Crystal structures of spinel-type Na2MoO4and Na2WO4 revisited using neutron powder diffraction

Fortes, Andrew Dominic (2015) Crystal structures of spinel-type Na2MoO4and Na2WO4 revisited using neutron powder diffraction. Acta Crystallographica Section E: Crystallographic Communications 71 (6), pp. 592-596. ISSN 2056-9890.

Abstract

Time-of-flight neutron powder diffraction data have been collected from Na2MoO4 and Na2WO4 to a resolution of sin ([theta])/[lambda] = 1.25 Å-1, which is substanti­ally better than the previous analyses using Mo K[alpha] 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 .-3m and are in octa­hedral coordination while the transition metal atoms have site symmetry -43m and are in tetra­hedral coordination.

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