Destabilized LiBH4−NaAlH4 Mixtures Doped with Titanium Based

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Destabilized LiBH4-NaAlH4 Mixtures Doped with Titanium Based Catalysts Qing Shi,†,‡ Xuebin Yu,†,§ Robert Feidenhans’l,‡ and Tejs Vegge*,† Materials Research DiVision, Risø National Laboratory for Sustainable Energy, Technical UniVersity of Denmark, DK-4000 Roskilde, Denmark, Niels Bohr Institute, UniVersity of Copenhagen, DK-2100 Copenhagen, Denmark, and Department of Materials Science, Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed: May 15, 2008; ReVised Manuscript ReceiVed: September 8, 2008

We investigate the hydrogen storage properties of the mixed complex hydride LiBH4-NaAlH4 system, both undoped and doped with a TiCl3 additive. The mixed system is found to initiate a transformation to LiBH4-NaAlH4 after ball-milling, and the doped system is found to have a significant lower hydrogen release temperature compared to the undoped system and that of titanium doped LiAlH4 and NaAlH4 systems. Three dehydrogenation steps with a total of 3.5 wt % released hydrogen are observed for samples with a 2:3 molar ratio, when heated to ∼210 °C under a constant heating ramp in a 1 bar helium atmosphere. A slow hydrogen release is also observed at room temperature in the freshly ball-milled powders due to decomposition of the formed LiAlH4. X-ray powder diffraction spectra shows that the first dehydrogenation step is completed after a couple of days at room temperature. LiNa2AlH6 is found in NaAlH4 rich mixtures at higher temperature, and this phase is found to reversibly discharge and recharge hydrogen at 80 bar and 180 °C. 1. Introduction Due to the limited supply of fossil fuels, hydrogen has attracted massive attention as an alternative energy carrier.1 The technical challenges of on-board hydrogen storage, however, still remain to be solved. The 2010 DOE system target of 6.0 wt % hydrogen for automobile applications can not be achieved by traditional metal-based hydrides.2 More interest has moved to complex hydrides as potential hydrogen storage materials, such as sodium alanate NaAlH43,4 and LiBH4.5-8 Sodium alanate doped with selected titanium based compounds can be reversibly discharged and recharged with hydrogen.3 NaAlH4 dehydrogenates in two steps:

3NaAlH4 T Na3AlH6 + 2Al + 3H2

(1)

3 Na3AlH6 T 3NaH + Al + H2 2

(2)

Abundant studies, both theoretical9,10 and experimental,11-14 have been performed to describe the catalytic effect of titanium in the doped alanate systems, but the role of the catalyst is still not fully understood. Dry doping has been found to be more effective to enhance the kinetics than wet doping with, e.g., Ti(OBu)4.15 Borohydrides, such as LiBH4 with a 18.3 wt % theoretical hydrogen storage capacity, have also received interest as potential high capacity hydrogen storage media. However, the onset of LiBH4 decomposition is 380 °C and only half of the hydrogen is released below 600 °C.5 Decomposition of LiBH4 releases 13.8 wt % hydrogen as

3 LiBH4 f LiH + B + H2 2

(3)

Recently, additives such as MgH27,8 and Al16 have been used successfully to reduce the enthalpy of reversible dehydrogena* Corresponding author. E-mail: [email protected]. † Technical University of Denmark. ‡ University of Copenhagen. § Fudan University.

