Synthesis, crystal structure and charge-distribution validation of a new alluaudite-type phosphate, Na2.22Mn0.87In1.68(PO4)3.

Na2.22Mn0.87In1.68(PO4)3, sodium manganese indium tris-(phosphate) (2.22/0.87/1.68), was obtained in the form of single crystals by a flux method and was structurally characterized by single-crystal X-ray diffraction. The compound belongs to the alluaudite structure type (space group C2/c) with general formula X(2)X(1)M(1)M(2)2(PO4)3. The X(2) and X(1) sites are partially occupied by sodium [occupancy 0.7676 (17) and 1/2] while the M(1) and M(2) sites are fully occupied within a mixed distribution of sodium/manganese(II) and manganese(II)/indium, respectively. The three-dimensional anionic framework is built up on the basis of M(2)2O10 dimers that share opposite edges with M(1)O6 octa-hedra, thus forming infinite chains extending parallel to [10]. The linkage between these chains is ensured by PO4 tetra-hedra through common vertices. The three-dimensional network thus constructed delimits two types of hexa-gonal channels, resulting from the catenation of M(2)2O10 dimers, M(1)O6 octa-hedra and PO4 tetra-hedra through edge- and corner-sharing. The channels are occupied by Na+ cations with coordination numbers of seven and eight.