Sodium manganese fluorosulfate with a triplite structure

The crystal structure of the NaMnSO4F fluorosulfate phase prepared by low-temperature solid-state synthesis has been solved and refined by the Rietveld analysis of synchrotron X-ray powder diffraction data. Isostructural to the naturally occurring triplite family of minerals, this compound crystallizes in monoclinic C2/c symmetry (No. 15) with unit-cell parameters of a = 13.77027 (17), b = 6.63687 (8), c = 10.35113 (14) Å, β = 121.4795 (3)° and V = 806.78 (2) Å(3). Its structure is built of edge-sharing chains of distorted MO4F2 octahedra, which are interconnected by constituent SO4 tetrahedra to form a robust three-dimensional polyanionic framework. MO4F2 octahedra are randomly occupied by Na and Mn with close to 1:1 occupancy. This random mixing of cations among polyhedral building blocks means that there are no channels for Na-ion conduction, rendering it electrochemically inactive. The structure is discussed and compared with other known alkali metal fluorosulfates as well as to naturally occurring triplite-type minerals.