TEMPERATURE-DEPENDENCE OF THE LOCAL-STRUCTURE IN SUPERCONDUCTING AND NONSUPERCONDUCTING TL2BA2CUOY STUDIED BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE

The local structure of Tl2Ba2CuOy (Tl2201) has been investigated by '' tended X-ray absorption fine structure (EXAFS) on the Cu K-edge. The r esult for the high-T(c) sample with T(c) = 70 K indicates a splitting of Cu-O bondlengths in the CuO2 plane, which shows a strong temperatur e dependence over a wide temperature range well above T(c), although t he crystallographical symmetry of the unit cell is tetragonal. The spl itting is evaluated from the filtered EXAFS by the beat-node method, a nd the maximum splitting distance is approximately 0.16 angstrom. The magnitude of the splitting abruptly decreases in the vicinity of T(c) by more than 10%. On the other hand, no splitting in the in-plane Cu-O bondlengths was observed in non-superconductor samples from room temp erature down to 30 K. No apparent change of axial oxygen sites was obs erved. These facts strongly demonstrate that the local structural dist ortion in the CuO2 plane may directly relate to the superconductivity.