Crystal structure of the thallium strontium cobaltite TlSr2CoO5 and its relationship to the electronic properties

The complex cobaltite TlSr2CoO5 undergoes a first-order metal-insulator pha se transition at room temperature. The structures of the high temperature ( HT) and low temperature (LT) phases have been refined on powder samples usi ng electron diffraction, X-ray diffraction and EXAFS. The HT-phase is isost ructural 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 size s and the distortions of these two sites differ in such a way that a differ ent electronic configuration of cobalt is stabilized in each. This effect i s described as a spin state disproportionation (SSD). Long range ordering o f these two spin states (SSO) is compared to the charge ordering (CO) effec t and SSO seems to play a role as important for cobaltites(iii) as CO does for nickelates or manganites.