Crystal structures of spinel-type Na2MoO4and Na2WO4revisited using neutron powder diffraction

Abstract

Time-of-flight neutron powder diffraction data have been collected from Na2MoO4and Na2WO4to a resolution of sin (θ)/λ = 1.25 Å−1, which is substantially better than the previous analyses using MoKα X-rays, providing roughly triple the number of measured reflections with respect to the previous studies [Okadaet 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 [Swansonet 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 Okadaet 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 accuracyandprecision for sodium tungstate. Both compounds adopt the spinel structure type. The Na+ions have site symmetry .-3mand are in octahedral coordination while the transition metal atoms have site symmetry -43mand are in tetrahedral coordination.