Sodium manganese fluorosulphate with a triplite structure

Abstract

The crystal structure of the NaMnSO4F fluorosulphate 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 (#15) with unit cell parameters of a = 13.77027(17), b = 6.63687(8), c = 10.35113(14) Å,  = 121.4795(3) and V = 806.78(2) Å3. 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 to other known alkali metal fluorosulphates as well as to naturally occurring triplite-type minerals.