGA measurements of Li3BN2H8 + 11 wt % NiCl2 suggest that of the original 8 H/formula unit, about 0.6 H remained in the IP, so we have

Li4BN3H10 f IP + LiNH2 + ∼3.7H2

(3)

The 7.4 wt % weight loss predicted for reaction 3, taking into account dilution by the NiCl2, is in excellent agreement with the observed value of 7.6 wt % for the first step in Figure 4 (conversely, the observed 7.6 wt % loss if entirely hydrogen would translate to 3.8 H2). The IP is unlikely to be caused by the incorporation of Ni, because (1) there are only 0.019 mol of Ni per mole of Li, and it is unlikely that such a small amount of Ni could account for the formation of a new majority phase, and (2) an X-ray absorption spectroscopy (XAS) study of Li3BN2H8 + 11 wt % NiCl2 found that the Ni environment, resembling that in Ni3B, was unchanged during the desorption process, including samples composed almost exclusively of the IP.25,26 LiNH2 is present as a secondary phase. Its identification

is made unambiguous by the two characteristic LiNH2 peaks at 2θ ) 17.4° and 19.6°, although coexistence of LiCl with a slightly contracted lattice giving coincident peaks cannot be excluded. The LiNH2 peak locations again indicate about a 1% lattice expansion relative to database values.22 The remaining minority phase is body-centered tetragonal (bct) Li3BN2, a new ternary polymorph that is observed in dehydrogenated materials.1,27 Its appearance is due to the long hold at 235 °C for this sample, during which slow weight loss continued, indicating further dehydrogenation of the IP toward Li3BN2. The second step above 280 °C is initially dominated by NH3, although some additional H2 is also produced. Curiously, above about 360 °C, the NH3 production falls off, and an experimentally significant peak occurs in the N2 signal, accompanied by a congruent increase in the H2 signal. Although the obvious shift in N2 baseline level makes quantitative assessment difficult, the relative sizes of the N2 and H2 peaks are at least qualitatively consistent with the decomposition of NH3 into N2 + 3H2, presumably as a consequence of the same Ni-based catalyst responsible for the depression of the dehydrogenation temperature. After full desorption, the sample consisted primarily of two polymorphs of Li3BN2, the monoclinic phase28 favored at high temperatures (peaks labeled “f” in Figure 1d) and a smaller quantity of the bct polymorph (“b”). A contribution to the peak at 44.5° due to pure Ni cannot be excluded. Including the NiCl2 dilution, if the NH3 were produced by the expected decomposition of the LiNH2 remaining in reaction 3 into Li2NH, the predicted weight loss would be 9.1 wt %; the observed weight loss of 8.3 wt % was somewhat smaller but still in reasonable agreement. The XRD pattern in Figure 1d, however, did not contain diffraction lines corresponding to pure Li2NH. Instead, a set of peaks labeled “c*” were observed corresponding to a cubic phase with lattice constant a ) 0.5175 nm. These have about the correct diffraction angles to be indexed as LiCl in the Fm3jm fcc crystal structure (a ) 0.5140 nm), but with relative peak intensities more closely resembling those of Li2NH. Although the precise crystal structure of Li2NH has been the subject of considerable recent study,29-31 the Li and N backbone of the Li2NH structure can also be represented by a disordered Fm3jm structure with lattice constant a ) 0.5047 nm. It is unlikely that the c* peaks arise from Li2NH alone, as this would require an unrealistically large lattice expansion of about 2.5%. Furthermore, off-stoichiometric compositions of Li2-xNH1+x have been studied in detail, and their cubic lattice parameter varies only between 0.500 and 0.508 nm.32 However, several other known ternary Li-N-Cl phases share the Fm3jm structure, with compositions corresponding to Li9N2Cl3 (a ) 0.5400 nm)33 or Li10N2Cl4 (a ) 0.5416 nm).34 Although the composition of the phase giving rise to the c* peaks is not known, it is consistent with the formation of a variant of the Fm3jm-type mixed Li-N-Cl compounds, possibly including a small amount of hydrogen as well. The second reaction step can then be represented as

