Fe3PO7, Un cas de coordinence 5 du fer trivalent,etude structurale et magnetique

The structure of Fe₃PO₇ is established from a single crystal. The cell is trigonal R3m, with, in hexagonal reference a = 8,006(5), c = 6,863(5)Å, Z = 3. The structure is determined with the direct method and refined to R = 0,027. The PO₄ tetraedra are isolated; the iron atoms are in a five coordinated site constituted of trigonal bipyramids sharing 2 edges and forming groups of 3 hexaedra; through these shared edges the iron atoms are at 3.13 Å a distance rather short which brings a repulsion causing the off centering of the cation along the pseudo axis of the bipyramid. The magnetic measurements and the Mössbauer spectroscopy show antiferromagnetism behaviour and, in the paramagnetic state, outstanding parameters (μ = 6.45 μ_B, θ_p = ?1707 K) recalling however Ca₂Fe₂O₅ or LaFeO₃. The Mössbauer spectroscopy gives, at room temperature, parameters classical for trivalent iron; through cooling, it points to a magnetic transition temperature of 160 ± 3 K below which the spectrum displays at least 2 hyperfine patterns. The magnetic interactions are discussed.