Pressure-induced structural and electronic changes in α-AlH3

Pressure-induced structural, electronic, and thermodynamic changes in alpha-AlH3 were investigated using synchrotron x-ray powder diffraction and density-functional theory. No first-order structural transitions were observed up to 7 GPa. However, increasing Bragg peak asymmetry with pressure suggests a possible monoclinic distortion at moderate pressures (1-7 GPa). The pressure-volume relationship was fit to the Birch-Murnaghan equation of state to give a bulk modulus of approximately 40 GPa. The reduced cell volume at high pressure is accommodated by octahedral tilting and a decrease of the Al-H bond distance. Ab initio calculations of the free energy indicate that hydrogenation becomes favorable at H-2 pressures above 0.7 GPa at 300 K. Electronic density of states calculations reveal a slight decrease in the band gap with pressure but no evidence of an insulator-to-metal transition predicted by previous high-pressure studies. Calculated Mulliken charges and bond populations suggest a mixed ionic and covalent Al-H bond at 1 atm with an increase in covalent character with pressure.