Structure and Mossbauer spectroscopy of barbosalite Fe2+Fe23+ (PO4)(2)(OH)(2) between 80 K and 300 K

Natural barbosalite Fe2+Fe23+ (PO4)(2)(OH)(2) from Bull Moose Mine, South D akota, U.S.A., having ideal composition, was investigated with single cryst al X-ray diffraction techniques, Mossbauer spectroscopy and SQUID magnetome try to redetermine crystal structure, valence state of iron and evolution o f Fe-57 Mossbauer parameter and to propose the magnetic structure at low te mperatures. At 298 K the title compound is monoclinic, space group P2(1)/n, a(o) = 7.3294(16) Angstrom, b(o) = 7.4921(17) Angstrom, c(o) = 7.4148(18) Angstrom, beta = 118.43(3)degrees, Z = 2. No crystallographic phase transit ion was observed between 298 K and 110 K. Slight discontinuities in the tem perature dependence of lattice parameters and bond angles in the range betw een 150 K and 180 K are ascribed to the magnetic phase transition of the ti tle compound. At 298 K the Mossbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe2+ and Fe3+ in octahedral coordinati on; the area ratio Fe3+/Fe2+ is exactly two, corresponding to the ideal val ue. Both the Fe2+ and the Fe3+ sublattice order magnetically below 173 K an d exhibit a fully developed magnetic pattern at 160 K. The electric field g radient at the Fe site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to V-zz, the main component of t he electric field gradient. The temperature dependent magnetic susceptibili ty exhibits strong antiferromagnetic ordering within the corner-sharing Fe3 +-chains parallel to [101], whereas ferromagnetic coupling is assumed withi n the face-sharing [110] and [-110] Fe3+-Fe2+-Fe3+ trimer, connecting the F e3+-chains to each other.