Structure and magnetism in rare earth strontium-doped cobaltates

Substantial interest has recently been generated by rare earth cobaltate compounds as cathode materials for solid oxide fuel cells. We have synthesised a wide range of single-phase perovskite-based rare earth cobaltates (Ln_1-xSr_xCoO_{3- δ}) (Ln=La³⁺-Yb³⁺). A combination of electron and X-ray diffraction of these phases reveals a complex family of tetragonal and orthorhombic superstructures. The nature of structural and magnetic ordering relies on both cation and oxygen vacancy distribution. Phase boundaries exists between compounds containing large, medium and small rare earths (between Nd³⁺ and Sm³⁺, and also between Gd³⁺ and Dy³⁺) and also at different Sr-doping levels. Powder neutron diffraction has been used in conjunction with the other techniques to reveal cation and oxygen vacancy ordering within these materials. These phases show mixed valence (3+/4+) cobalt oxidation states that increases with Sr content. A range of magnetic behaviours has been observed, including ordered antiferromagnetism at elevated temperatures (>300 K) in Ho_0.2Sr_0.8CoO_2.75.