Abstract

The complex cobaltite TlSr₂CoO₅ undergoes a first-order metal–insulator phase transition at room temperature. The structures of the high temperature (HT) and low temperature (LT) phases have been refined on powder samples using electron diffraction, X-ray diffraction and EXAFS. The HT-phase is isostructural with the so-called 1201-type cuprate. The unit cell is tetragonal (space group P4/mmm) and cobalt occupies a highly elongated octahedron. At low temperature, a commensurate modulation of the oxygen positions sets in such that two sites are available for cobalt, in the ratio 1 to 2. The sizes and the distortions of these two sites differ in such a way that a different electronic configuration of cobalt is stabilized in each. This effect is described as a spin state disproportionation (SSD). Long range ordering of these two spin states (SSO) is compared to the charge ordering (CO) effect and SSO seems to play a role as important for cobaltites(III) as CO does for nickelates or manganites.