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Determination of the Phase Behavior of (LiNH2)c(LiBH4)1-c Quaternary Hydrides through in Situ X-ray Diffraction Jonathan P. Singer,†,| Martin S. Meyer,‡ Richard M. Speer, Jr.,§ John E. Fischer,† and Frederick E. Pinkerton*,‡ Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104, Chemical Sciences and Materials Systems Laboratory, General Motors Research and DeVelopment Center, 30500 Mound Road, Warren, Michigan 48090, and Optimal Inc., 14492 Sheldon Road Suite 300, Plymouth Township, Michigan 48170. ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: September 8, 2009
Solid hydride materials combining high gravimetric and volumetric hydrogen capacity, rapid hydrogenation and dehydrogenation kinetics, and low operating temperature are highly desirable for hydrogen storage on fuel cell vehicles. Recently, a material of nominal composition Li3BN2H8, formed by combining LiNH2 and LiBH4 in a 2:1 ratio, has garnered much attention because of its high hydrogen release (up to 11.9 wt % hydrogen on heating above ∼200-250 °C). While the material is expected from previous experiments to exist in a single “Li3BN2H8” R-phase, here it is shown that single-phase 2:1 Li3BN2H8 samples decompose with time into a combination of an R-phase composition enriched in LiNH2 compared to Li3BN2H8 and the 1:1 β-phase Li2BNH6. Through in situ X-ray diffraction of samples from the (LiNH2)c(LiBH4)1-c system with c in the range 0.667-0.75 (corresponding to LiNH2:LiBH4 ) 2:1 to 3:1) prepared either by high-energy ball milling (HEBM) for 10 min (mixed) or HEBM for 300 min (milled), the equilibrium phase behavior of the quaternary R-phase was investigated. The complete equilibrium composition range of the R-phase at 50 °C was shown to be LiNH2:LiBH4 ) ∼2.62:1-2.83:1 or c ) 0.724-0.739. Also, an approximate phase diagram in the composition range c ) 0.5-0.75 and temperature range T ) 0-250 °C was generated. Introduction One of the largest obstacles for automotive application of fuel cells is effective on-board hydrogen storage. For a successful solution to this problem, material with both high gravimetric and volumetric hydrogen capacity must be obtained. Also, an ideal candidate material must have desorption kinetics that are fast enough in an appropriate range of pressures and temperatures.1 One promising group of materials is light metal hydrides. These have very high theoretical hydrogen capacities and operate at noncryogenic temperatures; however, as has been observed in the results of previous studies, the dehydrogenation process often leaves hydrogen-containing products, such as metal imides2,3 or lithium hydride,4-7 and in some cases will release other gas species, such as ammonia8-10 or diborane.11 In addition, light metal hydrides often decompose at temperatures much higher than the maximum operating temperatures of fuel cells. Studies using the Li-B-N-H system10,12-17 attempted to address the issues of high-temperature decomposition and production of unusable dehydrogenation products containing hydrogen. Compounds in this system were synthesized by thermal or mechanical reaction of lithium amide and lithium borohydride to form quaternary phases of the form * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Pennsylvania. ‡ Chemical Sciences and Materials Systems Laboratory, General Motors Research and Development Center. § Optimal Inc. | Current address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139.
