Crystal structure of a silver-, cobalt- and iron-based phosphate with an alluaudite-like structure: AgCoFe(PO).

The new silver-, cobalt- and iron-based phosphate, silver cobalt iron tris(ortho-phosphate), AgCoFe(PO), was synthesized by solid-state reactions. Its structure is isotypic to that of NaCoFe(PO), and belongs to the alluaudite family, with a partial cationic disorder, the Ag atoms being located on an inversion centre and twofold rotation axis sites (Wyckoff positions 4 and 4), with partial occupancies of 0.885 (2) and 0.7688 (19), respectively. One of the two P atoms in the asymmetric unit completely fills one 4 site while the Co and Fe atoms fill another 4 site, with partial occupancies of 0.86 (5) and 0.14 (5), respectively. The remaining Co and Fe cations are distributed on a general position, 8, in a 0.39 (4):0.61 (4) ratio. All O atoms and the other P atoms are in general positions. The structure is built up from zigzag chains of edge-sharing [O] ( = Fe/Co) octa-hedra stacked parallel to [101]. These chains are linked together through PO tetra-hedra, forming polyhedral sheets perpendicular to [010]. The resulting framework displays two types of channels running along [001], in which the Ag atoms (coordination number eight) are located.