Phonon polariton study ofCuCl1−xBrx

The optical-phonon spectrum of $\mathrm{Cu}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ at low temperature is very unusual in regard to many of its properties. Despite the apparent two-mode behavior our measurements illustrate anomalies in the Raman intensity and temperature-induced frequency shift, as well as in the oscillator strengths yielded by the polariton measurements. All these irregularities are connected to the anomalies of CuCl and they disappear rapidly with increasing Br concentration. The data are interpreted in terms of a previous model, which allows copper ions to occupy off-center sites. It is assumed that in $\mathrm{Cu}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ this may happen only in the tetrahedra where ${\mathrm{Cu}}^{+}$ is surrounded by chlorine ions alone.