SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY OF THE STRUCTURAL PHASE-TRANSITION IN BA0.95CA0.05TIO3

The tetragonal-to-cubic transition in ferroelectric Ba0.95Ca0.05TiO3 ( BCT) has been studied using a single-crystal neutron diffraction techn ique. The intensities of the Bragg profiles are observed to increase a nomalously with temperature above 110 degrees C are then to drop sharp ly before reaching the structural transition temperature T-0. The inte nsity variations and T-0 are sensitive to sample heating rate with T-0 = 145 degrees C at slow heating rates. Structural refinements indicat e that the observed intensity variations are due to changes in the dom ain structure and the thermal parameters B with the latter becoming an omalously large several degrees Celsius below T-0 compared with those above T-0 and at room temperature, indicating the onset of significant pretransition disorder for Ti ions along the polar c axis. The observ ed linewidths of the diffraction profiles reflect contributions from t he anisotropic domain sizes and the internal strains and an added cont ribution from the coexistence of tetragonal and cubic phases below T-0 . The results suggest that the fluctuations in the Ti positions existi ng well below T-0 could be responsible for the broadening of dielectri c response observed for BCT.