Stability and equation of state of MgGeO3 post-perovskite phase

A phase transition of MgGeO3 perovskite was examined at high-pressure and -temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIrO3-type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGeO3 is a low-pressure analogue toMgSiO(3), for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGeO3 post-perovskite phase obtained at 300 K during decompression from 79 to 6 GPa show that the b-axis is significantly more compressible than are the a- and c-axes, which could be due to the GeO6-octahedral sheet stacking structure along b. The bulk modulus was determined to be K-0 192(+/-5) GPa with a fixed pressure derivative of the bulk modulus, K-1, of 4.