Zero and First Sound in NaF and RbI

Zero and first sound as well as the transition region are investigated theoretically in RbI and NaF. The temperature dependence of the first sound, zero sound, and isothermal elastic constants is calculated on the basis of a breathing shell model for the harmonic lattice dynamics and anharmonic parameters from microscopic Grüneisen constants. The theoretical results are compared with ultrasonic data and inelastic neutron scattering results. Own recent neutron experiments in RbI are concerned with the zero sound region at room temperature while the NaF experiments cover also parts of the transition region between first and zero sound at various temperatures between 295 and 700 K. It is shown that the dispersion of sound is satisfactorily described by use of an averaged thermal phonon lifetime. In NaF for each temperature first sound elastic constants and averaged phonon lifetimes are deduced from an analysis of the experimental data.