Electron-density Fourier maps of an untwinned YBa2Cu3O6.877 single crystal by x-ray-diffraction data.

Abstract

The room-temperature structure of an untwinned orthorhombic ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystal was determined from x-ray-diffraction intensity data. The crystal was grown by the self-flux method, and detwinned by the application of a uniaxial stress under flowing oxygen at 450 \ifmmode^\circ\else\textdegree\fi{}C. A superconducting transition of 91.3 K (\ensuremath{\Delta}${\mathit{T}}_{\mathit{c}}$=1.4 K) was measured. Precession photographs showed no evidence of twinning, and no line splitting or broadening was apparent in high-angle reflections. The crystal structure was refined in the Pmmm space group, with a=3.8184(3), b=3.8857(2), and c=11.701(1) \AA{}. The overall oxygen content was determined to be 6.877(8). Electron-density Fourier maps and the anisotropic temperature factors do not support the double-well apical oxygen model proposed from extended x-ray absorption fine structure data. Distortions of the electron density of the planar Cu sites suggest the possibility of a nonhomogeneous ${\mathrm{CuO}}_{2}$ conducting plane. The anomalous thermal parameters of the chain oxygens appear to result from dynamic disorder rather than a buckling of the Cu-O chains.