Crystal structure refinement and magnetic properties of Fe4(P2O7)3 studied by neutron diffraction and Mossbauer techniques

Fe-4(P2O7)(3) was prepared from Fe(PO3)(3) and FePO4 at 940degreesC under oxygen. The unit cell is monoclinic, space group P2(1)\n, with a = 7.389(2) Angstrom, b = 21.337(1) Angstrom, c = 9.517(2) Angstrom, beta = 90(l)degrees, and Z = 4. The crystallographic structure has been determined from a single crystal through direct methods and difference Fourier synthesis and refined to R = 0.10 (R-w = 0.09). The three-dimensional framework is built up from Fe2O9 clusters of two face-sharing octahedra, linked by bent diphosphates P2O7 (P-O-P similar to 156degrees). Fe-4(P2O7)(3) is antiferromagnetic below T-N = 50 K. The magnetic structure has been determinated by means of powder neutron diffraction. There are four antiferromagnetic iron sublattices corresponding to the four crystallographically distinct iron atoms. The magnetic moments are antiferromagnetically coupled inside the Fe2O9 dimers, in agreement with the Goodenough rules. They are parallel to the c axis and have 4.55(5) mu(B) value at 1.7 K. The magnetic interactions are discussed. Mossbauer spectra are fitted with four doublets and sextuplets in the paramagnetic and antiferromagnetic states, respectively. Their rather high isomer shifts are explained by the inductive effect. (C) 2002 Elsevier Science.