SUPERCONDUCTING TRANSITION IN TL2BA2CACU2O8 OBSERVED BY ELECTRON-DENSITY DISTRIBUTIONS

We have examined the electron-density distributions of a Tl2Ba2CaCu2O8 superconductor (T-c=108 K) by the single-crystal diffraction method with synchrotron X-rays. Anomalous redistributions of the electron density, Delta rho(r), in the Tl-O(2)-Cu region were observed, that have some out-of-plane character when the temperature is reduced from 120 K to 104 K: asymmetrically shifts of apical O(2) toward Tl; the positive and negative Delta rho(r) peaks around Cu perpendicular to the basal plane; a positive Delta rho(r) peak at (0.5, 0.5, 0). The Delta rho(r) changes are supported by a temperature dependence of the interatomic distances: Tl-O(2) shortens when T < T-c, but Cu-O(2) and Tl-O(3) expand. Significant Delta rho(r) changes could not be detected in the Cu-O basal plane.