STRUCTURAL STUDY OF BARIUM-TITANATE BETWEEN 150 AND 425 K

The lattice parameters and structures of the four phases of pure bariu m titanate have been determined at 5 K intervals between 150 and 425 K . The experiments were performed on the high-resolution powder diffrac tometer at the neutron spallation source, ISIS facility, Rutherford Ap pleton Laboratory. Using the fact that strain couples to the square of the spontaneous polarisation, we calculated lattice parameters from t he magnitude of the spontaneous polarisation determined from the Gibbs free energy, giving excellent agreement with experimental values; the coefficients in the expansion were found by a least-squares technique . However, using these values for the coefficients, the potential pred icts transition temperatures between the rhombohedral, orthorhombic an d tetragonal phases which are very different from the experimentally d etermined ones. The structures were refined using the Rietveld techniq ue. The only distortion of the octahedron is through electrostrictive coupling to the polarisation. Using apparent charges and absolute atom ic displacement, the spontaneous polarisation can be calculated, which is in reasonable agreement with experiment over the whole temperature range. The bond valency of barium shows a small, abrupt change at the cubic-tetragonal transition.