Interplay between Local Electronic Configuration and the Occurrence of a Metallic State: An Experimental Approach

Assuming that structural distortions are involved in a large class of metal–insulator transitions as well as in the stabilization of the local electronic configuration of a given cation leads the authors to look for a chemical system appropriate for investigating the interplay between both effects. The chosen cobaltite, TlSr2CoO5, undergoes a first order phase transition near room temperature involving steep changes and a hysteresis loop in the temperature dependencies of the magnetic susceptibility and of the electrical resistivity. The low temperature o-phase is an antiferromagnetic semiconductor and the high temperature t-phase is a metal with dominant ferromagnetic interactions. Electronic structure was calculated using the extended Hückel tight-binding method. The metallic character of the t-phase can be explained by the delocalization of some of the d-electrons of Co3+-ions in a σ*x2−y2 band and the strong ferromagnetic interactions can be ascribed to an indirect coupling of the localized moments via itinerant electrons. The electronic configuration of cobalt ions can be regarded as intermediate between high-spin and intermediate-spin states in the t-phase. In the low temperature phase it disproportionates into a purely high-spin state and a purely intermediate-spin state for Co3+ ions located in two different sites, the features of which respectively accommodate each electronic configuration.