LOCAL-STRUCTURE OF TL-2 (BA1-XSRX)(2)CUO(6) (X=0.0, 0.4, 0.5) STUDIED BY X-RAY-ABSORPTION SPECTROSCOPY

We have determined the local structure of Tl-2(Ba1-xSrx)(2)CuO6, with x=0.0, 0.4, 0.5 and T-c=89 K, 57 K, 0 K, respectively, using X-ray absorption fine structure at both the Cu K and Tl L(III) edges. As the strontium concentration increases from x=0.0 to 0.5, the copper-in-plane oxygen distance decreases from 1.94 Angstrom to 1.90 Angstrom, the copper-copper distance from 3.88 Angstrom to 3.81 Angstrom, and the copper-axial oxygen (0(2)) distance from 2.71 Angstrom to 2.65 Angstrom, while the thallium-axial oxygen (0(2)) distance remains at 2.00 Angstrom, indicating an overall unit-cell contraction. We also find that the strontium atom is 0.14 Angstrom closer to the CuO2 plane than the barium atom. The contraction of the copper-in-plane oxygen bonds reflects an increase in the hole concentration in the CuO2 plane, leading to the observed decrease of T-c due to hole overdoping.