Pressure-induced structural phase transition inNaAlH4

Abstract

The structural stability of $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4}$, a promising hydrogen storage metal hydride system, has been investigated up to $27\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using in situ synchrotron x-ray diffraction and Raman experiments with a diamond anvil cell. Analysis of both diffraction and Raman data indicates a pressure-induced structural phase transition above $14\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from the ambient $I{4}_{1}∕a$ tetragonal phase $(\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4})$ to a high-pressure $P{2}_{1}∕c$ monoclinic $(\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4})$ phase which is accompanied by a 12% volume collapse. On decompression, $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4}$ reverts back to the ambient tetragonal phase with similar unit cell parameters observed prior to pressurization. Subsequent ab initio calculations for $\mathrm{Na}\mathrm{Al}{\mathrm{H}}_{4}$ and x-ray diffraction measurements on isostructural $\mathrm{Na}\mathrm{Al}{\mathrm{D}}_{4}$ compound up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ give further confirmation of the pressure-induced transition and its reversibility.