A POSITION-SPACE MODEL FOR SUPERCONDUCTIVITY IN YBA2CU3O7-X

M(r) = 666.2, orthorhombic, Pmmm, Z = 1, lambda(Mo K-alpha) = 0.71069 angstrom, F(000) = 294, T = 298 K. Data set (1): a = 3.847 (4), b = 3.875 (4), c = 11.70 (1) angstrom, V = 174.44 (5) angstrom 3, D(x) = 6.341 Mg m-3, mu = 29.24 mm-1, R = 0.055, wR = 0.041 (O5 excluded), R = 0.053, wR = 0.037 (O5 included) for 1132 unique reflections. Data set (2): a = 3.842 (1), b = 3.886 (1), c = 11.690 (4) angstrom, V = 174.55 (2) angstrom 3, D(x) = 6.338 Mg m-3, mu = 29.32 mm-1, R = 0.046, wR = 0.041 (O5 excluded), R = 0.046, wR = 0.040 (O5 included) for 1134 unique reflections. The structure is disordered with an O4 site occupancy of 0.54 (2) for data set (1) and 0.64 (2) for data set (2). The DELTA-rho maps for the structure have characteristics consistent with those of cubic perovskites, but modified as expected for the distorted structure. For both experiments, the nature of the depletion of electron density near Cu1 is as expected for a cooperative Jahn-Teller-distorted Cu system. Its radial dependence is consistent with depletion of paired spins along the Cu1-O1 bond, and of unpaired spins along the Cu1-O5-Cu1 vector. DELTA-rho associated with Cu2 is strikingly disimilar to that near Cu1. The density near the O5 site is not accounted for adequately by a disordered model since the mean-square amplitudes of vibration with refined occupation determined by least squares are non-physical. The main characteristics of DELTA-rho in the (001) plane containing the O5 site could be inferred from the DELTA-rho topology of the archetypal KCuF3 structure which has been accurately determined. The mechanism proposed for coupling the redistributed electron density with the vibrational motion of Cu1 in the [100] direction involves modulation by that motion of the antiferromagnetic exchange between adjacent Cu1 atoms.