Structure determination at room temperature and phase transition studies above T<SUB>c</SUB> in ABi<SUB>4</SUB>Ti<SUB>4</SUB>O<SUB>15</SUB> (A = Ba, Sr or Pb)

The room temperature structure of three compounds belonging to the Aurivillius family (n = 4), ABi4Ti4O15 (A = Ba, Sr or Pb) has been analysed. BaBi4Ti4O15 crystallizes in a tetragonal I4/mmm space group whereas SrBi4Ti4O15 and PbBi4Ti4O15 crystallize in the orthorhombic space group A21am. The starting model for the Sr and Pb analogues was derived from ab initio methods and refined using the Rietveld method. The cations Ba and Sr are disordered over the Bi sites while the Pb cation is found exclusively in the [Bi2O2]2+ layers. The TiO6 octahedra are tilted with the Ti-O bonds forming zigzag chains along the 'c' axis. The displacement of Bi atoms along the 'a' axis might be responsible for ferroelectricity in these compounds. The high temperature X-ray data above Tc indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure