STRUCTURES OF THE FERROELECTRIC PHASES OF BARIUM-TITANATE

The structures of the three ferroelectric phases of BaTiO3 have been determined by Rietveld refinement using powder diffraction data collected at a spallation neutron source. The correlation between refined atomic displacements and thermal parameters, which has hampered previous structure determinations, has been partially alleviated by using data which extend over a wide range of d spacings. Data collected at a large number of sample temperatures provide information about the temperature dependence of the ferroelectric displacements and changes in the oxygen octahedra which surround the Ti ions. The temperature dependence of the thermal parameters gives atomic Debye-Waller temperatures that are remarkably similar to those for the cations in the high-T(c) superconductors. Our results are insensitive to predictions of a soft-mode displacive model; however, values of the anisotropic thermal parameters do not support the order-disorder model suggested by Comes et al. Powder extinction and profile coefficients from the structural refinements show pronounced minima and maxima, respectively, near the phase transitions and provide information about the temperature dependence of the mosaic structure and the strain in the different phases.