Mossbauer investigation of Fe-57-doped TlSr2CoO5

The thallium cobaltite TlSr2CoO5 undergoes a metal-insulator transition nea r room temperature. Mossbauer investigation of a Fe-57-doped sample shows t hat iron actually enters into the lattice and that a long range antiferroma gnetic order sets in below 150 K explaining the origin of a peak previously observed in the temperature dependence of the magnetic susceptibility. An unexpected complex behaviour is found for the high temperature phase for wh ich previous electron diffraction and X-ray diffraction studies revealed a single site for Co atoms. The results are interpreted in terms of a dynamic al formation of two types of structural domains characterized by a fluctuat ion of Co-O bond lengths inducing a fluctuation of Co spin state. (C) 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.