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 electro nic configuration of a given cation leads the authors to look for a chemica l system appropriate for investigating the interplay between both effects. The chosen cobaltite, TlSr2CoO5, undergoes a first order phase transition n ear room temperature involving steep changes and a hysteresis loop in the t emperature dependencies of the magnetic susceptibility and of the electrica l resistivity, The low temperature o-phase is an antiferromagnetic semicond uctor and the high temperature t-phase is a metal with dominant ferromagnet ic interactions, Electronic structure was calculated using the extended Huc kel tight-binding method. The metallic character of the t-phase can be expl ained by the delocalization of some of the d-electrons of Co3+-ions in a si gma(x2-y2)* band and the strong ferromagnetic interactions can be ascribed to an indirect coupling of the localized moments via itinerant electrons. T he electronic configuration of cobalt ions can be regarded as intermediate between high-spin and intermediate-spin states in the t-phase, Tn the low t emperature phase it disproportionates into a purely high-spin state and a p urely intermediate-spin state for Co3+ ions located in two different sites, the features of which respectively accommodate each electronic configurati on. (C) 1999 Academic Press.