Raman and infrared studies of the CN- stretching-mode anharmonicity and its relation to phase transitions in pure alkali cyanides.

Abstract

A comprehensive ir and Raman study of the fundamental and overtone stretching modes of ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules in pure alkali cyanides has been performed. Peak position, half-width, and strength of these modes have been measured\char22{}as far as is accessible with both techniques\char22{}between 4 and 300 K through the phases of different order in pure NaCN, KCN, RbCN, and CsCN. From the main data, we first derive a general anharmonic-oscillator model for a single ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ in different hosts, determining anharmonicity constants, dissociation energy, polarizability derivatives, etc., and comparing them to calculated values and models. More detailed changes observed in frequency and width and strength of the transitions under temperature variation can be related to elastic deformations and changes in ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ rotational freedom and interaction effects in the different phases. In particular, a sideband with characteristic temperature dependence of splitting and strength observed in KCN and NaCN reveals the gradual buildup of internal electric field and antiferroelectric order by its pronounced effect on the ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ oscillator. The absence of this effect in RbCN and CsCN confirms the absence of electric order in these materials.