Oxygen vacancy ordering and magnetism in the rare earth stabilised perovskite form of "SrCoO_3-δ"

We have demonstrated that SrCoO_3-δ can be stabilised into phase pure perovskite forms by the introduction of small amounts ∼5% of certain rare earth ions (Sm³⁺-Yb³⁺). At the same doping levels, La³⁺ and Pr³⁺ crystallise with the same isostructural trigonal structure as Sr₆Co₅O₁₅; while the Nd³⁺ composition shows a mixture of both structure types. Powder X-ray diffraction showed only a simple cubic perovskite structure, however, a combination of electron and neutron diffraction has revealed a tetragonal (P4/mmm) a_p×a_p×2a_p superstructure. Strontium and the rare earth ions are disordered over a single site, while the oxygen vacancies are localised on the apical O2 sites. Magnetisation measurements show that these materials undergo transitions to a spin-glass state at temperatures below 150 K, and that significant coupling occurs between the rare earth ions and the mixed Co^3+/4+ ions. Magnetisation measurements as a function of applied field reveals that below the transition temperature ferromagnetic ordering takes place at relatively large fields.