Preparation and characterization of a new Tl-1222 layered cobaltite
Résumé
The preparation and the characterization of a new layered cobaltite of formula Tl0.900±0.005Sr3LaCo2O8.60±0.07 is described. This oxide is isostructural with the so-called 1222 member of the homologous cuprate series (Space Group I4/mmm). Oxygen vacancies are rather located in thallium layers or in apical positions. The six oxygen atoms surrounding the cobalt form an elongated octahedron of C4v symmetry. The magnetic susceptibility exhibits a complex behavior attributed to a change of the cobalt effective moment with temperature. The compound is a semiconductor. The temperature dependencies of the electrical conductivity and of the thermoelectric power are interpreted in terms of hopping and variable range hopping at low temperature. The behavior of this 1222 cobaltite is compared to that of the 1201-TlSr2CoO5, paying a particular attention to the interplay of electronic properties with structural distortion, disordering, and cobalt mixed valence.