First-principles prediction of low-energy structures for AlH₃

We report density-functional calculations that predict ten different low-energy structures for aluminum hydride AlH3 with space groups Pnma, P6/mmm, I4/mcm, P4/mbm, P4/nmm, Pm 3̄ m, P 21 /m, P 21 /c, Pbcm, and P4/n. Phonon calculations within harmonic approximation reveal unstable modes in the P6/mmm, I4/mcm, P4/mbm, P4/nmm, Pm 3̄ m, P 21 /m, and P 21 /c structures, indicating that they are unstable at low temperatures. The calculations show that the thermodynamic stabilities for AlH3 with space groups Pnma, Pbcm, and P4/n are overall close to the existing α - and γ -AlH3. From x-ray powder-diffraction patterns, the simulated main-peak positions for AlH3 (P4/n) are in good agreement with experimental δ -AlH3. A full Rietveld analysis reveals that the fitting space groups R 3̄ c, Pbcm, and Pnma to the experimental x-ray powder-diffraction pattern of α -AlH3 gives almost the same satisfactory result.