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The effect of pressure on optical phonon frequencies of MgB$_2$ has beencalculated using the frozen-phonon approach based on a pseudopotential method.Gr\"uneisen parameters of the harmonic mode frequencies are reported for thehigh-frequency zone-center $E_{2g}$ and $B_{1g}$ and the zone-boundary $E_{2u}$and $B_{2u}$ modes at $A$. Anharmonic effects of phonon frequencies and theimplications of the calculated phonon frequency shifts for the pressuredependence of the superconducting transition temperature of MgB$_2$ arediscussed. Also reported are Raman and optical reflectance spectra of MgB$_2$measured at high pressures. The experimental observations in combination withcalculated results indicate that broad spectral features we observed in theRaman spectra at frequencies between 500 and 900 cm$^{-1}$ cannot be attributedto first-order scattering by zone-center modes, but originate in part from achemical species other than MgB$_2$ at the sample surface and in part from amaximum in the MgB$_2$ phonon density of states. Low-temperature Raman spectrataken at ambient pressure showed increased scattering intensity in the regionbelow 300 cm$^{-1}$.Comment: 18 pages, 8 figures (EPS), revte

MgB2under pressure: phonon calculations, Raman spectroscopy, and optical reflectance

MgB₂under pressure: phonon calculations, Raman spectroscopy, and optical reflectance