Electron-density Fourier maps of an untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.877}$ single crystal by x-ray-diffraction data

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 °C. A superconducting transition of 91.3 K (Δ${\mathit{T}}_{\mathit{c}}$=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) Å. 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.