Localized Vibrations of H in Nb and Ta at High Concentration and Low Temperatures
Extensive measurements have been performed of the optic mode frequencies in the hydrides of Nb and Ta at concentrations between 70% and 95% using inelastic incoherent neutron scattering techniques at the WNR pulsed neutron source of the Los Alamos National Laboratory. The aim of this study was to gain new information on anharmonicities of the H potential as well as to relate observations of broading or splitting of the local mode peaks to details in the high concentration, low temperature portion of the NbHx phase diagram. The higher harmonic of the singlet excitation was observed in all cases at energies about 3–5% less than the harmonic value. For some cases a simultaneous excitation of the singlet and doublet is apparent, which provides additional information on the hydrogen potential when these vibrations are coupled by anharmonic interactions. A previously reported1 splitting in the optic mode peaks of NbH0.87, which occurs at temperatures below about 210K, as well as broadening of the optic mode peaks of NbH0.78 and NbD0.8 can be discussed using new information on the NbHX phase diagram in terms of coexisting phases with differing hydrogen sites.