Raman-scattering investigation of anharmonicity in potassium halides. Experiment and theory

Raman spectra of ${\mathrm{Tl}}^{+}$-doped KCl, KBr, and KI have been measured as functions of hydrostatic pressure up to 7 kbar at 30, 85, and 300 K, and as functions of temperature at zero pressure. The data were analyzed using pure crystal $p-V$, bulk modulus, and thermal-expansion data. Calculations of the spectra have been performed using unperturbed or very weakly perturbed pure-crystal phonons and host bulk anharmonicity. Calculated shifts of prominent features in the impurity-induced $E_g$ and $T_{2g}$ spectra are found to be in good agreement with experiment. In all cases, the peak shifts can be accounted for by considering the volume dependence of phonon frequencies. The net contribution to peak shifts from multiphonon effects appears to be negligible.