Raman spectroscopy of perovskite and post-perovskite phases of MgGeO₃ to 123 GPa

Raman spectra of the perovskite (Pv) and post-perovskite (PPv) phases of MgGeO₃ were measured in the laser-heated diamond cell up to 123 GPa. Between 14 and 51 GPa we observed a total of seven Pv modes, the frequencies and relative intensities of which are similar to MgSiO₃-Pv with systematically lower frequencies. We also observe a total of ten PPv modes which are much more intense than Pv modes, indicating strong polarizability of the bonds in PPv. The observed Raman mode frequency range of PPv extends to much higher frequencies than Pv, which can be related to the edge sharing of the GeO₆ octahedra in PPv which would make Ge-O bonds stronger. The spectroscopic Grüneisen parameters of Pv and PPv are constrained to be 1.56 ± 0.10 and 1.15 ± 0.06, respectively, at the PPv transition. We also estimated that the thermal expansion parameter decreases by 25 ± 10% across the PPv transition. If a similar change occurs in MgSiO₃, the low thermal expansivity would dynamically stabilize the PPv layer at the lowermost mantle. Using our Raman measurements and a Kieffer-type model, the Clapeyron slope of the PPv transition is constrained to be + 20 ± 10 MPa/K. The strong positive slope is consistent with the prediction for MgSiO₃ and is related to a positive shift of phonon density of states at high-frequency region resulting from the formation of the edge sharing among the GeO₆ (or SiO₆) octahedra in PPv.