A novel In3O16 fragment in Cs3In3(PO4)4

The double phosphate Cs(3)In(3)(PO(4))(4), prepared by a flux technique, features a fragment of composition In(3)O(16) formed by three corner-sharing InO(6) polyhedra. The central In atom resides on a twofold rotation axis, while the other two In atoms are on general positions. The O atoms in this fragment also belong to PO(4) tetrahedra, which link the structure into an overall three-dimensional anionic In-O-P network that is penetrated by tunnels running along c. Two independent Cs(+) cations reside inside the tunnels, one of which sits on a centre of inversion. In general, the organization of the framework is similar to that of K(3)In(3)(PO(4))(4), which also contains an In(3)O(16) fragment. However, in the latter case the unit consists of one InO(7) polyhedron and one InO(6) polyhedron sharing an edge, with a third InO(6) octahedron connected via a shared corner. Calculations of the Voronoi-Dirichlet polyhedra of the alkali metals give coordination schemes for Cs of [9+2] and [8+4] (1 symmetry), and for K of [8+1], [7+2] and [7+2]. This structural analysis shows that the coordination requirements of the alkali metals residing inside the tunnels cause the difference in the In(3)O(16) geometry.