Effects of isotopic composition on the lattice dynamics of CuCl

We have investigated the changes in the Raman spectra of CuCl resulting from isotope substitution. Our samples were made from the stable isotopes ${}^{63}\mathrm{Cu}$, ${}^{65}\mathrm{Cu}$, ${}^{35}\mathrm{Cl}$, and ${}^{37}\mathrm{Cl}$ as well as the natural elements. While the longitudinal-optic (LO) mode shifts according to the expected reduced mass behavior of a $\Gamma$-point phonon, the transverse-optic (TO) vibration shows pronounced deviations. Due to the strong anharmonicity of CuCl, the TO mode interacts with a two-phonon combination band of acoustic phonons and one obtains coupled excitations which have been attributed to a Fermi resonance. We describe this process using a shell model for the lattice dynamics of CuCl and only one cubic coefficient for the anharmonic interaction. The Fermi resonance results in an anomalous line shape of the TO phonon, which reflects properties of the zone center mode as well as of the two-phonon combination band. Isotope substitution allows us to discern the two contributions and to check the validity of the model. The off-center model proposed earlier to explain the TO anomaly cannot account for our findings.