An Exploration of Co\(^{3+}\) Magnetism in Hexagonal and Cubic Perovskites

Abstract

Cobalt has long been of interest to inorganic chemists due to its wide range of oxidation states and magnetic anomalies. Oxychloride hexagonal perovskites are a new sub-class of perovskites and were chosen as the host structure to test cobalt’s magnetic properties. This was done by synthesizing the novel titanium-doped perovskite Ba\(_{5}\)Co\(_{3}\)Ti\(_{2}\)O\(_{13}\)Cl. In this structure, Co\(^{3+}\) was shown to exhibit a mix of intermediate spin and low spin character as determined by its effective magnetic moment. During the synthesis of Ba\(_{5}\)Co\(_{3}\)Ti\(_{2}\)O\(_{13}\)Cl, it was discovered that oxygen levels have an effect of the magnetic properties of Ba\(_{6}\)Co\(_{6}\)O\(_{15.5}\)Cl, prompting the synthesis of two samples of Ba\(_{6}\)Co\(_{6}\)O\(_{15.5}\)Cl with different oxygen contents. Magnetic susceptibility testing of Ba\(_{6}\)Co\(_{6}\)O\(_{15.5}\)Cl yielded unexpected results, and previously published explanations of its behavior were unsatisfactory. Instead, a new model for the electronic structure of Ba\(_{6}\)Co\(_{6}\)O\(_{15.5}\)Cl is proposed, asserting that Ba\(_{6}\)Co\(_{6}\)O\(_{15.5}\)Cl is a rare itinerant antiferromagnet, few of which are known to exist. Cobalt magnetism analysis concluded with the synthesis of a more traditional cubic perovskite series, BaCo\(_{1-x}\)Nb\(_{x}\)O\(_{3}\). A \(d\)\(^{0}\) cation, Nb\(^{5+}\)—which was doped in place of cobalt—was thought to stabilize the intermediate spin state of Co\(^{3+}\). Magnetic susceptibility testing proved this prediction true, as the effective moment of BaCo\(_{0.75}\)Nb\(_{0.25}\)O\(_{3}\) and BaCo\(_{0.50}\)Nb\(_{0.50}\)O\(_{3}\) suggest the presence of intermediate spin Co\(^{3+}\).