Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi{sub 4}Ti{sub 4}O{sub 15} (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi{sub 4}Ti{sub 4}O{sub 15} (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2{sub 1}am. This orthorhombic structure is a consequence of a combination of rotation of the TiO{sub 6}, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi{sub 2}O{sub 2} layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca