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

Time-of-flight neutron powder diffraction data have been collected from Na2MoO4 and Na2WO4 to a resolution of sin (theta)/lambda = 1.25 angstrom(-1), which is substantially better than the previous analyses using Mo K - 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) angstrom, and shorter Na-O distances, 2.378 (8)angstrom, 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.