Structurally modulated magnetic properties in the A₃MnRu₂O₉ phases (A = Ba, Ca): the role of metal-metal bonding in perovskite-related oxides

Abstract

Ca₃MnRu₂O₉ and Ba₃MnRu₂O₉ were synthesized from transition metal dioxides and alkaline earth metal carbonates at 1100-1300°C. Ca₃MnRu₂O₉ adopts the prototypical GdFeO₃-type perovskite structure with Mn and Ru statistically disordered over the single metal atom site. The susceptibility shows Curie-Weiss behavior above 240 K with μ_eff = 3.14 μ_B/metal atom, which is in excellent agreement with the expected spin-only moment of 3.20 μ_B. Below 150 K, the compound shows spin-glass-like short-range ferrimagnetic correlations. The high-temperature region of the electrical resistivity reveals a small activation energy of 17(1) meV whereas the low-temperature region is nonlinear and does not fit a variable range hopping model. Ba₃MnRu₂O₉ crystallizes in the 9-layer BaRuO₃-type structure containing M₃O₁₂ face-shared trioctahedral clusters in which Mn and Ru are statistically disordered. Ba₃MnRu₂O₉ shows nonlinear reciprocal susceptibility at all temperatures and is described by a variable-spin cluster model with an S = ½ ground state with thermally populated excited states. The low spin value of this system (S = ½) is attributed to direct metal-metal bonding. Below 30 K, the compound shows short-range magnetic correlations and spin-glass-like behavior. The high-temperature region of the electrical resistivity indicates a small activation energy of 8.8(1) meV whereas the low-temperature region is nonlinear. The importance of metal-metal bonding and the relationships to other related compounds are discussed.