Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that $d{T}_c/dP<0$, 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, $d{T}_c/dP>0$.

• Received 29 April 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.025503