Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction

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Abstract

Time-of-flight neutron powder diffraction data have been collected from Na2MoO4 and Na2WO4 to a resolution of sin (θ)/λ = 1.25 Å-1, which is substantially better than the previous analyses using Mo Kα X-rays, providing roughly triple the number of measured reflections with respect to the previous studies [Okada et al. (1974). Acta Cryst. B30, 1872-1873; Bramnik & Ehrenberg (2004). Z. Anorg. Allg. Chem. 630, 1336-1341]. The unit-cell parameters are in excellent agreement with literature data [Swanson et al. (1962). NBS Monograph No. 25, sect. 1, pp. 46-47] and the structural parameters for the molybdate agree very well with those of Bramnik & Ehrenberg (2004). However, the tungstate structure refinement of Okada et al. (1974) stands apart as being conspicuously inaccurate, giving significantly longer W - O distances, 1.819 (8) Å, and shorter Na - O distances, 2.378 (8) Å, than are reported here or in other simple tungstates. As such, this work represents an order-of-magnitude improvement in precision for sodium molybdate and an equally substantial improvement in both accuracy and precision for sodium tungstate. Both compounds adopt the spinel structure type. The Na+ ions have site symmetry .3¯m and are in octahedral coordination while the transition metal atoms have site symmetry 4¯3m and are in tetrahedral coordination.

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Fortes, A. D. (2015). Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction. Acta Crystallographica Section E: Crystallographic Communications, 71, 592–596. https://doi.org/10.1107/S2056989015008774

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