Synthesis, crystal structure determination of a novel phosphate Ag1.64Zn1.64Fe1.36(PO4)3 with an alluaudite-like structure.

Single crystals of Ag1.64Zn1.64Fe1.36(PO4)3 [silver zinc iron phosphate (1.64/1.64/1.36/3)] have been synthesized by a conventional solid-state reaction and structurally characterized by single-crystal X-ray diffraction. The title compound crystallizes with an alluaudite-like structure. All atoms of the structure are in general positions except for four, which reside on special positions of the space group, C2/c. The Ag+ cations reside at full occupancy on inversion centre sites and at partial occupancy (64%) on a twofold rotation axis. In this structure, the unique Fe3+ ion with one of the two Zn2+ cations are substitutionally disordered on the same general position (Wyckoff site 8f), with a respective ratio of 0.68/0.32 (occupancies were fixed so as to ensure electrical neutrality for the whole structure). The remaining O and P atoms are located in general positions. The three-dimensional framework of this structure consists of kinked chains of edge-sharing octa-hedra stacked parallel to [10]. These chains are built up by a succession of [MO6] (M = Zn/Fe or Zn) units. Adjacent chains are connected by the PO4 anions, forming sheets oriented perpendicular to [010]. These inter-connected sheets generate two types of channels parallel to the c axis, in which the Ag+ cations are located. The validity and adequacy of the proposed structural model of Ag1.64Zn1.64Fe1.36(PO4)3 was established by means of bond-valence-sum (BVS) and charge-distribution (CHARDI) analysis tools.