General rules for predicting phase transitions in perovskites due to octahedral tilting

Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that dT(c)/dP < 0, contrary to a general rule previously proposed for such zone-boundary transitions. We show that the negative slope of the phase boundary is a consequence of the octahedra in these perovskites being more compressible than the extra-framework cation sites. Conversely, when the octahedra are stiffer than the extra-framework cation sites, the phase transition temperatures increase with increasing pressure, dT(c)/dP > 0.