tion and rehydrogenation reactions. However, the effect of aluminum is often limited by an oxide overlayer, which inhibits the role of the additive. Therefore, instead of Al particles, metal alanates can be used to introduce aluminum and act as a catalyst for the decomposition of LiBH4. The mixed LiBH4-LiAlH4 powder doped with transition metal halides significantly enhances the dehydrogenation kinetics.16 In the present study, the dehydrogenation properties of the mixed LiBH4-NaAlH4 system, both undoped and doped with 4 mol % TiCl3 (of the NaAlH4 content) were investigated. The actual weight percent of catalyst in the doped mixed system is thus less than the doped NaAlH4 previously studied. The molar ratio between LiBH4 and NaAlH4 was chosen as 1:1, 2:3, and 1:3, respectively, to study possible nonstoichiometric effects. Different phases of the compounds under various reaction temperatures were characterized by X-ray powder diffraction. Full dehydrogenation-hydrogenation cycles of the mixed powders are also studied. On the basis of this study, we identified the formation of a secondary complex hydride phase after ball-milling, which released hydrogen already at room temperature. We observed that the system is partially rechargeable with hydrogen for the NaAlH4 rich mixtures due to the formation of LiNa2AlH6. 2. Experimental Details All the commercial chemical compounds were used as received in powder form without further purification. LiBH4 (no. 62460, 95.0% gas-volumetric) was delivered from SigmaAldrich (Fluka & Riedel Co.). NaAlH4 (hydrogen-storage grade, 93%) and TiCl3 (hydrogen-storage grade, 99.999%) were purchased from Sigma-Aldrich. All sample handling and storage was performed in an inert gas (Ar) glovebox. A high energy Mixer Mill from Glen Creston Ltd., UK, was employed to mix and catalyze the samples17 under an argon atmosphere at 200 rmp; a Teflon O-ring sealed the stainless steel ball-mill vial, and five wolfram carbide balls (total weight 31 g) were used for the ball-milling. Each time, typically

10.1021/jp804311s CCC: $40.75  2008 American Chemical Society Published on Web 10/23/2008

Destabilized LiBH4-NaAlH4 Mixtures

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2 g of sample was prepared by ball-milling for 30 min using a 15:1 weight ratio between balls and sample powder. The freshly ball-milled powder was then filled into the reactor chamber of a Sieverts equipment and pressed as a pellet on the sample holder for X-ray powder diffraction (XRPD) characterization. The hydrogen release curves as a function of temperature were measured under 1 bar He atmosphere. In this study, the reactor in the Sieverts system is heated in a Carbolite furnace with an adjustable heating ramp. Around 0.75 g of powder sample was used for each experiment. A valve on the reactor prevents the sample powder from exposure to air during transportation from the glovebox to the instrument. The minimum operation time between completion of the ball-mill and start of the Sieverts experiment is 20 min, which includes sample loading and transportation. The XRPD spectra were recorded on a Bragg-Brantano STOE diffractometer (40 kV, 30 mA, Cu KR with λ ) 1.5418 Å). The powder diffraction data was collected at room temperature in steps of 0.05°/2θ. To avoid reactions between the powder and oxygen/moisture in the air, a specially designed airtight XRPD sample holder with an aluminum foil X-ray window was employed. A piece of Si is located on the sample holder at the sample position. This is used to determine the sample position relative to the incoming X-ray beam and serve as a reference for the peak positions. Therefore, every XRD spectrum presented also includes Si peaks at 28.44°, 47.30°, and 56.12°. 3. Results and Discussion In the following, we describe our absorption/desorption experiments on the mixed LiBH4-NaAlH4 system. 3.1. Desorption Experiments. The dehydrogenation curves as a function of temperature (Figure 1) show the hydrogen release properties of the ball-milled LiBH4-NaAlH4 systems, both undoped (molar ratio 2:3) and doped with 4 mol % TiCl3 (of NaAlH4) (molar ratios of 1:1, 2:3, and 1:3). The doped systems start to release hydrogen at room temperature and there is a total of 3.1, 3.6, and 3.9 wt % hydrogen released below 210 °C for samples with molar ratios of 1:1, 2:3, and 1:3, respectively. The undoped sample starts to release hydrogen at higher temperatures, around 110 °C, and a total of 4.2 wt % hydrogen is released above 250 °C. As expected, the amount of released hydrogen for the doped sample is smaller than that of the undoped, since some LiAlH4 has been consumed by the reduction reaction of TiCl3 during the ball-milling process.18 The dehydrogenation data for 4 mol % TiCl3 doped NaAlH4 is also included in Figure 1a as a reference. This data is obtained under the same experimental conditions as the mixed LiBH4-NaAlH4 systems. The undoped and doped systems (molar ratio 2:3 and 1:3) show a clear three step hydrogen release reaction pattern below 310 °C (see Figure 1b). However, only a two step hydrogen release is observed for the 1:1 molar ratio. XRD spectra obtained at different temperatures during the dehydrogenation for the 2:3 molar ratio shows NaAlH4 signals at 100 °C (see Figure 2), which are not observed in the 1:1 molar ratio system (XRD data not shown here). The third step of the desorption reaction occurs in the same temperature range as the second dehydrogenation step of TiCl3 doped NaAlH4. The third hydrogen release reaction is thus coming from the second step of the decomposition of the excess NaAlH4 (eq 2) in the mixed system. In the first step of hydrogen desorption, a maximum of 2 wt % hydrogen is released in a very fast reaction for the doped system and we expect the first step of hydrogen unloading starts already during ball-milling. Data from the hydrogen release experiments

