The electronic properties of mixed metal oxides

The properties of Fe and Mn in a variety of perovskite-related crystal structures have been studied by X-ray and neutron diffraction, magnetometry, high resolution electron microscopy and Moessbauer spectroscopy. The structure of Sr2FeTaO6 is of the GdFeO3 type with a disordered arrangement of Fe and Ta over the octahedrally coordinated sites in contrast to the partial ordering, 0.795(6)Fe/0.205(6)Sb, observed in Sr2FeSbO6. Sr2FeTaO6 is a spin glass below 23 K whereas Sr2FeSbO6 forms a type I antiferromagnetically ordered phase below TN = 37(2) K with an ordered moment of 3.06(9) μB Fe-1 at 1.5 K on the Fe-rich site. Susceptibility measurements in the magnetically dilute series Sr2Fe1-xGaxTaO6 indicate that magnetic ordering in these Fe3+ perovskites is partially controlled by next-nearest-neighbour superexchange. A new 15R perovskite structure containing face-sharing dimers of octahedra linked to one another by vertices or bridging octahedra has been - found for the composition SrMn0.915(5)Fe0.085(5)O2.979(3). The Mn4+ cations align antiferromagnetically below TN = 220(5) K showing an ordered moment of 2.25(3) μB at 3 K. The Fe cations remain disordered to 3 K. This composition forms a 6-layered hexagonal perovskite in the temperature range 1200 < T / deg. C < 1350 and distorts to monoclinic symmetry with increasing temperatures as a consequence of oxygen loss. The maximum Fe concentration in a face-sharing octahedral site, 4.0(5)%, is independent of the stacking sequence and can determine the adopted structure. Ba6Mn4MO15 (M = Cu, Zn) form pseudo 1-dimensional phases related to Ba6Ni5O15. The trigonal prismatic sites in this structure are preferentially occupied by Zn/Cu; the latter is displaced from the centre of the trigonal prism to give pseudo square-planar coordination. At 1.7 K antiferromagnetic superexchange within a highly frustrated crystal structure leads to a magnetic structure exhibiting rotation of 120 deg. between spins in neighbouring chains. The magnetic moments refined to 0.7(1) and 0.6(1) μB per octahedral site in the Cu and Zn compounds respectively. The magnetic susceptibilities have been rationalised in terms of 1-dimensional ordering of the octahedrally coordinated Mn. The trigonal prismatic Cu is paramagnetic in the range 5 ≤ T / K ≤ 300. The magnetic order in Ba6Mn4ZnO15 is lost by 6.2(2) K. The compounds Ba6Mn5-xNixO15 (1 ≤ x ≤ 4) adopt related but incommensurate structures. (author)