Synthesis, crystal structure, and magnetic properties of a new mixed-metal diphosphate of formula: Co5Fe2(P2O7)4

A new pyrophosphate iron (III) cobalt (II) with the formula Co5Fe2(P2O7)4, was synthesized via coprecipitation using Fe (NO)3.9H2O, Co(NO3)2.6H2O, and (NH4)H2(PO4) as starting materials. Single crystals of the new compound were grown by annealing at 1000 °C in open air for one week. Single-crystal X-ray diffraction revealed Co5Fe2(P2O7)4 crystallizes in the orthorhombic space group C2221 with unit cell parameters a = 8.358 Å, b = 9.679 Å, c = 23.699 Å, and Z = 4. Its crystal structure is isotypic to that of FeII5FeIII2(P2O7)4 and consists of a sequence of two types of polyhedra: CoO6 and FeO6 octahedra as well as PO4 tetrahedra. The cobalt(II) ions are distributed across four crystallographic sites Co(1), Co(2), Co(3) and Co(4), whereas the iron(III) ions occupy a single site position. Infrared spectroscopy was performed to identify the (P2O7)4- and M-O vibrations, with the assignments further corroborated by Raman analysis. Magnetic measurements indicate that, at room temperature, the compound is paramagnetic with a Néel temperature of approximately 20 K.