Layered cobaltites: Synthesis, oxygen nonstoichiometry, transport and magnetic properties

Complex cobalt oxide perovskite-derived compounds with a general formula LnBaCo₂O_5+x (Ln = rare earth) attracted considerable interests because of their interesting properties: magnetic and metal-insulator transitions, giant magnetoresistance, ionic conductivity, and a structural similarity to high temperature superconductors. All the compounds are oxygen nonstoichiometric (0 < x < 1) and the cobalt cations can adopt different oxidation and spin states. Compounds with x ≈ 0.5 display a metal-insulator transition. We found that this transition is affected by an oxygen isotope substitution and is accompanied by structural changes and melting of the orbital ordering. Studies of the metal-insulator transition qualitatively support the models assuming rather carriers delocalization than a spin-state transition in Co³⁺.