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Abstract: The two crystallographically non-equivalent Co atoms of the quasi-one-dimensional crystal structure of Ca3Co2O6 form chains with alternating, face-sharing polyhedra of Co2O6 trigonal prisms and Co1O6 octahedra. This compound forms a substitutional solid-solution phase with Sc, in which the Sc atoms enter the Co2 sublattice exclusively. The homogeneity range of Ca3Co2− v Sc v O6 (more specifically Ca3Co1Co21− v Sc v O6) extends up to v≈0.55. The crystal structure belongs to space group R3¯c with lattice parameters (in hexagonal setting): 9.0846(3)⩽a⩽9.1300(2)Å and 10.3885(4)⩽c⩽10.4677(4)Å. The magnetic moment decreases rapidly with increasing amount of the non-magnetic Sc solute in the lattice. [Copyright &y& Elsevier]

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase 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. [ABSTRACT FROM AUTHOR]