X-ray-scattering study of finite-range order in Ba2YCu3O6.7

We have observed evidence of finite-range order in quenched crystals of nominal composition Ba2YCu3O6.7. 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 Ba2YCu3O7. 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 AI for the a0, b0, and c0 directions, respectively.