Stability and equation of state of MgGe[O.sub.3] post-perovskite phase

A phase transition of MgGe[O.sub.3] perovskite was examined at high-pressure and--temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIr[O.sub.3]type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGe[O.sub.3] is a low-pressure analogue to MgSi[O.sub.3], for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGe[O.sub.3] 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 Ge[O.sub.6]-octahedral sheet stacking structure along b. The bulk modulus was determined to be [K.sub.0] = 192([+ or -]5) GPa with a fixed pressure derivative of the bulk modulus, K' of 4.