Studies on Rare Earth Cobaltites,

In rare earth cobaltites, the cobalt ions are present mainly in the diamagnetic low‐spin CoIII state at low temperatures. The CoIII ions transform to high‐spin Co3+ ions with increase in temperature. At higher temperatures, there is electron‐transfer from Co3+ to CoIII ions producing intermediate states. We see spin‐state transitions in these cobaltites in the range 250–870 K. At very high temperatures, the cobaltites show evidence for localizeditinerant electron transitions. In La1−xSrxCoO3, there is onset of ferromagnetism at x > 0.125, at which point there is a structural discontinuity and electrons become itinerant. The composition with x = 0.5 is metallic and Tc = 230 K. The ferromagnetic component in La1−xSrxCoO3, increases with x in the range 0.125–0.50. In LaCo1−xFexO3, the iron ions are always present in the high‐spin state and the d‐electrons get more localized with increase in x. Effect of substitution by ions like Th4+ and Ni3+ in LaCoO3 have also been examined. Catalytic properties of rare earth cobaltites appear to be related to the spin state equilibria.