IP + LiNH2 + ∼3.7H2 f Li3BN2 + [Li-N-Cl-(H?)] + ∼0.42NH3 + ∼4H2 (4) As a test of the above interpretation, mixed powders at the composition 3LiNH2 + LiCl were ball-milled for 5 h, heated at 5 C/min to 400 °C, and held for 135 min until weight loss ended. The observed weight loss, 15.3 wt %, was again less than that predicted for complete decomposition of the LiNH2 to Li2NH (22.8 wt %). Furthermore, as shown by Supporting

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Information Figure S4, both the as-milled starting material and the decomposed sample were dominated by a single set of cubic diffraction peaks with lattice constants a ) 0.5172 and 0.5228 nm, respectively, clearly demonstrating the formation of an fcc ternary Li-N-Cl or quaternary Li-N-Cl-H phase before and after decomposition. The larger lattice constant for the material after weight loss to 400 °C is consistent with the larger relative Li to N ratio after losing some nitrogen in the form of NH3. A second LiCl-rich composition at LiNH2/LiCl ) 1:1 also produced a Li-N-Cl-(H?) phase in the as-milled state (Supporting Information Figure S5a), but with pronounced shoulders on the low angle side of the diffraction peaks. Heat treatment to 400 °C produced a weight loss of 12.5 wt %, slightly smaller than the 13.0 wt % loss predicted for decomposition of the LiNH2. XRD of the heat treated sample (Figure S5b) again showed an fcc Li-N-Cl-(H?) phase with an even larger lattice constant of 0.5252 nm. A second set of fcc peaks were observed that closely correspond to LiCl, indicating that the homogeneity limit of the Li-N-Cl-(H?) phase occurs somewhere between the LiNH2/LiCl ) 3:1 and 1:1 compositions. More detailed study of the Li-N-Cl-(H?) product phase is beyond the scope of this paper. In conclusion, the TGA, RGA, and XRD results clearly show that the main decomposition of Li4BN3H10 into H2 and NH3 is not the result of a single decomposition reaction but rather is two separate H2 and NH3 decompositions that are commensurate in temperature in the uncatalyzed material. Incorporating a suitable catalyst such as NiCl2 lowers the H2 release reaction in Li4BN3H10 to less than 235 °C, below the temperature at which the NH3 reaction becomes thermally activated. Together with similar previous results for catalyzed Li3BN2H8,8,10 we have strong experimental evidence that reducing the H2 release temperature of the R-phase Li-B-N-H can suppress NH3 cogeneration during the dehydrogenation process. Acknowledgment. The authors wish to thank Richard Speer, Jr., and Michael Balogh for providing the X-ray diffraction results. We also thank Jan Herbst for useful conversations and support. Supporting Information Available: Figures showing XRD spectra of Li4BN3H10 and 3LiNH2 + LiCl, and TGA weight loss of LiNH2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pinkerton, F. E.; Meisner, G. P.; Meyer, M. S.; Balogh, M. P.; Kundrat, M. D. J. Phys. Chem. B 2005, 109, 6. (2) Aoki, M.; Miwa, K.; Noritake, T.; Kitahara, G.; Nakamori, Y.; Orimo, S.; Towata, S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1409.

