Co-operative and frustration effects in novel perovskite-related phases
1 Solid State Chemistry, Martin-Luther University Halle-Wittenberg, 06099 Halle (Saale), Germany
2 Solid State Chemistry, University of Augsburg, 86135 Augsburg, Germany
We report on magnetic and electronic properties of various perovskite-type oxides containing 4d- and 5d-transition metals. The compounds under investigation crystallize in (distorted) cubic, layered, and hexagonal perovskite-related structures. These changes in structural dimensionality are reflected by different ordering phenomena. (Pseudo-) cubic perovskites ACu3B4O12 (with A = alkali, alkaline earth or rare earth; B = Ru, Ti) possess an A-site ordered structure with copper on modified A-positions. Structural investigations as well as XANES (X-ray absorption near edge structure) measurements indicate a valence degeneracy, which is keeping the oxidation state of Ru close to +4. Upon replacing Ru by Ti, the itinerant magnetism and metallic conductivity of the pure ruthenates successively change to a localized magnetic moment and a semiconducting behavior. The pure titanates like Ln2/3Cu3Ti4O12 or CaCu3Ti4O12are insulators with colossal dielectric constants. The cation-deficient Cu2+xTa4O12+δ shows a large compositional flexibility with 0.125 ≤ x ≤ 0.500. Both copper content and cooling speed have a strong impact on the crystal structure and the observed magnetic ordering. This behavior can be explained by uncompensated Cu2+-moments resulting from different site occupations. Quasi-2D La2RuO5 undergoes a structural and magnetic phase transition at roughly 160 K, leading to a diminishing magnetic moment and a semiconductor-semiconductor transition. LDA calculations reveal an antiferromagnetic coupling within pairs of neighboring Ru4+-ions, leading to a spin-Peierls like transition. New hexagonal perovskites containing Ru, Ir, and Pt crystallize in the [AO1+δ][A2BO6] structure type and contain peroxide ions (O) in the [AO1+δ] layers. La1.2Sr2.7IrO7.33 exhibits a small temperature-independent paramagnetism, which can be explained on basis of the crystal-field splitting and the strong spin-orbit coupling. The isostructural La1.2Sr2.7RuO7.33 shows a frustrated magnetic ordering at roughly 6 K. The frustration results from the alignment of Ru5+-ions, which form elongated, edge-sharing Ru4-tetrahedra. Substituting La3+ by the smaller Nd3+ results in shorter Ru–Ru distances and leads to an increase of the frustrated magnetic interaction.
© EDP Sciences, Springer-Verlag, 2010