XAFS and XRD Study of the Atomic Displacements in Aurivillius Phase Ferroelectric Bi2.25Ca0.5Na0.25Nb2O9

The novel layered perovskite-like oxide with Aurivillius phase structure Bi2.25Ca0.5Na0.25Nb2O9 has been synthesized by solid-state reaction method. This compound is a ferroelectric with the Curie temperature 972 K. The crystal structure of Bi2.25Ca0.5Na0.25Nb2O9 has been determined from powder diffraction data recorded at room temperature. The parameters of the orthorhombic cell (space group A21am) are: a=5.4845 A, b=5.4549 A, c=24.9195 A. Using the profile of the X-ray diffraction pattern the atomic coordinates have been refined by Rietveld method and the Nb ion position in the oxygen octahedra has been found. It was shown that the Nb ion is displaced from the center of the oxygen octahedra by ∼0.15 A. Nb K-edge EXAFS spectra were measured over the temperature range 295 K - 960 K. The interatomic distances Nb-O and the mean-square relative displacements σ2 have been determined. The interatomic distances Nb-O at room temperature obtained from XAFS analysis are in a good agreement with those found from XRD data. It was revealed that the temperature increase results in the decreasing of Nb ion displacements from the center of the oxygen octahedra. However, in the vicinity of the ferroelectric phase transition the displacement of Nb ion was preserved