(LiNH2)c(LiBH4)1-c. At least two quaternary phases, R and β, have been shown to exist between c ) 0.5 and c ) 0.75. A major effect of forming the quaternary phases is a change in the decomposition behavior from those of the separate LiNH2 and LiBH4 components. The onset of hydrogen release for uncatalyzed Li-N-B-H compounds is ∼250 °C, a decrease from >400 °C for unreacted lithium borohydride. Moreover, the Li-N-B-H compounds release a relatively large quantity of hydrogen (8-11 wt %);10,15 however, because of the amide content, some ammonia is released during the main decomposition and also in separate events that occur near 100 °C. Motivated by the existence of a ternary hydrogen-free compound, Li3BN2,10 early studies in Li-B-N-H tried to avoid ammonia release and access the full theoretical hydrogen content by focusing on the c ) 0.667 composition, corresponding to a 2:1 ratio of amide-to-borohydride,10 via the reaction 2LiNH2 + LiBH4 f Li3BN2H8 f Li3BN2 + 4H2. In a compositional study of samples with c ) 0.33-0.8 formed through high-energy ball milling (HEBM),15 thermogravimetric analysis (TGA) results showed that the c ) 0.667 composition had the greatest hydrogen release (11 wt %) and one of the lowest ammonia releases (∼2 mol % of the evolved gas). In situ X-ray diffraction (XRD) of the dehydrogenation process confirmed the formation of Li3BN2 upon decomposition.15 Also, XRD analysis indicated that the milled mixture formed a single cubic R-phase, referred to as Li3BN2H8. Other XRD results showed that the R-phase crystal structure persisted up to the c ) 0.75 (3:1) composition (Li4BN3H10), distinguishable from Li3BN2H8 by a 1% smaller cubic lattice constant. Samples with c > 0.75 became mixtures of Li4BN3H10 R-phase and lithium amide. Another single phase, β, was identified at c ) 0.5 (1:1, Li2BNH6), having a different crystal structure later identified as hexagonal.17,18 This phase
10.1021/jp905970h CCC: $40.75 2009 American Chemical Society Published on Web 09/30/2009
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was found to be a line compound; all samples with c < 0.5 contained a mixture of Li2BNH6 β-phase and lithium borohydride and all samples having 0.5 < c < 0.667 contained β-phase and Li3BN2H8 R-phase. This single composition has been attributed to the β-phase structure being made up of equal parts Li[BH4-]1[NH2-]3 and Li[BH4-]3[NH2-]1 tetrahedra and the structural instability of Li[BH4-]2[NH2-]2 tetrahedra in the β lattice. As a result, the only possible substitutions in the borohydride-rich direction result in the formation of large Li[BH4-]4 tetrahedra, which segregate from the lattice (forming lithium borohydride), and the only substitutions in the amiderich direction form Li[NH2-]4 tetrahedra within the lattice (forming R-phase).17 A few unexplained issues arose from this and other studies of the Li-N-B-H system. First, single-crystal structural determinations showed that the equilibrium composition of the R-phase in 2:1 (c ) 0.667) samples after recrystallizing from the melt was Li4BN3H10, corresponding to a 3:1 (c ) 0.75) composition.13,14,17 Another issue was that β-phase would frequently appear in XRD results of the 2:1 samples.9 Finally, during dehydrogenation, ammonia release was observed, but if only Li3BN2 was in fact being produced there should be no generation of ammonia. These observations suggested that the R-phase at the 2:1 composition was not in fact a stable compound and was decomposing into R-phase of a different composition and β-phase. It will be shown in this work that this phase separation can be observed through in situ XRD, where the increase in intensity of β-phase peaks can be correlated to shifts in the position of the R-phase peaks. This process occurs even at room temperature and can be accelerated through heating. To reconcile these results, a detailed examination of the phase behavior and stability of samples in the range c ) 0.667-0.75 (2:1-3:1) was performed through time-dependent in situ XRD analysis at 50 °C. This temperature, 50 °C, was selected to accelerate the decomposition while still being below the melting temperature of β-phase (around 80 °C). The study approached the equilibrium composition of the R-phase through two different routes: (i) annealing of homogeneously mixed relatively unreacted samples (“mixed”) and (ii) annealing of samples completely reacted by HEBM (“milled”). In this way, the equilibrium R-phase composition was approached both by the “relaxation” from the nonequilibrium state created by HEBM (ii) and from a more gentle heat-induced reaction (i). An approximate phase diagram was determined by analysis of peak positions and intensities of the R-phase, β-phase, and starting phases. Experimental Details Samples were made from commercially available LiNH2 (Aldrich, 95% purity), LiBH4 (Aldrich, 95% purity), and LiCl (Aldrich, 99% purity). Samples were ball-milled using a SPEX high-energy ball mill in a sealed steel jar in 0.250 g batches with one large steel ball (1.27 in. diameter) and two small steel balls (0.635 in.). All samples were handled in a recirculating argon glovebox to avoid oxidation and reaction with ambient water. A preliminary experiment was performed on Li3BN2H8 samples produced by ball milling a 2:1 mixture of LiNH2 and LiBH4 for 300 min. After milling, a few weight percent of LiCl was mixed into the Li3BN2H8 powder to serve as an inert internal standard for calibrating the XRD peak positions. For this preliminary experiment in situ XRD was performed using a Bruker AXS General Area Detector Diffraction System
Singer et al. (GADDS). The sample was sealed in the glovebox into a quartz capillary tube, then transported to the XRD apparatus, and mounted in a furnace for elevated temperature measurements. XRD data were collected as a function of time at 50 °C. After the preliminary experiment, LiCl was not added to the main set of samples because of the possibility that LiCl might not be completely inert. The main set of experiments consisted of six compositions in the (LiNH2)c(LiBH4)1-c system with c ) 0.667, 0.688, 0.706, 0.722, 0.737, and 0.75 (corresponding to LiNH2:LiBH4 ) 2:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1, and 3:1). Expected errors in composition were less than 1%. Samples were produced by two methods: (i) mixing LiNH2 and LiBH4 during 10 min of HEBM (“mixed”) and (ii) thorough milling for 300 min of HEBM (“milled”). All samples were removed as white powder, indicating that the material did not melt during milling. For in situ XRD measurements samples were loaded anaerobically in the glovebox into 1 mm quartz capillaries. To avoid oxidation of samples during measurement, the X-ray capillaries were sealed with 3 M Super Silicone Sealant. In initial experiments, a bead of sealant was applied to the top of the capillaries; however, it was found that this led to leaks. Capillaries closed with sealant-covered wood dowels were found more effective, providing samples that showed no evidence of oxidation over several weeks. In situ X-ray experiments for the main samples were conducted using an INEL CPS 120 detector in a Debye-Scherrer geometry. This geometry allows for simultaneous collection of a full range of 2θ between ∼10° and ∼100°. The level of detail in these measurements is controllable by the accumulation time of diffracted intensity. In situ X-ray measurements were conducted at 50 ( 3 °C. All measurements were started within 2 h after milling so as to capture as much of the sample development as possible and consisted of a series of consecutive accumulations. In situ experiments began with a series of twenty-four 10-min accumulations, followed by twenty-four 15min accumulations. These were followed by twelve to seventytwo 1-h accumulations, depending on how quickly the sample stabilized. In this way rapid changes that occurred early on were captured, while the more gradual later behavior of the samples was observed with greater sensitivity. Heating for the in situ experiments was provided by a homebuilt capillary oven with X-ray transparent kapton windows. Temperature was found to be 50 ( 3 °C in every preliminary experiment and was further confirmed every few measurements, rather than monitored in every experiment, to avoid collisions between the thermocouple and capillary. Through the execution of the previously discussed series of accumulations, in situ X-ray results consisted of 60-120 data sets per sample. To avoid lengthy analysis, it was necessary to be able to rapidly analyze large numbers of files for peak positions and intensities, a capability that was not available with INEL software. To perform the analysis, software was written in Python programming language utilizing open-source peakfitting software. Peaks were fitted to a Gaussian function with a simple linear-splined background, allowing for fitting to be automated without requiring user feedback. One consequence of this simplified fitting is an increased level of noise caused by the fit cutting off portions of the peak. For this reason, all reported single-peak position and area values were separately fit using Origin (OriginLab) peak-fitting software with a cubicsplined background.
(LiNH2)c(LiBH4)1-c Quaternary Hydrides Phase Behavior
Figure 1. R-Phase peak position (open squares) and β-phase peak area (filled circles) for a 2:1 milled sample at 50 °C. The growth of the β-phase and the relaxation of lattice strain in the R-phase are highly correlated, as shown in the inset, after some initial deviations caused by the time required to stabilize the temperature of the sample.
