PLANAR OXYGEN-CENTERED LATTICE INSTABILITIES IN TL-BASED HIGH-TEMPERATURE SUPERCONDUCTORS

Analysis of the copper K-edge X-ray absorption fine structure on Tl2Ba 2CuO6, Tl2Ba2CaCu2O8, and Tl2Ba1.2Sr0.8CuO6 indicates that the copper- equatorial oxygen local structure changes in the vicinity of the super conducting transition. These data also show that the radial distributi on function for these bonds is complex with several oxygen positions r elative to the copper, indicating that the local structure of the CuO2 planes differs significantly from the average crystal structure. Thes e results suggest the presence of polarons in these planes, whose stru cture changes across the superconducting transition, and are thus coup led to the hole pairing states.