Vibration Spectra of Vanadium Hydride in Three Crystal Phases by Inelastic Neutron Scattering

Abstract

The vibration spectra of vanadium hydride in three crystal phases have been investigated by the energy-gain scattering of cold neutrons. The measured spectra are generally split into two broad bands above and below about 300 cm− 1, which are primarily associated with optical hydrogen vibrations and metal-atom vibrations, respectively. Pseudofrequency distributions for the hydrogen vibrations were derived from the measured neutron spectra. These indicate broad optical vibration bands peaked at 970 ± 50 and ∼1400 cm− 1 in the α (bcc) phase, 440 ± 20, 970 and ∼1400 cm− 1 in the β (bct) phase, and ∼1300 cm− 1 in the γ (fcc) phase. The width of the 440-cm− 1 band increases with hydrogen concentration, possibly due to hydrogen–hydrogen interactions. The “metal-atom” vibration spectra also show changes in peak positions and widths as the distribution of phases is changed. The changes in the neutron spectra with temperature and as a function of composition between VH0.20 and VH1.71 correlate quite well with the previously measured phase diagram. A comparison of the present results with neutron spectra for hydrides with known hydrogen positions suggests that in the α phase hydrogens only occupy tetrahedral-type sites, while in the β phase both octahedral and tetrahedral sites are occupied. The results for VH1.49 and VH1.71 indicate that in the γ phase the hydrogens occupy regular tetrahedral sites corresponding to the fluroine positions in a CaF2-type structure.