THE LOCAL-STRUCTURE OF HIGH-TEMPERATURE SUPERCONDUCTORS

We show how XAFS has been successfully used in the determination of th e local crystal structure of high-temperature superconductors, with ad vantages over traditional diffraction techniques. We review the experi mental results that yielded the first evidence for an axial-oxygen-cen tered lattice instability connected with the superconductivity transit ion. The interpretation of this instability in terms of a dynamical tu nneling model suggests the presence of polarons in these materials. XA FS on Tl2Ba2CuO6 and other Tl-based superconductors indicate the prese nce of local instabilities in the CuO2 planes of these materials, in a ddition to axial-oxygen instabilities.