Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

Abstract

The structure of Bi_2.34U_0.33La_0.33O₅ has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P, Z = 1, and lattice parameters a_H = 4.01395(8), c_H = 9.5423(2) Å. X-Ray diffraction data are in favour of a model in which U and La atoms are situated in different crystallographic positions: i.e. 1a and 2d of the P space group respectively, forming mixed Bi/U and Bi/La layers which alternate along the c axis. The structure of this bismuth-based mixed oxide can be described as built up of interlocked edge-sharing (Bi/U)O₈ and (Bi/La)O₈ polyhedra. The co-ordination polyhedron around the Bi/U atoms is a hexagonal bipyramid, the equatorial plane being a puckered hexagon. The Bi/La atoms are co-ordinated to eight oxygen atoms forming a slightly distorted cube. Electron diffraction studies showed that the structure can be described as a fluorite-type superstructure, the relation to the fluorite sub-cell is a*_H ≈ ⅓[4]a*_C, b*_H ≈ ⅓[ 22]a*_C, c*_H ≈ ⅓[111]a*_C. This compound is a good oxide ion conductor (σ_300 °C = 2.5 × 10^–5 S cm^–1). On the basis of the anion vacancies distribution determined from neutron diffraction refinement, a pathway for the oxide ion conduction is proposed.