Results and Discussion The composition change of R-phase Li3BN2H8 over time toward a higher LiNH2 content by segregation of β-phase was first observed in preliminary in situ XRD experiments conducted at 50 °C. Figure 1 demonstrates the time evolution of a freshly milled 2:1 sample containing LiCl as an internal standard. While the sample began as single-phase Li3BN2H8, the angular positions of the R-phase peaks increased as a function of time (i.e., the lattice constant, a, decreased toward Li4BN3H10), contemporaneous with the growth of β-phase peaks. Also, when the relative changes were plotted (relaxation percentage and growth percentage), the two effects were shown to be nearly linearly correlated (Figure 1 inset). To quantitatively analyze the change in the R-phase with respect to development of secondary phases, it was necessary to run a systematic set of samples. Analysis of in situ experiments performed on this main series of samples produced fits for peak position and intensity as a function of time for all phases present in the samples. Composition of the R-phase manifests in two ways. The first is in the lattice constant; the composition of the R-phase is taken to be linear in the lattice constant contraction between freshly prepared single-phase 2:1 and equilibrium 3:1 samples, consistent with Vegard’s Law. In this work the angular position of the main R-phase (222) peak near 28.9° is used as a surrogate for the lattice constant. The second manifestation of the R-phase composition is the intensity of the main R-phase (222) peak relative to those of nearby peaks from other phases. This approach is simplified by the chemistry of the system; the high reactivity of lithium borohydride is such that the most prevalent secondary phase, if present, was either β-phase (in borohydride-rich compositions) or lithium amide (in amide-rich compositions). In the cases where no other phases were present, it could be taken that the R-phase composition was the same as the overall composition of the sample. To quantitatively assess peak positions of evolving phases, the use of an internal standard was desirable. Fortunately, the experiments naturally contained three line compounds that could be considered standards: lithium amide, lithium borohydride, and the β-phase.15,17 These phases, however, were not always present throughout any given experiment, and after their disappearance the data collection was terminated. Peak Positions. The peak position of the R-phase is highly dependent on its composition, with lower angle positions corresponding to an expanded R-lattice consistent with higher borohydride content. As the borohydride content of the R-phase
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Figure 2. Final R-phase main peak positions relative to the nearest β-phase peak for all in situ samples, with compositions ranging from 2:1 to 3:1 (0.667-0.75) in increments of 0.2, in which a secondary phase was present. Shown are 10-min HEBM “mixed” (black) and 300min “milled” (red) HEBM samples. It can be seen that sample final positions are dependent both on the sample composition and on the preparation technique.
decreased during in situ heating at 50 °C, either through the formation of β (c e 0.706) or through incorporation of additional amide (c > 0.722) (as will be discussed below), the R-phase peak position shifted toward higher angles, as in Figure 1. This shift in R-peak positions occurred for a majority of samples. Final post-heat-treatment positions of the main R-phase (222) peak relative to the position of the nearest β-phase peak ((311), ∼27.6°) are plotted in Figure 2 for all in situ samples. Points for amide-containing samples (c > 0.722) were placed on the plot by measuring the R-phase peak position relative to the nearest amide peak ((112), ∼30.7°) and correcting for the distance between the amide and β-phase peaks (3.15 ( 0.01°) obtained through measurement of a mixed standard of amide and β-phase. As a point of reference, were the samples to have a 2:1 or 3:1 R-phase composition, the peak distance from β(311) would be roughly 1.33° or 1.72°, respectively. Not included are samples that were entirely composed of R-phase, as there was no secondary phase to act as a standard and these samples’ R-phase composition could be taken as the total sample composition. These samples, 2.4: 1, 2.6:1, and 2.8:1 milled, were all 300-min milled samples. It can be seen from Figure 2 that the final R-phase (222) peak position, and therefore the final R-phase composition, appears to depend not only on the initial composition but also on the preparation technique. The three compositions for which both sample preparations contain a second phase (2:1, 2.2:1, and 3:1) have differing final peak positions for the mixed and milled samples. In general, mixed or milled samples could be easily differentiated based on their R-phase composition, with the mixed samples having roughly the same R-phase peak position at ∼1.59° separation from the position of the β-phase peak. Milled samples containing β-phase (2:1 and 2.2:1) have a slightly lower R-phase peak position at ∼1.56° separation. One exception to both behaviors is the mixed sample at 2.4:1 composition, which shows only β-phase for a limited time during the experiment. During this time, the peak position of R-phase is located roughly between where the mixed and milled sample peaks are. Finally, the position of the 3:1 milled sample’s R-phase peak at ∼1.68° is approximately 0.1° farther away from the β-phase peak (i.e., higher angle) than the other observed R-phase peak positions. It should be noted that this peak position is still ∼0.04° away from the expected main peak position of
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Figure 3. (a) Relative intensity of R-phase and lithium amide peaks during in situ measurement as a function of time. Rapidly increasing values indicate samples annealing into a single R-phase.(b) Relative intensity of R- and β-phase peaks during in situ measurement as a function of time. Some samples possess a period of stability before an exponential decrease in the amount of β-phase.
