Structural study of barium titanate between 150 and 425 K

Abstract

Abstract The lattice parameters and structures of the four phases of pure barium titanate have been determined at 5 K intervals between 150 and 425 K. The experiments were performed on the high-resolution powder diffractometer at the neutron spallation source, ISIS facility, Rutherford Appleton Laboratory. Using the fact that strain couples to the square of the spontaneous polarisation, we calculated lattice parameters from the 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 predicts transition temperatures between the rhombohedral, orthorhombic and tetragonal phases which are very different from the experimentally determined ones. The structures were refined using the Rietveld technique. The only distortion of the octahedron is through electrostrictive coupling to the polarisation. U...