Structural phase transition and octahedral tilting in the calcium titanate perovskite CaTiO3

Crystal structure of the calcium titanate perovskite CaTiO3 was refined by the Rietveld analysis of neutron diffraction data collected over the temperature range of 296 to 1720 K. The orthorhombic Pbnm-tetragonal I4/mcm phase transformation reversibly occurred at 1512 ± 13 K and the tetragonal I4/mcm-cubic Pm3̄m transition was reversibly observed at 1635 ± 2 K. The structural phase transitions are characterized by the tilt of the TiO6 octahedron. The tilt systems of the Pbnm, I4/mcm, and Pm3̄m CaTiO3 are ϕx−ϕy−ϕz+ (ϕx− = ϕy−), ϕx0ϕy0ϕz−, and ϕx0ϕy0ϕz0, respectively. Here the ϕx−, ϕz+, and ϕx0 stand for the out-of-phase tilt angle of TiO6 octahedron along x axis, the in-phase octahedral tilt angle along z axis, and no octahedral tilting along x axis, respectively. All the tilt angles, ϕx−, ϕz+, and ϕz− decreased with increasing temperature where the critical exponents were about 0.25. The ϕx− and ϕy− of the orthorhombic structure decreased discontinuously to ϕz− of the tetragonal structure or 0° through the Pbnm-I4/mcm transition. The tilt angle, the unit-cell parameters and unit-cell volume discontinuously changed at the Pbnm-I4/mcm transition temperature, indicating the first-order nature. The increase of the cell volume in the Pbnm-I4/mcm transition was 0.088 vol.%. In contrast, the unit-cell volume continuously increased and the tilt angle ϕz− continuously decreased with increasing temperature and became 0° at the I4/mcm-Pm3̄m transition temperature, indicating a continuous nature of the transformation. The Pbnm-I4/mcm and I4/mcm-Pm3̄m phase transitions are induced by the tilting of TiO6 octahedra, indicating the displacive nature of the transitions. The larger thermal motion of O2 atom in I4/mcm CaTiO3 indicates the larger positional disorder of O2 atom, which is consistent with the tilt system ϕx0ϕy0ϕz−.