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

Abstract

The continuing controversy about the true space group and structure of koechlinite y-Bi,O,-,MoO, due to uncertainties in the light atoms positions, has been resolved by neutron powder profile refinement. The original structure of Van den &en is correct, but contrary to his coordinates there are no unusually short oxygen-oxygen distances.The structure may be related to a series consisting of Bi,O, layers interleaved with perovskite like layers having the formula Bin-lMnX3,,+l (Aurivillius, 1952;Wells, 1975). For n = 1, M = Mo and X = 0 the perovskite layer is reduced to MOO,, which corresponds to the actual composition of the koechlinite Bi,MoO,. Complex oxides and oxyhalides of bismuth formed according to that principle have a pseudetetragonal symmetry. Deviations from tetragonal symmetry are due to MOO, octahedra tilts. Further distortions are revealed by octahedra rotations and displacement of molybdenum atoms from the centers of the octahedra.

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

“…5. The displacement vector of the tungsten atom from the centre of the octahedron is almost entirely in the polar axis direction (r = 0.151i + 0.043j -0.986k, with i, j, k unit vectors parallel to a, b, c respectively), but the magnitude of the displacement, 0.278 A, is less than the 0.357 A determined for koechlinite (Theobald et al, 1984). The displacement vector can be seen to approximately bisect a pseudo 2-fold axis in the WO6 octahedra and suggests that russellite is a one-dimensional ferroelectric, similar to many oxide perovskites (Lines and Glass, 1977).…”

“…Unpublished X-ray diffraction data determined by Blasse (1966) showed that there was a strong similarity between the diffraction patterns of koechlinite and russellite and Blasse proposed that BizMoO 6 and Bi2WO6 were isostructural. The correct space group for koechlinite has also been an area of some controversy (for a discussion see for example Theobald et al, 1984) which has only recently been resolved by Theobald et al to be Pca21 using neutron powder diffractometry. Subsequent time-of-flight neutron powder diffraction studies have confirmed the space group as Pca21 (Teller et al, 1985).…”

Synthesis and Structural Characterization of a New Family of Layered Intergrowth Phases Based on Antimony(III) Oxide

“…This compares with only an ȁ10Њ rotation in crystal structures consisting of ordered intergrowths of both Bi 2 WO 6 and Bi 2 MoO 6 and accounts for the significant lamellar sheets of simpler structure types. The bismuth difference in a and b basal plane cell dimensions in the oxide-based family of Aurivillius phases (1), many of case of Sb 2 WO 6 . Second, the distortion of the Sb 2 O 2ϩ 2 layer which are displacive ferroelectrics, is one such example.…”

“…WO 2Ϫ 4 , is severely distorted from the ideal perovskite type, by an ȁ25Њ rotation about a twofold axis of the WO 6 There are many families of compounds with layered octahedra. This compares with only an ȁ10Њ rotation in crystal structures consisting of ordered intergrowths of both Bi 2 WO 6 and Bi 2 MoO 6 and accounts for the significant lamellar sheets of simpler structure types.…”

Synthesis of g-Bi2MoO6 by Co-precipitation Method and Evaluation for Photocatalytic Degradation of Rhodamine B, Crystal Violet and Orange II Dyes Under Visible Light Irradiation

“…Of these γ-Bi 2 MoO 6 phase has excellent intrinsic physical and chemical properties, including ion conduction, dielectric and gas sensing properties [18,19] and also shows excellent catalytic activity towards oxidation of propylene [20], ammoxidation of propene [21]. The structure of γ-Bi 2 MoO 6 , which is built upon [Bi 2 O 2 ] 2+ layers alternated with layers of corner sharing MoO 6 octahedra (Aurivilliustype structure) [22] Bismuth molybdates can be prepared by several methods that include solid state reaction [23], spray drying [24,25], reflux method [26], ultrasonic assisted synthesis [27], hydrothermal synthesis [28,29], co-precipitation method [30] and sol-gel [31], etc. Among these methods, many researchers reported the co-precipitation route is one the most effective method for synthesizing bismuth molybdate nanoparticles [32][33][34].…”