Synthesis and Structure of Cu 1.35Fe 3(PO 4) 3 and Cu 2Mg 3(PO 4) 3: New Mixed Valence Compounds of the Alluaudite Structure Type.

Abstract

The first example of a phase in the copper-iron-phosphorus-oxygen quaternary system (Cu I,Cu II) 1.35 (Fe II,Fe III) 3(PO 4) 3, has been prepared by a high temperature synthesis technique. The crystal structure was determined from single-crystal data and shown to be of the alluaudite type, but with differences in copper coordination and occupancy compared to the alkali and alkaline earth metal analogues. It crystallizes in the monoclinic space group C2/ c with lattice constants a = 11.894(4) Å, b = 12.629(4) Å, c = 6.440(1) Å, β = 115.24(2)°, Z = 4. The structure contains chains of edge sharing FeO 6 octahedra linked together via the common corners of PO 4 tetrahedra, yielding two distinct one-dimensional channels for copper ion occupation. The compound exhibits both copper and iron mixed valency. The copper-magnesium phosphate analogue Cu ICu IIMg 3(PO 4) 3 has been prepared and is shown to be of the same structure type, a = 11.85(2) Å, b = 12.55(2) Å, c = 6.38(1) Å, β = 115.9(2)°. Magnetic susceptibility measurements confirm the presence of one Cu 2+ ( d 9) ion per unit formula with μ eff = 1.78 μ B (gt; 50 K).