Cation Disorder in the Ferroelectric Oxides ABi2Ta2O9, A=Ca, Sr, Ba

The crystal structures of the layered ferroelectric oxides ABi2Ta2O9 (A=Ca, Sr, and Ba) have been refined using multiple-wavelength synchrotron powder diffraction data. At room temperature CaBi2Ta2O9 and SrBi2Ta2O9 have an orthorhombic structure with space group A21am. Bulk samples of the two oxides exhibit small, but significant amounts of disorder between the A-type and Bi atoms. BaBi2Ta2O9 has a tetragonal structure, space group I4/mmm. Again significant amounts of cation disorder are detected