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

Abstract

The determination of the $k\ensuremath{\approx}0$ longitudinal-optic-mode frequencies and lifetimes from the dielectric [$\ensuremath{\epsilon}(\ensuremath{\omega})$] and inverse dielectric [$\ensuremath{\eta}(\ensuremath{\omega})$] response functions of simple cubic solids is discussed. Experimental results of the temperature dependence, over the range 5-400\ifmmode^\circ\else\textdegree\fi{}K, of the $k\ensuremath{\approx}0$ 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\ifmmode^\circ\else\textdegree\fi{}K of the LO frequencies of RbI, CsBr, and CsI is reported. At 5\ifmmode^\circ\else\textdegree\fi{}K the Lyddane-Sachs-Teller relation holds within \ifmmode\pm\else\textpm\fi{}3% for all the salts, and even at 290\ifmmode^\circ\else\textdegree\fi{}K the relation is obeyed within \ifmmode\pm\else\textpm\fi{}10%. The temperature and pressure results are used to determine the self-energies and Gr\"uneisen 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.