Fe2PO5, un phosphate de fer de valence mixte. Préparation et études structurale, mössbauer et magnétique

Fe₂PO₅ is prepared from diverse components of the Fe---P---O system at 900°C in sealed silica tubes in vacuum. The study of a single crystal shows an orthorhombic cell, space group Pnma, with a = 7.378, B = 6.445, C = 7.471 Å and Z = 4. The structure is determined through direct methods and Fourier syntheses and refined to R = 0.027. The phosphorus fills isolated tetrahedra. The Fe^II and Fe^III ions are ordered in distorted octahedra: the octahedra surrounding Fe^II build strings parallel to the b axis and share edges; the octahedra containing Fe^III are connected on both sides of these strings alternatively with the PO₄ tetrahedra. The two types of octahedra share one face. Such an arrangement strongly recalls two out of the three Al₂SiO₅ polymorphs, namely, the kyanite and sillimanite. The Mössbauer spectroscopy exhibits a magnetic transition at 220 K. Below, the spectrum shows a six-line hyperfine pattern for Fe^III and an eight-lines one for Fe^II with a rather weak hyperfine field; above, there are two well differentiated doublets, confirming the absence of electronic charge transfer at room temperature. The magnetic susceptibility, recorded from 90 to 300 K, is typically that of an antiferromagnetic compound with C_M = 7.12, θ_p = −350 K and T_N 250 K. The magnetic interactions are discussed. The electronic localization is explained through structural and crystal field considerations; the electrostatic potential difference between the Fe^II and Fe^III sites is calculated.