Lattice Sidebands of Vibrational Spectra and Their Pressure Dependence

Vibrational spectra of solids which show the general form ${\nu }_{\mathrm{int}}\pm {\nu }_{\mathrm{ext}}$ are discussed, and the information that they contain about the lattice modes of the solid is considered. Small polyatomic impurity ions isolated in alkali halides are responsible for most of the spectra considered. The importance of torsional motion of the impurity ion is shown, and a directional selection of ${\nu }_{\mathrm{ext}}$ controlled by the ${\nu }_{\mathrm{int}}$ transition moment is suggested. The pressure dependence of some of these spectra, up to 50 kbar at 100°K, is presented and discussed in terms of the host-lattice dispersion curves. For instance, it is shown that there is no acoustic-to-optic energy gap in the high-pressure CsCl-type structures of KBr, KI, RbBr, and RbI. Some of the wider applications of this type of spectrum are also indicated.