Influence of Lattice Anharmonicity on the Longitudinal Optic Modes of Cubic Ionic Solids

Abstract

The determination of the $k\approx 0$ longitudinal-optic-mode frequencies and lifetimes from the dielectric [$\epsilon \left(\omega \right)$] and inverse dielectric [$\eta \left(\omega \right)$] response functions of simple cubic solids is discussed. Experimental results of the temperature dependence, over the range 5-400°K, of the $k\approx 0$ LO frequencies and lifetimes of 18 alkali and thallium halides are given, as determined from Kramers-Kronig analyses of near-normal-incidence single-crystal reflectance data and from small-grazing-angle reflectance data from thin films on conducting substrates. In addition, the pressure dependence up to 5 kbars at 290°K of the LO frequencies of RbI, CsBr, and CsI is reported. At 5°K the Lyddane-Sachs-Teller relation holds within ±3% for all the salts, and even at 290°K the relation is obeyed within ±10%. The temperature and pressure results are used to determine the self-energies and Grüneisen constants for the LO modes. At room temperature, the self-energy contributions are positive for those salts crystallizing in the NaCl structure and negative for those salts taking the CsCl structure. This is interpreted in terms of competing three-phonon and four-phonon decay and scattering processes.