Figure 1. (a) Dehydrogenation curves as a function of temperature for the mixed LiBH4-NaAlH4 (molar ratio 2:3) systems, undoped and doped with TiCl3. To compare the hydrogen release properties of the mixed system, a 4 mol % TiCl3 doped NaAlH4 dehydrogenation curve is also included. The heating ramps for all systems are 0.5 °C/min. (inset) Full temperature range hydrogen release curves of the mixture, showing the release is first complete above 550 °C. (b) Dehydrogenation curves as a function of temperature for the 4 mol % TiCl3 (of NaAlH4) doped LiBH4-NaAlH4 systems with molar ratios of 1:1, 2:3, and 1:3.

on the system with the 2:3 molar ratio was recorded at various times after ball-milling (see Figure 3). After keeping the sample for a couple of hours in a glovebox at room temperature, the amount of hydrogen released in the first step is decreased. It is clear that the first step reaction releases hydrogen at room temperature after ball-milling. Due to the sample delivery and loading, the minimum time required to start the experiment after ball-milling is 20 min. There is a 0.5 wt % lower release of hydrogen 6 h after ball-milling compared to 20 min after ballmilling. If the recording time is started 20 h after ball-milling, the first step released hydrogen has lost an additional 0.7 wt %. The first step of hydrogen release will be completed in a couple of days at room temperature. The inset of Figure 3 shows the XRD spectra of a ball-milled sample left in the glovebox for 7 days and a fresh powder heated to 100 °C; both have completed the first step reaction and the two spectra are almost identical. Dehydrogenation curves for the doped and undoped systems obtained using different heating ramps are shown in Figure 4. Faster heating ramps are seen to shift the hydrogen release curves to higher temperatures. There is a 20 °C difference

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NaAlH4 + LiBH4 f NaBH4 + LiAlH4

Figure 2. XRD spectra of the 4 mol % TiCl3 (of NaAlH4) doped LiBH4-NaAlH4 (molar ratio 2:3) system after dehydrogenation at different temperatures. The spectra were measured after cooling to room temperature.

(4)

The XRD spectra of powders with different molar ratios but same ball-milling time, and the mixed powders without doping are shown in Figure 5b. It is clearly seen that the reaction above (eq 4) happens for different molar ratios and even in the system without doping. In agreement with thermodynamic calculations,19,20 the formation of NaBH4 is observed following eq 4. A big difference in the amount of released hydrogen from the first step reaction is observed for different molar ratios of LiBH4 and NaAlH4 (see Figure 1b); this is related to the amount of LiBH4 in the original mixture powder, which is confirmed by the intensities of NaBH4 peaks in XRD spectra (see Figure 5b). The powder diffraction spectrum for the sample with extended ball-milling shows signals of LiH/Al. The LiH/Al signals get stronger when the mixed powder is heated to higher temperatures (see Figure 2). By comparing to the temperature dependent hydrogen release curves in Figure 3, the decomposition of LiAlH4 is seen to start already during ball-milling. The peak positions of LiH and Al are very close and can not be distinguished in this setup. However, the XRD signal around 77.5° splits into two peaks (not show here), which correspond to LiH and Al signals, respectively. Therefore, we believe that both LiH and Al signals contribute the intensity at LiH/Al peak positions. There are two close peaks in the lower angle range of the XRD spectrum from the powder heated to 100 °C (see Figure 2), which correspond to the two main peaks of Li3AlH6. These signals can also be found in the XRD spectrum of powder kept in the glovebox for 7 days (see inset of Figure 3). The presence of an intermediate hydride phase indicates a second decomposition reaction of LiAlH4 in this mixed system:

3LiAlH4 f Li3AlH6 + 2Al + 3H2

(5)

