Abstract
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 $0.73(5)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$ $(T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. 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.
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