Raman scattering in osmium under pressure

The effect of pressure and temperature on the Raman-active phonon mode of osmium metal has been investigated for pressures up to 20 GPa and temperatures in the range 10-300 K. Under hydrostatic conditions (He pressure medium) the phonon frequency increases at a rate of 0.73(5) cm(-1)/GPa (T=300 K). A large temperature-induced and wavelength-dependent frequency shift of the phonon frequency is observed, of which only a small fraction can be associated with the thermal volume change. The main contribution to the temperature dependence of the phonon frequency is rather attributed to nonadiabatic effects in the electron-phonon interaction, which explains also the observation of an increasing phonon linewidth upon cooling. The phonon linewidth and the pressure-induced frequency shift were found to be unusually sensitive to shear stress.