Crystal structure of a sodium, zinc and iron(III)-based non-stoichiometric phosphate with an alluaudite-like structure: Na1.67Zn1.67Fe1.33(PO4)3

Abstract

The new title compound, disodium dizinc iron(III) tris(phosphate), Na1.67Zn1.67Fe1.33(PO4)3, which belongs to the alluaudite family, has been synthesized by solid-state reactions. In this structure, all atoms are in general positions except for four, which are located on special positions of theC2/cspace group. This structure is characterized by cation substitutional disorder at two sites, one situated on the special position 4e(2) and the other on the general position 8f. The 4esite is partially occupied by Na+[0.332 (3)], whereas the 8fsite is entirely filled by a mixture of Fe and Zn. The full-occupancy sodium and zinc atoms are located at the Wyckoff positions on the inversion center 4a(-1) and on the twofold rotation axis 4e, respectively. Refinement of the occupancy ratios, bond-valence analysis and the electrical neutrality requirement of the structure lead to the given composition for the title compound. The three-dimensional framework of this structure consists of kinked chains of edge-sharing octahedra stacked parallel to [10-1]. The chains are formed by a succession of trimers based on [ZnO6] octahedra and the mixed-cation FeIII/ZnII[(Fe/Zn)O6] octahedra [FeIII:ZnIIIratio 0.668 (3)/0.332 (3)]. Continuous chains are held together by PO4phosphate groups, forming polyhedral sheets perpendicular to [010]. The stacked sheets delimit two types of tunnels parallel to thecaxis in which the sodium cations are located. Each Na+cation is coordinated by eight O atoms. The disorder of Na in the tunnel might presage ionic mobility for this material.