Structural phase transition and octahedral tilting in the calcium titanate perovskite CaTiO 3

Abstract

Crystal structure of the calcium titanate perovskite CaTiO 3 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 Pm3̄ m transition was reversibly observed at 1635 ± 2 K. The structural phase transitions are characterized by the tilt of the TiO 6 octahedron. The tilt systems of the Pbnm, I4/ mcm, and Pm3̄ m CaTiO 3 are ϕ x − ϕ y − ϕ z + ( ϕ x − = ϕ y −), ϕ x 0 ϕ y 0 ϕ z −, and ϕ x 0 ϕ y 0 ϕ z 0, respectively. Here the ϕ x −, ϕ z +, and ϕ x 0 stand for the out-of-phase tilt angle of TiO 6 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, ϕ x −, ϕ z +, and ϕ z − decreased with increasing temperature where the critical exponents were about 0.25. The ϕ x − and ϕ y − of the orthorhombic structure decreased discontinuously to ϕ z − 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 ϕ z − continuously decreased with increasing temperature and became 0° at the I4/ mcm- Pm3̄ m transition temperature, indicating a continuous nature of the transformation. The Pbnm- I4/ mcm and I4/ mcm- Pm3̄ m phase transitions are induced by the tilting of TiO 6 octahedra, indicating the displacive nature of the transitions. The larger thermal motion of O2 atom in I4/ mcm CaTiO 3 indicates the larger positional disorder of O2 atom, which is consistent with the tilt system ϕ x 0 ϕ y 0 ϕ z −.