Pressure-Induced Phase Transformations in LiAlH₄

Abstract

The pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.

Pressure-Temperature Phase Relations in Complex Hydrides

“…Ex situ X-ray diffraction studies confirmed that the β phase had a tetragonal lattice. The structure could not be solved in detail, but the density of the β phase 4 , drawn from literature data [14,[16][17][18][19][20] for increasing temperature as discussed in the text. Squares and circles denote thermal anomalies observed by Konovalov et al [14]; open circles are unidentified anomalies attributed to impurities.…”

“…Squares and circles denote thermal anomalies observed by Konovalov et al [14]; open circles are unidentified anomalies attributed to impurities. Open triangle Talyzin and Sundqvist [16,17], filled triangle Chellappa et al [18,19]; bars show observed transition ranges. Phase boundaries have been redrawn as the best-fitted straight lines or, for the Li 3 AlH 6 boundary, a parabolic curve through the zero-pressure point.…”

Structural study of ball-milled sodium alanate under high pressure

“…This transformation has been reported to occur between 2.2 and 3.5 GPa [15]. Alternatively, NaBH 4 under pressure shows a phase transformation from ␣-cubic to ␤-tetragonal structure starting at 5.4 GPa with an orthorhombic phase stabilized at pressures above 8.9 GPa [16].…”

Structural Characterization of Metal Hydrides for Energy Applications

“…In this study, XRD measurements of unmilled or as received NaAlH 4 showed the phase transformation to an orthorhombic structure which is in agreement with the prediction of Vajeeston et al [74], followed by an amorphization at 13.6 GPa. A close analogue, LiAlH 4 , has a monoclinic structure at ambient conditions with space group P21/c and undergoes a slow transition to a tetragonal phase between 2.2 and 3.5 GPa [69].…”

“…The tetragonal and orthorhombic phases reported in these early studies were observed only when high pressure was combined with hightemperature treatment. High pressure phase transitions of pure LiAlH 4 investigated using in situ Raman spectroscopy and computation which shows that ~3 GPa the ambient monoclinic-P21/c phase of LiAlH 4 transform to a high pressure phase with a distorted AlH 4 -tetrahedron [69,70]. The phase transition is reversible below 1.4 GPa on decompression.…”