Anharmonicity in the Silver and Thallium Halides: Far-Infrared Dielectric Response

Abstract

The isobaric temperature dependence of the $q\ensuremath{\approx}0$ transverse- and longitudinal-optic modes of AgCl, AgBr, TlCl, and TlBr is reported in the range 4-450 K and the isothermal pressure dependence of the transverse-optic modes for pressures up to 10 kbar. These measurements are used to determine the thermal-strain and anharmonic contributions to the self-energy for the $q\ensuremath{\approx}0$ transverse-optic mode for these materials. The results reveal a near cancellation of the cubic and quartic anharmonic contributions to the self-energy for AgCl and AgBr with the cubic contribution dominating at all temperatures. In the thallium halides the quartic contributions are dominant at all temperatures and are sufficiently large as to dominate the thermal-strain contribution to the self-energy, thus causing the mode to be weakly soft.