Observation of oxygen-vacancy-ordered domains in oxygen-deficient single-crystal Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.7}$

Ordered domains due to oxygen vacancies in oxygen-deficient Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.7}$ single crystals have been imaged by dark-field electron micriscopy using the diffuse diffraction spots centered at (½,0,0). When projected along the $c$ axis, the ordered domains have longitudinal and transverse dimensions of ∼200 and ∼20 Å, respectively. The longitudinal direction of the domains is parallel to the Cu-O chain direction (i.e., $b$ axis). The vacancy-ordered domains are estimated to occupy ≲3% of the sample volume.