X-Ray scattering study of finite-range order in Ba sub 2 YCu sub 3 O sub (6.7).

We have observed evidence of finite-range order in quenched crystals of nominal composition Ba sub 2 YCu sub 3 O sub (6.7). X-ray scattering shows diffuse peaks with a reduced wave vector of (1/200) in the oxygen deficient compound but not in fully oxidized Ba sub 2 YCu sub 3 O sub 7. Although the crystals are twinned, we find that the diffuse satellites occur symmetrically around (h00) peaks rather than (0k0) 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 9angstroms for h, k, and l directions respectively.