Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivilliusphase ferroelectrics Bi5Ti3FeO15 (at 25°C) and SrBi4Ti4O15 (at a series of temperatures up to 800°C). At 25°C both materials adopt the polar orthorhombic space group A21am, in common with two-layer analogues such as SrBi2Ta2O9. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650°C and above, with Tc~550°C) SrBi4Ti4O15 transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 550>650°C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO6 octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi4Ti4O15, which parallels that found in SrBi2Ta2O9, are discussed.