STRUCTURE ANALYSIS OF THE TWIN-FREE ORTHORHOMBIC YBA2CU3O7-X, SINGLE CRYSTALS

摘要

The electron density maps of the twin-free orthorhombic YBa2Cu3O7-x single crystal were calculated from the refinement of X-ray structure analysis. The YBa2Cu3O7-x single crystal was grown by the top-seeded crystal pulling method, Uniaxial pressure was applied in the oxygen annealing process to manufacture a twin-free orthorhombic single crystal. The crystal structure of YBa2Cu3O7-x with a superconducting transition temperature of 91 K was determined by a four-circle X-ray diffractometer, Crystal data are as follows: M(r), 666.4; orthorhombic; Pmmm; a = 3.8278(7), b = 3.8952(7), c = 11.711(2) Angstrom; V = 174.61(5) Angstrom(3); Z = 1; D-x = 6.317 Mg-3; lambda(MoK alpha) = 0.71069 Angstrom; mu(MoK alpha) = 282.77 cm(-1). The final values of the weighted reliability factor (R(W)) and unweighted factor (R) were 0.031 and 0.026, which enables us to analyze Fourier difference maps. In the Fourier difference maps it was found that two electrons in the 3d(z2) orbital of Cu2 are nor dumbbell-like but are localized as a lone pair and that the inclined electron density around the O2 atom gives polarization along the c axis but not around the O3 atom. (C) 1996 Academic Press, Inc. [References: 21]