Low-pressure Structural Modification of Aluminum Hydride

Report No. ARL-TR-5463
Authors: Jennifer A. Ciezak-Jenkins
Date/Pages: February 2011; 22 pages
Abstract: In-situ isothermal high-pressure synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of alpha-aluminum hydride (AlH3). The x-ray measurements show that the pressure-volume relations remain smooth to near 50 GPa. Although there is no evidence for a first-order phase transition, the unit cell axial ratio (c/a) is anomalous near 2.5 GPa, which correspond to the onset of a monoclinic structural distortion. Infrared measurements conducted within a similar pressure range show a distinct discontinuity in the frequencies of the Al-H lattice modes. The combined data provide evidence for a monoclinic structural distortion beginning near 2.5 GPa. The results are consistent with recent synchrotron x-ray diffraction measurements and theoretical calculations. All structural changes are reversible upon pressure release.
Distribution: Approved for public release
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Last Update / Reviewed: February 1, 2011