Disproportionation of Co 2+ in the Topochemically Reduced Oxide LaSrCoRuO 5 .

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, LaSrCoRuO ₆ , using Zr, yields LaSrCoRuO ₅ . This reduced phase contains an ordered array of apex-linked square-based pyramidal Ru ³⁺ O ₅ , square-planar Co ¹⁺ O ₄ and octahedral Co ³⁺ O ₆ units, consistent with the coordination-geometry driven disproportionation of Co ²⁺ . Coordination-geometry driven disproportionation of d ⁷ transition-metal cations (e.g. Rh ²⁺ , Pd ³⁺ , Pt ³⁺ ) 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 d ⁷⁺ Co ²⁺ should be stable to disproportionation in oxide environments, so the presence of Co ¹⁺ O ₄ and Co ³⁺ O ₆ units in LaSrCoRuO ₅ is surprising. Low-temperature measurements indicate LaSrCoRuO ₅ adopts a ferromagnetically ordered state below 120 K due to couplings between S= ¹ / ₂ Ru ³⁺ and S=1 Co ¹⁺ .
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