STRUCTURE REFINEMENT OF THE 0201-1201 INTERGROWTH-TYPE FERRITE PBSR4FE2O9 - POWDER NEUTRON-DIFFRACTION AND MOSSBAUER-SPECTROSCOPY STUDIES

The structure of the tetragonal ferrite PbSr4Fe2O9 (a = 3.8484 Angstrom, c = 30.683 Angstrom) has been refined from powder neutron diffraction data. The intergrowth of the two structures 0201 and 1201 with the La2CuO4 and the TlSr2CuO5 types is confirmed as is the oxygen stoichiometry (O-9), which involve oxidation states III and IV for iron and lead, respectively. The structure of the [PbO]infinity layers consists of two sets of Pb sites (at the origin and on 8i (x00) split positions) and one set of oxygen atoms O(1) on 8j (x1/20) split positions. According to neutron diffraction, Mossbauer spectroscopy, and electric field gradient calculations, a splitting of the axial oxygen [O(2)] along the c direction appears also as most probable. It results in two highly distorted octahedral coordinations for Fe(III), which can be better described as almost regular [FeO5] pyramids. Similarly two coordination states are found for Pb(IV): a flattened [PbO6] octahedron and a distorted [PbO4] tetrahedron. (C) 1994 Academic Press, Inc.