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

Abstract

The crystal structure of the displacive ferroelectric mineral russellite, Bi2WO6, has been determined using Rietveld profile refinement of high-resolution, time of flight, neutron powder diffraction data on the synthetic compound. Russellite is orthorhombic, Pca21, with a 5.43726(2) ~, b 16.43018(5) ~, c 5.458422) A, Z = 4 and is isostructural with the bismuth molybdate mineral koechlinite, Bi2MoO 6. The structure consists of layers of tilted WO6 octahedra sandwiched between layers of bismuth and oxygen with the tungsten displaced from the centre of the octahedron by 0.278 ~. The orientations of the lone-pair electrons in the Bi 3+ cations have been inferred from the 3.0 A coordination 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 WO6 octahedra. New laboratory source X-ray powder diffraction data are presented for russellite, which, with supplementary synchrotron powder diffractometry, corroborate the new space group and structure determination.

Stability and range of the type II Bi 1−x W x O 1.5+1.5x solid solution

“…ions are coordinated to 4 and 6 oxygens in Bi14WO24 (i.e., tetrahedral environment [27,28]) and Bi2WO6 (i.e., octahedral environment [17,18]) respectively. W L3-edge XANES spectra originate predominantly from the dipole transition of the W 2p3/2 core electrons into the empty 5d states in the conduction band, which is known to be sensitive to both the W oxidation state and its local coordination environment [29,30].…”

“…The structure of the line phase Bi2WO6 is an n = 1 Aurivillius phase, composed of alternating perovskite-and fluorite-like layers [17,18].…”

Conductividad eléctrica y difusión de oxígeno en el sistema Bifevox

“…En el dominio de composición O 10 , la capa perovskita puede considerarse constituida por pirámides de base cuadrada (21) como en el compuesto Bi 4 V 2 O 10 , y las posiciones de oxígeno que antes eran vacantes, se convierten en posiciones vacías. Para el salto de los iones, está situación es equivalente a tener todos los átomos metálicos en coordinación octaédrica, es decir, sin vacantes aniónicas, como en el compuesto Bi 4 MoO 12 (22). Por ello, se espera que la conductividad esté fuertemente restringida en estos dominios, disminuyendo su contribución al mecanismo de conducción.…”

Oxygen Diffusion Mechanisms in Two Series of Oxide Ion Conductors: BIMEVOX and Brownmillerite Materials

“…The structure of Bi 2 WO 6 was described in the Pca2 1 space group 22 with the oxide ions located on six different crystallographic sites. With a migration energy of 1.6eV, the diffusion of oxide ion within the Bi 2 O 2 layers is difficult.…”