High-temperature structural phase transitions in perovskite (CaTiO₃)

High-temperature powder x-ray diffraction data are presented for CaTiO₃ perovskite between 293 and 1523 K. The temperature-dependence of superlattice intensities and cell parameters suggests a sequence of phase transitions from the room temperature orthorhombic (Pbnm) structure to a tetragonal (I4/mcm) polymorph at temperatures in the range 1373-1423 K, followed by transformation to the cubic (Pm3m) aristotype at 1523 ± 10 K. The intensity of the diffuse background increases on transformation to the cubic structure and is associated with disorder (and anionic mobility) of the oxygen sub-lattice. The I4/mcm-Pbnm transition induces a large spontaneous strain, but the tetragonal spontaneous strain in the I4/mcm phase due to the Pm3m-I4/mcm transition is small, below the resolution of this experiment. These results add weight to suggestions from recent computer simulations that orthorhombic MgSiO₃ may transform to a tetragonal (rather than a cubic) polymorph under the conditions of the Earth's mantle, in which case the effects on electrical conductivity would not be expected to be as great as for a transition to a cubic polymorph, although the consequences for elastic properties may be more significant.