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The crystal structure of BaBi<inf>4</inf>Ti<inf>4</inf>O<inf>15</inf>

Abstract:

The room temperature structure of BaBi 4 Ti 4 O 15 is determined by means of high resolution synchrotron x-ray powder diffraction. At Stanford Synchrotron Radiation Lab., the sample was mounted on a zero background holder and data was collected in reflection geometry at 10 KeV (1.24 ) from 2 to 138 in 2. Rietveld refinement was performed with programs FULLPROF and GSAS. Starting space group followed Aurivillius' choice 14/mmm. This tetragonal symmetry group does not explain observed peaks splitting, so orthorhombic symmetry was selected. Symmetry F2mm was considered, for it is consistent with observed electric dipole moment in the elementary cell. The selected space group allowed stable and accurate convergence of the refinement. Treatment of the experimental data included diffuse scattering characterization. Structured background fitting gave place to the derivation of the radial distribution function, as a measurement of short range segregation. BaBi 4 Ti 4 O 15 is orthorhombic, polar and with partial short-range ordering of the Ba and Bi ions. Variations of atomic coordinates, relative to those corresponding to the tetragonal paraelectric condition, give place to the appearance of spontaneous electric polarisation. © 2002 Taylor and Francis.