Disproportionation of Co²⁺ in the Topochemically Reduced Oxide LaSrCoRuO₅

Complex transition-metal oxides exhibit a wide variety of chemical and physical properties which are a strong function the local electronic states of the transition-metal centres, as determined by a combination of metal oxidation state and local coordination environment. Topochemical reduction of the double perovskite oxide, LaSrCoRuO₆ , using Zr, yields LaSrCoRuO₅ . This reduced phase contains an ordered array of apex-linked square-based pyramidal Ru³⁺ O₅ , square-planar Co¹⁺ O₄ and octahedral Co³⁺ O₆ units, consistent with the coordination-geometry driven disproportionation of Co²⁺ . Coordination-geometry driven disproportionation of d⁷ transition-metal cations (e.g. Rh²⁺ , Pd³⁺ , Pt³⁺ ) is common in complex oxides containing 4d and 5d metals. However, the weak ligand field experienced by a 3d transition-metal such as cobalt leads to the expectation that d⁷⁺ Co²⁺ should be stable to disproportionation in oxide environments, so the presence of Co¹⁺ O₄ and Co³⁺ O₆ units in LaSrCoRuO₅ is surprising. Low-temperature measurements indicate LaSrCoRuO₅ adopts a ferromagnetically ordered state below 120 K due to couplings between S=¹ /₂ Ru³⁺ and S=1 Co¹⁺ .