The crystal structure of BaBi4Ti4O15

The room temperature structure of BaBi4Ti4O15 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 Angstrom) from 2degrees to 138degrees in 2theta. 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. BaBi4Ti4O15 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.