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.

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Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two-phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E high 2 phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two-phonon DOS provide an explanation for this variation of the FWHM: the E high 2 frequency falls on a sharp ridge of the 2-DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E high 2 frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discussed for the E low 2 phonons. Applications of the isotope and pressure techniques to the elucidation of two-phonon spectra will be presented.Introduction Since the pioneering work of Weinstein for GaP [1] it is known that the widths and lineshapes of phonons can, in some cases, be strongly modified by application of pressure. The diamond anvil cell is an excellent tool for such investigations [2] which yield detailed information about the anharmonic decay mechanisms responsible for linewidths and lineshapes. The reason for the drastic lineshape changes observed by Raman spectroscopy in CuCl [3], CuBr [4], GaP [5] and, to a lesser extent Si [6] and CuI [7] is that, in these cases, the anharmonic self-energy is determined by decay into two phonons close to Van Hove singularities at the edge of the Brillouin zone. The fact that the corresponding two-phonon density of states (DOS) has strong singularities near Raman LO (CuBr) or TO (CuCl, GaP) phonons, together with strong anharmonic coupling constants, gives rise to complex lineshapes which are determined not only by the imaginary part but also by the real part of the selfenergy. Similar effects have been observed for the Raman LO phonons in CuBr. In these examples the application of hydrostatic pressure leads to a ''hardening" of the optical modes, which then shift to higher frequencies, and a ''softening" of the transverse acoustic (TA) ones.