Structural, thermal, magnetic and electrical studies of the iron oxophosphate Rb7Fe7(PO4)8O2·2H2O

A new iron oxophosphate of composition Rb7Fe7(PO4)8O2·2H2O has been synthesized and studied by X-ray diffraction, TG and DTA analysis, magnetic susceptibility, neutron diffraction, Mössbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic system with the P21/c space group and the unit cell parameters a = 8.224(8) Å, b = 22.162(6) Å, c = 9.962(6) Å and β = 109.41(8)°. Its structure is built up from Fe7O32 clusters of edge- and corner-sharing FeO5 and FeO6 polyhedra. Neighboring clusters are connected by the phosphate tetrahedra to form a three-dimensional framework. The Rb+ cations and the water molecules are occupying intersecting tunnels parallel to a and c. The presence of water molecules was confirmed by TG and DTA analysis. The magnetic susceptibility measurements have shown the existence of antiferromagnetic ordering below 22 K with a weak ferromagnetic component. Additionally, these measurements show evidence for a strong magnetic frustration characterized by |θ/TN| ≈ 12. Powder neutron diffraction study confirms the presence of a long range antiferromagnetic order coupled to a weak ferromagnetic component along the b-axis. The strongly reduced magnetic moments extracted from the refinement support the existence of a magnetically frustrated ground state. The Mössbauer spectroscopy results confirmed the presence of only Fe3+ ions in both five and six coordination. The ionic conductivity measurements led to activation energy of 0.81 eV, a value that agrees with the obtained for other rubidium phosphates.