Brillouin scattering studies of LaF3at high temperatures

Brillouin scattering studies of the fast-ion conductor La${\mathrm{F}}_3$ have been carried out in the temperature range 300-1400 K. The frequency shifts of the inelastically scattered light corresponding to phonon modes of longitudinal, transverse, and mixed character in selected symmetry directions show a linear decrease with temperature at lower temperatures followed by a more marked decrease above about 1100 K. The results are discussed in terms of anharmonic effects and the cooperative generation of disorder at high temperatures, and are compared with previous determinations of the high-temperature heat capacity.