NEUTRON-SCATTERING STUDIES OF ACOUSTIC PHONON MODES IN PBF2 UP TO HIGH-TEMPERATURES

The results of recent neutron scattering measurements of the energies and line-widths of long-wavelength acoustic-phonon modes in PbF2 at temperatures between 10 and 898K are described, and then interpreted in terms of theoretical expressions for the effects of anharmonicity, the hopping motions of the anions, and the time-averaged defect structure. The neutron data show that the elastic constants decrease linearly with temperature as Tc is approached from below, but decrease more rapidly in the vicinity of T c. This is particularly marked for C11 which halves in value between 600K and 800K. At 570K and above, the linewidths of transverse acoustic phonons increase as the square of the phonon wavevector, approximately independently of direction, implying that the measurements are in the hydrodynamic regime.