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DOI: 10.1039/A806343E (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 1137-1142

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

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Jose Manuel Amarilla, Jose Antonio Alonso and Rosa M. Rojas

Abstract

The structure of Bi2.34U0.33La0.33O5 has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P[3 with combining macron], Z = 1, and lattice parameters aH = 4.01395(8), cH = 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[3 with combining macron] 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[[2 with combining macron][2 with combining macron]4]a*C, b*H ≈ ⅓[ [4 with combining macron]22]a*C, c*H ≈ ⅓[111]a*C. This compound is a good oxide ion conductor (σ300 °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.

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