Pinkerton and Meyer (3) Meisner, G. P.; Scullin, M. L.; Balogh, M. P.; Pinkerton, F. E.; Meyer, M. S. J. Phys. Chem. B 2006, 110, 4186. (4) Filinchuk, Y. E.; Yvon, K.; Meisner, G. P.; Pinkerton, F. E.; Balogh, M. P. Inorg. Chem. 2006, 45, 1433. (5) Chater, P. A.; David, W. I. F.; Johnson, S. R.; Edwards, P. P.; Anderson, P. A. Chem. Commun. 2006, 2439. (6) Noritake, T.; Aoki, M.; Towata, S.; Ninomiya, A.; Nakamori, Y.; Orimo, S. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 277. (7) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J.; Yildirim, T. Chem. Mater. 2008, 20, 1245. (8) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P. J. Alloys Compd. 2007, 433, 282. (9) Tang, W. S.; Wu, G. T.; Liu, T.; Wee, A. T. S.; Yong, C. K.; Xiong, Z. T.; Hor, A. T. S.; Chen, P. Dalton Trans. 2008, 2395. (10) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P. J. Phys. Chem. B 2006, 110, 7967. (11) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002, 420, 302. (12) Luo, W. F. J. Alloys Compd. 2004, 381, 284. (13) Leng, H. Y.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. J. Phys. Chem. B 2004, 108, 8763. (14) Xiong, Z. T.; Hu, J. J.; Wu, G. T.; Chen, P.; Luo, W. F.; Gross, K.; Wang, J. J. Alloys Compd. 2005, 398, 235. (15) Hino, S.; Ichikawa, T.; Tokoyoda, K.; Kojima, Y.; Fujii, H. J. Alloys Compd. 2007, 446-447, 342. (16) Luo, W. F.; Stewart, K. J. Alloys Compd. 2007, 440, 357. (17) Luo, W. F.; Wang, J.; Stewart, K.; Clift, M.; Gross, K. J. Alloys Compd. 2007, 446-447, 336. (18) Liu, Y. F.; Hu, J. J.; Wu, G. T.; Xiong, Z. T.; Chen, P. J. Phys. Chem. C 2008, 112, 1293. (19) Ichikawa, T.; Isobe, S.; Hanada, N.; Fujii, H. J. Alloys Compd. 2004, 365, 271. (20) Sudik, A.; Yang, J.; Halliday, D.; Wolverton, C. J. Phys. Chem. C 2007, 111, 6568. (21) Balogh, M. P.; Tibbetts, G. G.; Pinkerton, F, E.; Meisner, G. P.; Olk, C. H. J. Alloys Compd. 2003, 350, 136. (22) International Centre for Diffraction Data (ICDD) PDF-4+ (2007). (23) Pinkerton, F. E. J. Alloys Compd. 2005, 400, 76. (24) Chater, P. A.; Anderson, P. A.; Prendergast, J. W.; Walton, A.; Mann, V. S. J.; Book, D.; David, W. I. F.; Johnson, S. R.; Edwards, P. P. J. Alloys Compd. 2007, 446-447, 350. (25) Graetz, J.; Chaudhuri, S.; Salguero, T. T.; Vajo, J. J.; Meyer, M. S.; Pinkerton, F. E. Nanotechnology 2009, 20, 204007. (26) Ignatov, A. Yu.; Graetz, J.; Chaudhuri, S.; Salguero, T. T.; Vajo, J. J.; Meyer, M. S.; Pinkerton, F. E.; Tyson, T. A. In X-ray Absorption Fine StructuresXAFS13; Hedman, B., Pianetta, P., Eds.; American Institute of Physics: New York, 2007; Vol. CP882, pp 642-644. (27) Pinkerton, F. E.; Herbst, J. F. J. Appl. Phys. 2006, 99, 113523. (28) Yamane, H.; Kikkawa, S.; Horiuchi, H.; Koizumi, M. J. Solid State Chem. 1986, 65, 6. (29) Ohoyama, K.; Nakamori, Y.; Orimo, S. I.; Yamada, K. J. Phys. Soc. Jpn. 2005, 74, 483. (30) Noritake, T.; Nozaki, H.; Aoki, M.; Towata, S.; Kitahara, G.; Nakamori, Y.; Orimo, S. J. Alloys Compd. 2005, 393, 264. (31) Balogh, M. P.; Jones, C. Y.; Herbst, J. F.; Hector, L. G., Jr.; Kundrat, M. J. Alloys Compd. 2006, 420, 326. (32) David, W. I. F.; Jones, M. O.; Gregory, D. H.; Jewell, C. M.; Johnson, S. R.; Walton, A.; Edwards, P. P. J. Am. Chem. Soc. 2007, 129, 1594. (33) Sattlegger, H. Z. Anorg. Allg. Chem. 1970, 379, 293. (34) Marx, R.; Mayer, H.-M. J. Solid State Chem. 1997, 130, 90.

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