X-ray Diffraction and Raman Spectroscopy of SrGeO3 at High Pressure

Strontium germanate, SrGeO₃, is a structural analog of calcium silicate perovskite (CaSiO₃) and is useful for understanding its possible structural evolution with increasing pressure, since SrGeO₃ will tend to undergo phase transitions at lower pressures, where the phenomena are more experimentally accessible. In an effort to discover what distortion pathway cubic CaSiO₃ perovskite might take at high pressure, we investigated the high-pressure behavior of SrGeO₃ with both synchrotron x-ray diffraction and Raman spectroscopic techniques. A phase transition in SrGeO₃ from a hexagonal pseudowollastonite structure to the cubic perovskite phase was observed at 12 GPa and 300 K, allowing a constraint of the phase boundary between the two phases. Further increases in pressure up to 75 GPa at ambient temperature revealed no distortion of the cubic perovskite phase. This suggests marked differences in the crystal chemistry between SrGeO₃ and other perovskite analogs such as SrTiO₃, which transforms into the tetragonally-distorted perovskite phase at a much lower pressure. Semi-empirical molecular orbital calculations indicate that these differences result from the increased covalent nature of the Ge-O-Ge bond as compared to the Ti-O-Ti bond as a result of strong pπ -dπ bonding. This causes chain-linked GeO₆ octahedra to resist the tilting or twisting motions necessary for a displacive phase transition to the tetragonal or orthorhombic phase, and implies that SrGeO₃ may be a better analog for CaSiO₃ perovskite than more ionic analogs that do not allow efficient pπ -dπ overlap.