Temperature and pressure dependence of the transverse-optic modes of ionic crystals and their associated anharmonic self-energies

The temperature dependence (2-300 K) and hydrostatic pressure dependence (0-7 kbar) of the $q\approx 0$ transverse-optic mode frequency ${\omega }_t$ and its associated damping $2\Gamma (0,t,{\omega }_t)$ are reported for the alkali, silver, and thallium halides. These results are analyzed to yield the temperature dependence of the thermal strain ${\Delta }^E(0,t)$ and far-infrared anharmonic self-energies ${\Delta }_T^A(0,t,{\omega }_t)$ associated with the mode at ${\omega }_t$. In addition, calculations of ${\Delta }^E(0,t)$, ${\Delta }_T^A(0,t,\Omega )$ at $\Omega =0\mathrm{}$ and $\Omega ={\omega }_t$, and $\Gamma (0,t,{\omega }_t)$ are presented for the alkali halides crystallizing in the NaCl structure using current many-body thermodynamic Green's-function anharmonic theories, and these are compared with the experimental results.