AN EVALUATION OF ANHARMONICITY IN THE EXTERNAL AND INTERNAL MODES OF IONIC CRYSTALS

Abstract

Recent advances in the methodology of measuring infrared transmission and reflection data coupled with similar advances in dispersion analysis techniques has offered an opportunity to evaluate quantitatively the nature, extent and degree of anharmonicity in the lattice modes of simple ionic crystals, such as LiF, and of the lattice and internal mode of a series of alkali halogenates. This evaluation indicates the following. (1) The anharmonicity in the main resonance band of LiF is primarily mechanical in nature, while the subsidiary maximum appears to exhibit a significant degree of electrical anharmonicity. (2) There is a slight degree of anharmonicity in some of the internal modes of a polyatomic ionic substance, such as $NaC10_{3}$. (3) The external modes of alkali halogenates are essentially harmonic, even though the lifetimes of the excited states for these modes are considerably shorter than those for LiF. (4) The short lifetimes for alkali halogenate lattice modes are rationalizable in terms of internal-external mode coupling, and this has been verified qualitatively by direct experimental evidence. This research was supported by the National Science Foundation.