Change in crystal structure and electron density by introducing oxygen in YBa2Cu3Oy single crystal

The electron density map of tetragonal YBa2Cu3O6 (YBCO6) single crystal was investigated, in comparison with that of orthorhombic YBa2Cu3O6.9 (YBCO6.9). The crystal structure was determined by using four-circle X-ray diffraction data. The final values of the weighted reliability factor (Rw) and unweighted factor (R) were 0.027 and 0.024 for YBCO6, which are small enough for analysis of the Fourier difference maps. The obtained Fourier difference map shows a wide negative electron density region around Cu1 and no clear peak in the vicinity of Cu2-O3 plane, except for a negative peak between Cu2 and O1. The introduction of O4 between two Cu1 atoms gives rise to substantial electron redistribution, creating negative and positive peaks around Cu1 and Cu2. There is no remarkable peak between Cu2 and O3 in YBCO6, while a negative peak corresponding to the 3d(x2-y2) orbital electrons is found in YBCO6.9. The former result for YBCO6 indicates that there is no clear bonding state within the CuO2 plane. (C) 1997 Academic Press.