Dispersive phonon linewidths: the E2 phonons of ZnO.

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.