First observation of superstructure reflections by neutron diffraction due to oxygen ordering in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.35}$

We present results of a structural investigation of oxygen ordering in a 0.1-${\mathrm{cm}}^3$ ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.35}$ single crystal. For the first time nonmagnetic superstructure reflections have been observed by neutron diffraction. From the intensities a 2 √2 a×2 √2 a×c oxygen ordering model is derived. In this structure ‘‘half-filled’’ CuO chains alternate with ‘‘quarter-filled’’ chains. Only 15% of the basal-plane oxygen atoms contribute to the superstructure.