Raman study of phonon anharmonicity inLaF3

The first-order Raman spectrum of ${\mathrm{LaF}}_3$ was studied as a function of hydrostatic pressure (0–10 GPa) and temperature (60–900 K). The calculated values for the mode Grüneisen parameters are between 1 and 2, in reasonable agreement with those derived from uniaxial-stress experiments. Deviations from the isotropic approximation, in handling the temperature dependence of the long-wavelength optical-phonon frequencies, are found to be smaller than 15%. Anharmonicity contributions to the phonon frequencies and half-widths are calculated from the joint data. The results indicate that for most of the phonons it is the volume contributions that are mainly responsible for the temperature-induced frequency shifts. Anharmonicity appears to be important for fewer phonons and also for the increase of linewidth with temperature for all phonons studied. Other observations are also discussed, such as the pressure dependence of the phonon linewidths and the frequency dependence of the frequency-versus-pressure slopes.