Structure and Mössbauer spectroscopy of barbosalite Fe²⁺Fe³⁺ ₂ (PO₄)₂(OH)₂ between 80 K and 300 K

Abstract

 Natural barbosalite Fe²⁺Fe³⁺ ₂ (PO₄)₂(OH)₂ from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of ⁵⁷Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298 K the title compound is monoclinic, space group P2₁/n, a _o  = 7.3294(16) Å, b _o  = 7.4921(17) Å, c _o  = 7.4148 (18) Å, β = 118.43(3)°, Z = 2. No crystallographic phase transition was observed between 298 K and 110 K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150 K and 180 K are ascribed to the magnetic phase transition of the title compound. At 298 K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe²⁺ and Fe³⁺ in octahedral coordination; the area ratio Fe³⁺/Fe²⁺ is exactly two, corresponding to the ideal value. Both the Fe²⁺ and the Fe³⁺ sublattice order magnetically below 173 K and exhibit a fully developed magnetic pattern at 160 K. The electric field gradient at the Fe²⁺ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to V_zz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe³⁺-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1 1 0] and [−1 1 0] Fe³⁺-Fe²⁺-Fe³⁺ trimer, connecting the Fe³⁺-chains to each other.