an R-phase with a 3:1 composition. Save for this final observation, the differences in peak positions mentioned thus far consist of differences on the order of 1-2% shifts and therefore should be treated with some degree of caution. To confirm these results, peak intensities were analyzed as an independent probe of phase composition. Peak Intensities. Peak intensities were also used to analyze the composition of the R-phase. Plots of relative intensity, IR/Iβ or IR/Iamide of the primary (R) and secondary (β or amide) phase are presented in Figure 3. The secondary phase was chosen as the denominator in the ratios to make changes in relative intensity easier to observe. The substantial changes in the intensity ratios at times 50 °C) and 80 °C. In the case of lithium amide, which has a melting temperature ∼380 °C, the energy of HEBM should not be enough to cause any significant nonequilibrium behaviors, such as forcing a single phase. At the same time, it should be enough energy to drive the reaction of lithium amide and borohydride, while simultaneously mixing the sample, preventing any compositional shifts. This implies that the amiderich solubility limit is, in fact, ∼2.83:1, consistent with 2.8:1 milled being a single phase. This is also consistent with the decrease of the amide content in 3:1 milled samples at higher temperatures seen in separate experiments (Figure 5), showing an increase in the amide solubility from the 2.83:1 composition further toward 3:1 upon annealing at 120 °C, and formation of the 3:1 composition in samples recrystallized from 220 °C. Were the actual equilibrium R-phase composition much lower, the increased mobility at higher temperatures would allow for the extra amide to leave the R-phase, causing the evinced amount of amide to increase. This makes the complete equilibrium R-phase solubility range roughly 2.62:1-2.83:1, far reduced from the 2:1-3:1 range previously reported in the literature. The calculated R-phase compositions for the examined samples based on the peak intensity analysis are plotted in Figure 6. This information allows the LiBH4:LiNH2 phase diagram proposed in ref 15 to be revisited with additional detail. A proposed simplified diagram in the c ) 0.5-1 composition range
(LiNH2)c(LiBH4)1-c Quaternary Hydrides Phase Behavior
Figure 6. Composition of the R-phase plotted against sample composition for all samples. Single-phase line (green) and expected solubility region (blue) plotted for reference, in addition to dashed lines at the 2:1 and 3:1 compositions.
and T ) 0-250 °C temperature range (below the R-phase liquid decomposition) is presented in Figure 7. It should be noted that this diagram works only in heating as the β-phase does not recrystallize and is formed only by reaction or HEBM below its melting temperature. It also leaves out the solid phases that form out of the β-phase liquid, such as γ, as they are not well understood. The R-phase solubility range begins at the 2.62: 1-2.83:1 range found in this study, then follows a peritectic behavior, shifting and narrowing as it approaches higher temperatures, thus accounting for both the decreasing amide peak in the 3:1 milled sample at 120 °C (Figure 5) and the fact that heat-treated 2:1 milled samples are repeatedly characterized as having a 3:1 R-phase.13,17 Additionally, there exists an R, β binary eutectic region, which corresponds to samples in the range c ) 0.556-0.636 melting during milling as evinced by the appearance of the γ-phase.15 Neither the eutectic composition nor temperature is known at this time and will require additional study. The eutectic temperature, however, must be above 50 °C or the β-phase in all in situ experiments would have melted. Furthermore, from the compositional study, it is likely that the eutectic composition falls in the range c ) 0.556-0.6, as this is where the dominant phase in the partially melted sample switches from β to R.15 The behavior of the R and lithium amide mixture was not investigated in this study; thus, behavior
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Figure 8. TGA weight loss from a 2:1 sample made by mixing a 3:1 milled sample and lithium borohydride in a 2:1 ratio. Approximately 12 wt % hydrogen release is observed much the same as in 2:1 milled samples. This indicates that overall sample composition may be more critical for hydrogen release than sample preparation.
