Synthesis, crystal structure, Mössbauer spectroscopy, optical and magnetic properties of Cs2M2Fe(PO4)3 (M = Mn, Co, Ni, Cu) ordered pollucite structure

Abstract

Four isomorphous iron phosphates Cs2M2Fe(PO4)3 (M = Mn, Co, Ni, Cu) have been synthesized as powder by conventional solid state technic and characterized by X-ray diffraction, magnetic susceptibility, diffuse reflectance and Mössbauer spectroscopies. All these compounds crystallize in the space group I4132 of the cubic system with similar cell parameters: a = 13.8268(1) Å, 13.7624(1) Å, 13.5917(1) Å or 13.5834(1) Å for M = Mn, Co, Ni or Cu, respectively and eight formula units per cell. Their structure closely related to that of pollucite, is built up from [M2/3,Fe1/3]O4 tetrahedra connected to each other through the common corners with PO4 tetrahedra. The resulting anionic three-dimensional framework leads to the formation along [1 1 1] direction, of channels where the large cesium ions occupy two independent crystallographic sites. A Mössbauer study at room temperature, confirmed the +3 oxidation state of iron and its tetrahedral coordination. Diffuse reflectance spectroscopy also confirmed the occurrence of isolated Fe3+ ions: low energy gap due to oxygen to iron (III) inter-electronic transition and absence of d-d transition linked to iron; the colorimetric properties of the compounds were shown to be mainly governed by the visible d-d transitions linked to the M2+ cation in tetrahedral coordination. Magnetic susceptibility measurements reveal that the title compounds are weakly magnetically ordered at very low temperature due to super-superexchange magnetic interaction. The thermal evolution of χm follows in a wide temperature range, a Curie-Weiss law χ = C/(T-θ) with θ = − 59, −36, −50 and −39 K for Mn, Co, Ni and Cu phases, respectively.