Structure determination at room temperature and phase transition studies above T_c in ABi₄Ti₄O₁₅ (A = Ba, Sr or Pb)

Abstract

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 T_c indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure.