Sergey V. Krivovichev,*,†,‡ Olivier Mentre,́ Oleg I. Siidra,† Marie Colmont, and Stanislav K. Filatov† †St. Petersburg State University, Department of Crystallography, University Emb. 7/9, 199034 St. Petersburg, Russia ‡Institute of Silicate Chemistry, Russian Academy of Sciences, Makarova Emb. 6, 199034 St. Petersburg, Russia UCCS, Equipe de Chimie du Solide, UMR CNRS 8181, ENSC LilleUST Lille, BP 90108, 59652 Villeneuve d’Ascq Cedex, France

Anion-centered tetrahedra in inorganic compounds.

The crystal structure of the displacivc ferroelectric mineral russellite, Bi2WOc" has bcen determined using Rietveld profile refinement of high-resolution, time of flight, neutron powder diffraction data on the synthetic compound. Russellite is orthorhombic, Pca2!, with a 5.43726(2) A, b 16.43018(5) A, c 5.458422) A, Z = 4 and is isostructural with the bismuth molybdate mineral koechlinite, Bi2MoOc,. The structure consists of layers of tilted WOc, octahedra sandwiched between layers of bismuth and oxygen with the tungsten displaced from the centre of the octahedron by 0.278 A. The orientations of the lone-pair electrons in the Bi"+ cations have been inferred from the 3.0 Acoordination shells of both crystallographically independent bismuths, and have been found to be non-centrosymmetric, an effect which may give rise to the tilting of the WOc, octahedra. New laboratory source X-ray powder diffraction data are presented for russcllite, which, with supplementary synchrotron powder diffractometry, corroborate the new space group and structure determination.

The crystal structure of russellite; a re-determination using neutron powder diffraction of synthetic Bi2WO6

In this work the oxygen reduction reaction on Bi0.5Sr0.5FeO3-delta (BSF) and La0.5Sr0.5FeO3-delta (LSF) as cathodes for intermediate temperature solid oxide fuel cells (600-750 degrees C) is compared in detail. Partial substitution of Sr in SrFeO3-delta with 50 mole % of Bi or La results in the distinct structural features which strongly impact its electrochemical characteristics due to the presence of a lone electron pair in Bi3+, which is not available in La3+. Cubic structure for BSF favours higher ionic conductivity and enhanced formation of oxygen vacancies relative to rhombohedral structure for LSF. Large oxygen nonstoichiometry for BSF, nevertheless, leads to the dominance of Fe3+ and subsequent low electronic conductivity relative to LSF. It was found that upon cathodic polarization, the oxygen vacancies are created on LSF which helps reducing its interfacial resistance afterward, which is not the case for BSF. Overall, BSF demonstrates good electrochemical performance which can be further optimized by enhancing its electronic conductivity. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3521316] All rights reserved.

https://iopscience.iop.org/article/10.1149/1.3521316/pdf

A Comparative Study of Oxygen Reduction Reaction on Bi- and La-Doped SrFeO3 − δ Perovskite Cathodes

Since their discovery in 1988, the BIMEVOXes have been the subject of significant research due to their high oxide ion conductivity at relatively low temperatures. The development of these materials is briefly reviewed. The defect structure of the BIMEVOXes is discussed and used to construct general defect equations for solid solution formation. Two limiting models are proposed by which solid solution formation can occur. In the Equatorial Vacancy
(EV) model, vacancies are located exclusively in bridging sites in the vanadate layer. In contrast, the Apical Vacancy
(AV) model assumes vacancies are located exclusively in non-bridging apical sites in the vanadate layer. The general defect equations can be used to predict theoretical solid solution limits for all types of substitutions for vanadium in Bi4V2O11−δ. These limits are found to vary not only with the charge of the dopant ion, but also with the coordination number of the metal dopant. In most cases it is found that the EV model yields theoretical solid solution limits close to those observed. The EV model is also used to present a mechanism for ionic conduction in BIMEVOXes, which involves movement of equatorial oxide ions/vacancies between vanadium octahedra and tetrahedra with the formation of a five-coordinate vanadium intermediate.

Defect chemistry of the BIMEVOXes

A sample of the new BiO-perovskite superconductor has been synthesized. X-ray diffraction has been used to locate the cations while neutron diffraction has been used to determine the precise oxygen co-ordination. The stricture contains CuO 2 planes with Cu coordinated to four oxygens at 1.92A within the plane, plus more distant oxygen at 2.65A perpendicular to the plane. Two such copper oxide (perovskite) planes are intercalated with planes of BiO. The stricture is described in an Fmmm subcell (5.4A, 5.4A, 30.8A), except for the oxygen within the BiO layers, and to a lesser extent the Bi itself, which require a x5 larger b-axis. The X-ray refinement in Bbmm indicates that this Bi displacement is 0.27A.

Powder X-ray and neutron diffraction study of the superconductor Bi2Sr2CaCu2O8

The continuing controversy about the true space group and structure of koechlinite γ-Bi2O3-MoO3 due to uncertainties in the light atoms positions, has been resolved by neutron powder profile refinement The original structure of Van den Elzen is correct, but contrary to his coordinates there are no unusually short oxygen-oxygen distancesThe structure may be related to a series consisting of Bi2O2 layers interleaved with perovskite like layers having the formula Bin-1MnX3n + 1 (Aurivillius, 1952; Wells, 1975) For n = 1, M = Mo and ' = O the perovskite layer is reduced to MoO4, which corresponds to the actual composition of the koechlinite Bi2MoO6 Complex oxides and oxyhalides of bismuth formed according to that principle have a pseudo-tetragonal symmetry Deviations from tetragonal symmetry are due to MoO6 octahedra tilts Further distortions are revealed by octahedra rotations and displacement of molybdenum atoms from the centers of the octahedra

Ahmed Laarif

Papers

The structure of koechlinite bismuth molybdate—a controversy resolved by neutron diffraction