Abstract
A new iron oxophosphate of composition Rb7Fe7(PO4)(8)O-2 center dot 2H(2)O has been synthesized and studied by Xray diffraction, TG and DTA analysis, magnetic susceptibility, neutron diffraction, Mossbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic system with the P2(1)/c space group and the unit cell parameters a = 8.224(8) angstrom, b = 22.162(6) angstrom, c = 9.962(6) angstrom and beta = 109.41(8)degrees. 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 vertical bar theta/T-N vertical bar approximate to 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 Mossbauer 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. (C) 2010 Elsevier Ltd. All rights reserved.