We performed a combined study using inelastic neutron scattering (INS) and first-principles calculations of the vibrational properties of CaAlH5 and α-AlH3 with different AlH6 networks, a zigzag one-dimensional AlH6 network for CaAlH5, and a three-dimensional AlH6 network for α-AlH3. Both materials showed qualitatively similar INS spectra, in which CaAlH 5/α-AlH3 was mainly divided into three regions: (i) the translational modes (318/316 cm-1), (ii) the librational modes of the octahedral AlH6 units (external molecular motion) in the lower frequency range and H-Al-H bond-bending modes (intra molecular motion) at a higher frequency (420-1157/513-1038 cm-1), and (iii) the Al-H bond-stretching modes (1238-1750/1486-1942 cm-1). In region ii, the appearance of both librational and bond-bending modes was determined by the networked nature of the octahedral AlH6 units. In addition, the librational modes of AlH6 on α-AlH3 exhibit higher frequencies than CaAlH5 due to the tighter bonding between the octahedral AlH6 units. With regard to average frequencies for the Al-H, ωS, bond-stretching modes, and average Al-H bond distances on the aluminum-based hydrides including CaAlH5 and α-AlH3, ωS showed lower frequencies that correlate with lengthening of the Al-H bond distances.
ASJC Scopus subject areas