Vibrational spectrum of MgSiO3 perovskite: Zero‐pressure Raman and mid‐infrared spectra to 27 GPa

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Abstract
MgSiO3 in the perovskite structure, thought to be the dominant mineral phase of the earth's mantle, has been studied by both zero‐pressure Raman spectroscopy and by infrared spectroscopy to pressures of 27 GPa. A total of eight bands are observed: four in the Raman spectrum between 250 and 500 cm−1, and four in the mid‐infrared between 500 and 800 cm−1. These bands are divisible into three separate groupings: high frequency (∼800 cm−1), caused by asymmetric silicon‐oxygen stretching motions; intermediate frequency (370–690 cm−1), modes predominantly due to octahedral stretching and bending vibrations; and low frequency (200–300 cm−1), involving Mg2+ motion. The pressure shift of the four mid‐infrared modes yields an average mode Grüneisen parameter of 1.36 ± 0.15. Systematic trends observed between mode Grüneisen parameters and mode frequency are used to derive an estimate of the thermodynamic Grüneisen parameter from the spectroscopic data. In this way, we derive an estimated value of ∼1.9, in good agreement with the thermodynamic value of 1.77 measured at zero pressure.