First-principles investigations of aluminum hydrides: M3AlH6 (M=Na,K)

The structural stability of Na3AlH6 and K3AlH6 has been systematically investigated using accurate density-functional total-energy calculations. The experimentally known crystal structure of α-Na3AlH6 is reproduced and α-K3AlH6 is predicted to be isostructural (α-Na3AlF6 type; space group P21/n). This structure contains a pseudo-face-centered-cubic Al sublattice and each Al atom is surrounded by distorted octahedra of hydrogen atoms with the long octahedral axis tilted from the [001] direction toward <111>. We predict that the α modifications will not be stable at higher pressures. On application of pressure to Na3AlH6, the α phase transforms into a Cs3NdCl6-type structure at 19.8 GPa. Similarly α-K3AlH6 transforms into two high-pressure forms: (1) α transforms to β with Rb3TlF6-type structure at 53.4 GPa and (2) β transforms to γ with U3ScS6-type structure at 60.2 GPa