proposed through in situ XRD in the initial composition study was retained.15 One additional change was to extend the L + LiNH2 region past the 3:1 composition to account for amide that is evinced in in situ experiments conducted above the R-phase melting temperature. Summary Through in situ XRD of samples from the (LiNH2)c(LiBH4)1-c system with c in the range 0.667-0.75 (corresponding to LiNH2: LiBH4 2:1-3:1) prepared by HEBM for 300 min (milled) or HEBM for 10 min (mixed), the equilibrium solubility range of the quaternary R-phase was investigated. It was found that the mixed samples allowed for diffusion-limited formation of the R-phase. This provided an accurate measure of the equilibrium behavior of β-containing samples, giving a borohydride-rich phase boundary at 50 °C of ∼2.62:1. Milled samples, on the other hand, created nonequilibrium conditions for β-containing samples, while allowing for homogeneous reaction of amidecontaining samples. While milled amide-containing samples were not as uniform as the β-containing ones, they were close enough to place the amide-rich phase boundary at ∼2.83:1. The complete 50 °C solubility range of the R-phase has been shown to be ∼2.62:1-2.83:1, a much narrower range than the 2:1-3:1
Figure 7. Proposed phase diagram in the composition range c ) 0.5-1 and temperature range T ) 0-250 °C. Compositions examined in detail as a part of this study fall in the 2:1-3:1 (or 0.667-0.75) range. It should be noted that both the eutectic composition and temperature of the R-β solid solution are not yet determined but are expected to fall within the displayed ranges (0.556-0.6 and 50-80 °C, respectively).
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reported in the literature. Utilizing this along with information from other studies, an approximate phase diagram (omitting β decomposition) in the composition range c ) 0.5-1 and temperature range T ) 0-250 °C was proposed (Figure 7). One implication of these results is that the 2:1 milled samples reported to give highly desirable hydrogen release properties were likely aged samples and therefore no longer singlephase Li3BN2H8 materials; much the same result could be obtained from a mixed sample without the extra processing required for a milled sample. This has since been shown to be the case through TGA of a 2:1 sample made by mixing a 3:1 milled sample and lithium borohydride in a 2:1 ratio (Figure 8). This could also have been done with just amide and borohydride, although the former mixture avoids formation of the β-phase while the latter can form a β-R mixture during heating. It is also likely that this new understanding of phase behavior will lead to some insight on the release of ammonia by these samples, which were expected to be a hydrogen-only reaction path. Ammonia release remains a point of continued investigation. Acknowledgment. The authors thank Steven Szewczyk for his assistance in repair and maintenance of X-ray equipment and Dr. Michael Balogh for his advice on X-ray analysis. References and Notes (1) Pinkerton, F. E.; Wicke, B. G. Ind. Physicist 2004, 10, 22.
Singer et al. (2) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (3) Luo, W. J. Alloys Compd. 2004, 381, 284. (4) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; Tolle, J. J. Alloys Compd. 2000, 302, 36. (5) Pinkerton, F. E. J. Alloys Compd. 2005, 400, 76. (6) Meisner, G. P.; Tibbetts, G. G.; Pinkerton, F. E.; Olk, C. H.; Balogh, M. P. J. Alloys Compd. 2002, 337, 254. (7) Vajo, J. J.; Skeith, S. L.; Mertens, F. J. Phys. Chem. B 2005, 109, 3719. (8) Luo, W.; Stewart, K. J. Alloys Compd. 2007, 440, 357. (9) Meisner, G. P.; Pinkerton, F. E.; Meyer, M. S.; Balogh, M. P.; Kundrat, M. D. J. Alloys Compd. 2005, 404-406, 24. (10) Pinkerton, F. E.; Meisner, G. P.; Meyer, M. S.; Balogh, M. P.; Kundrat, M. D. J. Phys. Chem. B 2005, 109, 6. (11) Jeon, E.; Cho, Y. J. Alloys Compd. 2006, 422, 273. (12) Aoki, M.; Miwa, K.; Noritake, T.; Kitahara, G.; Nakamori, Y.; Orimo, S.; Towata, S. Appl. Phys., A: Mater. Sci. Process. 2005, 80, 1409. (13) Filinchuk, Y. E.; Yvon, K.; Meisner, G. P.; Pinkerton, F. E.; Balogh, M. P. Inorg. Chem. 2006, 45, 1433. (14) Chater, P. A.; David, W. I. F.; Johnson, S. R.; Edwards, P. P.; Anderson, P. A. Chem. Commun. 2006, 2439. (15) Meisner, G. P.; Scullin, M. L.; Balogh, M. P.; Pinkerton, F. E.; Meyer, M. S. J. Phys. Chem. B 2006, 110, 4186. (16) Chater, P. A.; David, W. I. F.; Anderson, P. A. Chem. Commun. 2007, 4770. (17) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J.; Yildirim, T. Chem. Mater. 2008, 20, 1245. (18) 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.
JP905970H
