The Change of Electron Density from YBa₂Cu₃O₆ to Twin-free YBa₂Cu₃O_6.9 Single Crystals

Abstract

The electron density maps of YBa₂Cu₃O₆ (YBCO₆) and twin-free orthorhombic YBa₂Cu₃O₆₉ (YBCO₆₉) single crystals were calculated from the refinement of X-ray structure analysis. The YBCO_7-x single crystal was grown by the top-seeded crystal pulling method. Uniaxial pressure was applied in oxygen annealing process to manufacture a twin-free orthorhombic single crystal. The YBCO₆ sample was obtained by annealing in Ar gas. The crystal structures were determined by a four-circle X-ray diffractometer. The final values of the weighted reliability factor (Rw) and unweighted factor (R) were 0.027, 0.024 for YBCO₆ and 0.032, 0.027 for YBCO₆₉, which enabled us to analyze Fourier difference maps. The difference of the Fourier difference maps for YBCO₆₉ from that for YBCO₆ was found that two electrons in the 3d _z2 orbital of Cu2 are not dumbbell-like but localized as a lone pair which results from the electron shift from the negative region with Ap = -2.1 eÅ^-3 between Cu2 and O2 and that the inclined electron density around the 02 atom gives polarization along the c-axis.