Abstract
Microstructures of YBa2Cu3Oy with y=6.50, 6.75 and 6.85 as studied by high-resolution electron microscopy all consist of two kinds of thin parallel plates having different 2(b-a)/(b+a) values of 0% and 3-4%, where a and b are the lattice constants of the basal plane. These plates appear alternately along the [110] direction in a period of several tens of nm, while the ordinary orthorhombic twin structure with 2(b-a)/(b+a)=2% results at y=6.9. A two-phase region spreading over a wide temperature range for 6.50_??_y_??_6.85 has been concluded.
