X-ray-scattering study of finite-range order in ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{6.7}$

We have observed evidence of finite-range order in quenched crystals of nominal composition ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_{6.7}$. X-ray scattering shows diffuse peaks with a reduced wave vector of (1/2,0,0) in the oxygen-deficient compound but not in fully oxidized ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_7$. Although the crystals are twinned, we find that the diffuse satellites occur symmetrically around (h,0,0) peaks rather than (0,k,0) peaks indicating a doubling of the unit cell along the a axis (transverse to the Cu-O chains). The width of the diffuse peaks yields coherence lengths of 21, 16, and 9 Å for the $a_0$, $b_0$, and $c_0$ directions, respectively.