Search

Samples of highly conducting Bi12.5Lu1.5ReO24.5 and Bi12.5Er1.5ReO24.5 have been studied by neutron powder diffraction at room temperature for both phases and at 25°C≤T≤500°C in the case of Bi12.5Er1.5ReO24.5. Both materials crystallize in the cubic δ-Bi2O3 related system, space group Fm-3m. Changes in the oxygen sublattice at 25°C≤T≤500°C have been investigated by the Rietveld and maximum entropy methods. [ABSTRACT FROM AUTHOR]

Abstract: The crystal structure of fluorite-related Bi3ReO8 containing isolated ReO4 tetrahedra has been studied by powder X-ray and neutron diffraction at elevated temperatures. The phase is cubic and undergoes a phase transition between T ~150°C and 250°C passing from space group P213 at 25°C to I213 at higher temperature. The material is an oxide ion conductor and presents a sharp increase in conductivity at T ~450°C, with σ ~8×10−3 Scm−1 above 500°C. Two different long range oxide ion diffusion mechanisms involving vacancy and interstitial defects are proposed. Rotation of isolated ReO4 tetrahedra seem to facilitate the oxide ion conduction. [Copyright &y& Elsevier]

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]