Li3AlH6 can also be found in doped powder with a molar ratio of 1:1 when heated to 100 °C and in the powder without doping when heated to 150 °C (see the inset of Figure 5b). Li3AlH6 can further release hydrogen at temperatures above 100 °C in the ball-milled system with titanium doping: Figure 3. Dehydrogenation curves of the 4 mol % TiCl3 (of NaAlH4) doped LiBH4-NaAlH4 (molar ratio 2:3) system (blue recorded 20 min after ball-milling; green recorded 6 h after ball-milling; and red recorded 20 h after ball-milling). (inset) XRD spectra of the mixed system after dehydrogenation at 100 °C (a) and for the mixture after 7 days in an argon glovebox at room temperature (b).

between the 0.5 and 2 °C/min heating ramp data, which indicates that the process is limited by the reaction kinetics and/or heat conduction. To understand the first step reaction mechanism, XRD spectra of the mixed samples (molar ratio 2:3) without ball-milling and for different ball-milling times are recorded (see Figure 5a). Compared to the powder which has not been ball-milled, the XRD spectra of the ball-milled samples show new peaks which correspond to the formation of LiAlH4 and NaBH4. The intensities of the LiAlH4 and NaBH4 peaks increase as the ballmilling is extended; meanwhile, the original powder signals from LiBH4 and NaAlH4 are reduced. Upon increased ball-milling, this reaction goes to completion. The diffraction spectrum for the powder that was ball-milled for 30 min does not contain any LiBH4 signals, which are still remaining in the spectrum after shorter ball-milling. Therefore, the first reaction of the mixed system during ball-milling is

3 Li3AlH6 f 3LiH + Al + H2 2

(6)

In the XRD spectra (Figure 6), we notice that the intensity of the NaAlH4 peaks weakens as the temperature is increased,

Figure 4. Hydrogen release curves for different heating ramps for both undoped and doped (4 mol % TiCl3) LiBH4-NaAlH4 (molar ratio 2:3) systems. The heating ramps are 0.5 and 2 °C/min, respectively.

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Figure 6. XRD spectra of the 4 mol % TiCl3 (of NaAlH4) doped LiBH4-NaAlH4 (molar ratio 2:3) mixed system after dehydrogenation at 200 °C and the recharged powder which had been dehydrogenated at 300 °C and rehydrogenated at 180 °C under 80 bar H2. Both spectra were recorded after cooling to room temperature.

Figure 5. (a) XRD spectra of LiBH4-NaAlH4 (molar ratio 2:3) mixtures (blue: the undoped system simply mixed without ball-milling; green: the 4 mol % TiCl3 (of NaAlH4) doped system under 5 min ballmilling; and red: the 4 mol % TiCl3 (of NaAlH4) doped system after 30 min ball-milling). (b) XRD spectra of undoped and doped LiBH4-NaAlH4 mixtures (different molar ratio) under 30 min ballmilling. (inset) Mixed system after dehydrogenation at different temperatures. The spectra were measured at room temperature after cooling (blue undoped sample (molar ratio 2:3) heated to 150 °C; green doped sample (molar ratio 1:1) heated to 100 °C).

and new peaks of NaH appear at ∼200 °C; this indicates that hydrogen is released from the excess NaAlH4 in the doped system (eqs 1 and 2).3,4 As the temperature is further increased to 400 °C, the doped system releases additional hydrogen (see inset of Figure 1) from the formed NaBH4. This temperature is too high for practical applications, and the destabilizing effect of oxide-free Al on NaBH4 is thus insufficient. The XRD spectrum of the doped system (see Figure 2) at 600 °C shows pure Na, while the NaBH4 and NaH peaks have disappeared, indicating that both phases have released hydrogen at this temperature. 3.2. Absorption Experiment. A reloading experiment was performed on the 2:3 (molar ratio) LiBH4-NaAlH4 sample at a hydrogenation pressure of 80 bar H2 and 180 °C. Figure 6 shows the XRD spectra of the doped sample (molar ratio 2:3) heated to 200 °C together with the recharged sample, which had initially released hydrogen at 300 °C and was then (re)hydrogenated at 180 °C. In both spectra, LiNa2AlH6 peaks

Figure 7. Dehydrogenation curves as a function of temperature for the 4 mol % (of NaAlH4) TiCl3 doped LiBH4-NaAlH4 (molar ratio 2:3) system (blue) and the sample which has been recharged at 80 bar H2 (green). The heating ramp is 2 °C/min.

