High-Resolution Electron Microscopy Structure of Ba sub 2 YCu sub 3 O sub 7 Twin Boundaries

High resolution electron microscope images of twin boundaries in Ba sub 2 YCu sub 3 O sub 7 show that the twin plane is the (110) plane containing the Cu, Ba, Y and O atoms and not the plane containing only O atoms. This is the model which involves least rearrangement of the atoms for a displacement of the twin boundary. The angular difference between (110) planes on opposite sides of the twin is measured to be 1.6+-0.2degrees from optical diffraction patterns. This is less than the value of 1.9degrees for Ba sub 2 YCu sub 3 O sub 7. This difference probably corresponds to a loss of oxygen during high resolution electron microscope observation.