Pressure-Induced Phase Transformations in Li-based Complex Hydrides
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0987-PP04-10
Pressure-Induced Phase Transformations in Li-Based Complex Hydrides Raja Chellappa1, Dhanesh Chandra1, Stephen Gramsch2, Maddury Somayazulu2, and Russell Hemley2 1 Chemical & Metallurgical Engineering, University of Nevada, Reno, 1664 N. Virginia Street (MS 388), Reno, NV, 89557 2 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch, NW, Washington, DC, 20015
ABSTRACT An overview of the pressure-induced transformations and order-disorder phenomena in LiAlH4 (up to 7 GPa) and LiNH2 (up to 25 GPa) are presented. The analyses of pressure-induced changes in Raman spectra suggest a phase transition at ~3 GPa for LiAlH4 and ~14 GPa for LiNH2. New results on the metastable recovery of the high pressure β-LiAlH4 phase are also presented. An examination of the lattice translational and librational modes reveals that the high pressure β-LiAlH4 phase is disordered while there is evidence of orientational ordering in the high pressure β-LiNH2 phase.
INTRODUCTION The catalyzed Li-based aluminohydrides (LiAlH4), amides (LiNH2), borohydrides (LiBH4), and others are potential candidates for on-board hydrogen storage applications. The state-of-the-art and challenges in using catalyzed complex hydrides for hydrogen storage is described in Chandra et al. [1] and references there in. The preferred method of catalyst addition is in solid-state via ball milling; a dynamic, non-equilibrium process during which the samples can experience instantaneous pressures of up to 6 GPa [2]. The increase in surface area (due to reduction in particle size) in the milled complex hydrides (with or without catalysts) may be accompanied by metastable structural distortions as well [3, 4]. Enhancements in the hydrogenation/ dehydrogenation properties due to ball milling can be correlated with a weakened metal-hydrogen bonding in the anion complexes caused by high pressure effects as suggested by our previous study on LiAlH4 [5]. In addition to the structural and bonding changes, considerable densification in high pressure phase (~20% volume collapse in LiAlH4) has also been reported. If the high pressure phases do indeed possess enhanced properties, the possibility of metastable recovery of these compounds to ambient conditions may open a new direction of pressure-tuned hydride synthesis. In this study, we present previously unreported results from pressure quenching experiments on LiAlH4. We also present preliminary results from an ongoing study on pressure-induced transformations in LiNH2 [6] and compare it with the behavior of LiAlH4 [5]. An analysis of the vibrational lattice modes as a function of pressure suggests that LiAlH4 undergoes pressure-induced disorder while LiNH2 displays strong orientational ordering in the high pressure phase.
EXPERIMETAL DETAILS Reagent grade LiNH2 and LiAlH4 powder of 95% purity was purchased from Sigma Aldrich Inc. and used without further purification. The Diamond Anvil Cells (DAC) used for the Raman spectroscopy studies had diamonds with either 300 or 500 µm culets and sampl
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