Lattice Dynamics of Silver Chloride

Abstract

The dispersion relation for the normal modes of vibration in AgCl at 78°K has been determined from neutron coherent inelastic scattering data obtained on a triple-axis neutron spectrometer at the Oak Ridge High Flux Isotope Reactor. Some difficulty in obtaining a satisfactory fit to the results with a simple shell model was encountered. Only a fair fit to the data is obtained with an 11-parameter model, which includes next-nearest-neighbor interactions for both types of ions, a variable ionic charge, and the electrical and mechanical polarizabilities for both ions. A 13-parameter model, while providing an improved fit to the data, gives values for several of the parameters that have no obvious physical significance. The neutron data are in reasonably good agreement with the results of low-temperature measurements of the elastic constants and the infrared absorption. A frequency-distribution function has been computed, together with the lattice heat capacity and the mean-square displacements for each ion. A combined density-of-states function has also been obtained for comparison with the relevant optical data.