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

Abstract

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⁺.

Abstract

Topochemical reduction of LaSrCoRuO ₆ yields LaSrCoRuO ₅ a phase containing Co ³⁺O ₆, Co ¹⁺O ₄ and Ru ³⁺O ₅ units, consistent with the coordination‐geometry‐driven disproportionation of Co ²⁺ in an extended oxide.