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 Aurivillius-phase ferroelectrics Bi5Ti3FeO15 (at 25degreesC) and SrBi4Ti4O15 (at a series of temperatures up to 800degreesC). At 25degreesC both materials adopt the polar orthorhombic space group A2(1)am, in common with two-layer analogues such as SrBi2Ta2O9. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650degreesC and above, with T(c)similar to550degreesC) 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>650degreesC. 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. (C) 2002 Elsevier Science (USA).