are observed. Recalling the XRD spectra at 100 and 300 °C (see Figure 2), a reaction occurs between Li3AlH6 and NaH to form LiNa2AlH6 and release hydrogen around 200 °C. The LiNa2AlH6 phase releases additional hydrogen as the temperature is increased. When the dehydrogenated sample is kept at 80 bar H2 pressure and 180 °C, LiH, NaH, Al, and H2 is found to form LiNa2AlH6. This part of the hydrogen uptake/release is reversible under these conditions:

3 LiNa2AlH6 T LiH + 2NaH + Al + H2 2

(7)

The hydrogen absorption/desorption data in Figure 7 display a release of 0.6 wt % hydrogen from the recharged sample below 300 °C. We notice that a small amount of hydrogen is also released below 100 °C from the recharged sample. Due to the excess NaAlH4 in the system, we expect that there is a competitive reaction to form NaAlH4 (eq 1) during the (re)hydrogenation process. However, the small amount of recharged NaAlH4 can not be detected by XRD.

18248 J. Phys. Chem. C, Vol. 112, No. 46, 2008 LiAlH4, with a total of 10.6 wt % hydrogen, is kinetically stable in the solid state at room temperature.21,22 However, above its melting temperature, the tetrahedral [AlH4]- ion easily transforms into the octahedral [AlH6]3 - ion23 by which LiAlH4 decomposes and releases 5.3 wt % hydrogen at 112-175 °C upon thermal decomposition.24,25 In the present study, we observe that the formed LiAlH4 decomposes quickly in the doped, mixed systems during/after ball-milling at a temperature much lower than its melting point. Balema et al.26 have previously reported that rapid solid-state transformation of LiAlH4 into Li3AlH6 and Al can be achieved by the presence of titanium-based catalyst during high energy ball-mill treatment, but the reported onset of decomposition was still above 100 °C. In our mixed LiBH4-NaAlH4 system, this process occurs readily at room temperature when doped with a titanium catalyst. Previous investigations show that the dehydrogenation kinetics of LiAlH4 into Li3AlH6 is also effected by ball-milling on crystallite size.27 Therefore, both the titanium dopant and ballmilling improves the decomposition of LiAlH4 in the titanium doped LiBH4-NaAlH4 system, and we expect the presence of NaAlH4 compounds to facilitate the decomposition, but further investigations are needed. The hydrogen reloading experiment on the ball-milled and doped LiBH4-NaAlH4 system shows only limited rehydrogenation (eq 7) resulting from the formation of LiNa2AlH6. This reaction has previously been proved reversible upon addition of titanium.3 Under our experimental condition, the hydrogen reloading data does not show direct hydrogenation of LiAlH4 and Li3AlH6. This is expected since calculations show that the reversible reaction between LiAlH4 and Li3AlH6 is effectively impossible under practically accessible temperatures and hydrogen pressures.28 A small hydrogen release can be observed at 80-100 °C, which we ascribe to the formation of NaAlH4. 4. Conclusion The mixed LiBH4-NaAlH4 system is found to form LiBH4-NaAlH4 during prolonged ball-milling. This reaction is confirmed by mixing different molar ratios, with and without the addition of a TiCl3 dopant. The formed LiAlH4 releases hydrogen already at room temperature in the titanium doped mixed systems forming Li3AlH6. The Li3AlH6 releases additional hydrogen just above 100 °C; possibly due to the onset of LiNa2AlH6 formation. This temperature is significantly lower than observed for pure LiAlH4 powders doped with titanium. At higher temperatures, the presence of oxide-free Al is found to be unable to destabilize NaBH4 sufficiently to reach release temperatures which are suitable to practical applications. If there is excess NaAlH4 in the initial system, NaH, decomposed from NaAlH4, can react with LiH and Al to form LiNa2AlH6, which can be reversibly dis- and rechargeable with hydrogen in the doped system. The mechanism behind the fast hydrogen release at room temperature and the role of the catalyst and other species

Shi et al. in the mixed system are still unclear. Further detailed studies are necessary to understand the behavior of this system. Acknowledgment. The authors would like to thank our diligent technician, Mike Wichmann, for his professional technique support. This project was financial supported by the Copenhagen Graduate School for Nanoscience and Nanotechnology (C:O:N:T) and the Danish Energy Research Program (ENS-33031-0098). References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 353, 414. (2) http://www1. eere.energy.gov/ hydrogenandfuelcells/ storage/pdfs/ targets_onboard_hydro_storage.pdf. (3) Bogdanovic´, B.; Schwickardi, M. J. Alloys Comp. 1997, 253-254, 1–9. (4) Bogdanovic´, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M. J. Alloys Comp. 2000, 302, 36–58. (5) Zu¨ttel, A.; Rentsch, S.; Fisher, P.; Wenger, P.; Sudan, P.; Mauron, Ph.; Emmenegger, Ch. J. Power Sources 2003, 118, 1–7. (6) Lodziana, Z.; Vegge, T. Phys. ReV. Lett. 2004, 93, 145501. (7) Vajo, J. J.; Skeith, S. L.; Mertens, F. J. Phys. Chem. B 2005, 109, 3719. (8) Yu, X. B.; Grant, D. M.; Walker, G. S. Chem. Commum. 2006, 37, 3906–3908. (9) Vegge, T. Phys. Chem. Chem. Phys. 2006, 8, 4853. (10) Voss, J.; Shi, Q.; Jacobsen, H. S.; Zamponi, M.; Lefmann, K.; Vegge, T. J. Phys. Chem. B 2007, 111, 3886–3892. (11) Felderhoff, M.; Klementiev, K.; Gru¨nert, W.; Spliethoff, B.; Tesche, B.; Bellosta von Colbe, J. M.; Bogdanovic´, B.; Hrtel, M.; Pommerin, A.; Schu¨th, F.; Weidenthaler, C. Phys. Chem. Chem. Phys. 2004, 6, 4369– 4374. (12) Brinks, H. W.; Sulic, M.; Jensen, C. M.; Hauback, B. C. J. Phys. Chem. B 2006, 110, 2740. (13) Le´on, A.; Rothe, J.; Fichtner, M. J. Phys. Chem. C 2007, 111, 16664–16669. (14) Lohstroh, W.; Fichtner, M. Phys. ReV. B 2007, 75, 184106. (15) Jensen, C. M.; Zidan, R.; Mariels, N.; Hee, A.; Hagen, C. Int. J. Hydrogen Energy 1999, 24, 461–465. (16) Jin, S. A.; Shim, J. H.; Cho, Y. W.; Yi, K. W.; Zabara, O.; Fichtner, M. Scripta Mater. 2008, 58, 963–965. (17) Huot, J.; Boily, S.; Gu¨ther, V.; Schulz, R. J. Alloys Comp. 1999, 283, 304. (18) Balema, V. P.; Wiench, J. W.; Dennis, K. W.; Pruski, M.; Pecharsky, V. K. J. Alloys Comp. 2001, 329, 108–114. (19) Cho, Y. W.; Shim, J. H.; Lee, B. J. CALPHAD 2006, 30, 65–69. (20) Alapati, S. V.; Johnson, J. K.; Sholl, D. S. Phys. Chem. Chem. Phys. 2007, 9, 1438–1452. (21) Bastide, J. P.; Bonnetot, B. M.; Letoffe, J. M.; Claudy, P. Mater. Res. Bull. 1985, 20, 999. (22) Dymova, T. N.; Konoplev, V. N.; Aleksandrov, D. P.; Sizareva, A. S.; Silina, T. A.; Sizareva, A. S. Russ. J. Coord. Chem 1995, 21 (3), 175–182. (23) Bureau, J. C.; Bastide, J. P.; Claudy, P.; Letoffe, J. M.; Amri, Z. J. Less-Common Met. 1987, 22, 185. (24) Brinks, H. W.; Hauback, B. C.; Norby, P.; Fjellvåg, H. J. Alloys Comp. 2003, 351, 222–227. (25) Dymova, T. N.; Aleksandrov, D. P.; Konoplev, V. N.; Silina, T. A.; Sizareva, A. S. Russ. J. Coord. Chem 1994, 20, 279–285. (26) Balema, V. P.; Dennis, K. W.; Pecharsky, V. K. Chem. Commum. 2000, 17, 1665–1666. (27) Andreasen, A.; Vegge, T.; Pedersen, A. S. J. Solid State Chem. 2005, 178, 3672. (28) Jang, J. W.; Shim, J. H.; Cho, Y. W.; Lee, B. J. J. Alloys Comp. 2005, 420, 286–290.

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