Optical vibrations of hydrogen in metals

Abstract

Neutron energy loss measurements provide a powerful method of studying the modes of vibration of hydrogen in metal lattices. The crosssection for neutron scattering is proportional to the amplitude-weighted frequency distribution. Both the frequency distribution and the amplitudeweighting function can be obtained from the interatomic force constants by the standard methods of lattice dynamics. These calculations have been performed for rare earth dihydrides and it is found that hydrogen-nearesthydrogen forces have to be introduced to obtain agreement with experiment.The extension of this method directly to the case of β-PdH shows considerable differences between the calculated frequency distribution obtained from the dispersion curves for β-PdD measured by Rowe et al., and the measured cross-section over the optical modes. However, the latter is in satisfactory agreement with a recent extension of the lattice dynamical methods by Sköld et al., which allow for the defect nature of the structure.Based on this approach, a number of further measurements have been made to examine the effect of changes in the system. Thus, the measured cross-section has been shown to have a significant temperature dependence which must be due either to changes in the local order or to anharmonic effects. Further, attempts have been made to produce the structural change which has been reported to arise from a 50 K anneal, but no corresponding change in the cross-section was observed. Finally, the effect of alloying palladium with silver and cerium was examined. In the former case, little change is observed whereas, in the latter case, it is evident that the hydrogen-metal force has increased substantially while the hydrogen-second hydrogen force becomes correspondingly smaller.