The effect of pressure on optical phonon frequencies was calculated using the frozen-phonon approach based on a pseudopotential method. Grüneisen parameters of the harmonic mode frequencies were reported for the high-frequency zone-center E2g and B1g and the zone-boundary E2u and B2u modes at A. Anharmonic effects of phonon frequencies and the implications of the calculated phonon frequency shifts for the pressure dependence of the superconducting transition temperature were considered. Also reported were Raman and optical reflectance spectra measured at high pressures. The experimental observations, combined with calculated results, indicated that the broad spectral features observed in the Raman spectra at 500 to 900/cm could not be attributed to first-order scattering by zone-center modes, but partly originated from a chemical species other than MgB2 at the sample surface and partly from a maximum in the phonon density of states. Low-temperature Raman spectra, at ambient pressure, exhibited increased scattering intensity in the region below 300/cm.

MgB2 Under Pressure - Phonon Calculations, Raman Spectroscopy and Optical Reflectance. K.Kunc, I.Loa, K.Syassen, R.K.Kremer, K.Ahn: Journal of Physics - Condensed Matter, 2001, 13, 9945-62