Lattice dynamics of α-Al H3 and α-Al D3 by inelastic neutron scattering: High-energy band of optical bond-stretching vibrations

Inelastic neutron scattering (INS) spectra of trigonal α-Al H3 and α-Al D3 have been measured in a wide range (5-150 meV) of energy transfers with a much better statistical accuracy than previously using the inverted-geometry NERA-PR spectrometer at JINR, Dubna. The studied energy interval covers the range of low-frequency lattice vibrations and a broad band of optical H vibrations. By using a direct-geometry HRMECS spectrometer at ANL, Argonne, the Al H3 hydride was also examined by INS at energy transfers up to 315 meV in the regime of small momentum transfers. This significantly enhanced the contribution from the one-phonon neutron scattering and allowed an observation and accurate examination of one more, high-energy band of optical H vibrations predicted theoretically. Combining the obtained experimental data, the entire spectrum of phonon density of states was reconstructed for both α-Al H3 and α-Al D3.