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+) .