Tetrahedral and Octahedral Site Occupancy and Optic Mode Vibrations Mode Vibrations of Hydrogen in Yttrium
The yttrium-hydrogen system exhibits both tetrahedral and octahedral site occupancy prior to reaching the dihydride concentration. Using the WNR pulsed neutron source at the Los Alamos National Laboratory, temperature dependence studies from 15K to 275K of the optic modes of YH1.99 were made by inelastic incoherent neutron scattering. Room temperature neutron powder diffraction patterns were also taken. The powder data yielded a cell parameter for YH 1.99 of 5.2021 ± 0.0004A and the occupation numbers of tetrahedral and octahedral sites were 1.95 ± 0.08 and 0.05 ± 0.02, respectively. The inelastic scattering data clearly showed the vibrational components due to tetrahedral and octahedral protons and a two peaked structure for the fundamental tetrahedral vibration. Several higher harmonics of the tetrahedral peak and one harmonic of the octahedral were evident. The observed energies are 117 and 127 meV for the tetrahedral site split fundamental and 81 meV for the octahedral site. The higher harmonics were a few percent lower than the expected harmonic value. The occupation fractions obtained from the elastic and inelastic scattering were compared to a model which predicts occupation factors as a function of temperature.