We studied grain boundaries in calcined and sintered YBa2Cu3Ox transmission electron microscopy (TEM) and found complex Ba-Cu oxides. The complex oxide was polycrystalline and composed of grains smaller than 100 nm in diameter and it often included minor phases rich in Y. Some stoichiomet-ric oxides such as BaCuO2, CuO and BaO were also observed along grain boundaries.

Ordering of oxygen atoms in YBa2Cu3O6+x has been analysed by employing statistical thermodynamics ana the results are compared with the experimental data. The axial lengths a and b at room temperature were determined experimentally as a function of oxygen content and were found to show good agreement with the calculated lengths a and b. Special care was taken in the above experiment to dope oxygen atoms into the sample at low temperatures, typically 400° C. The superconducting transition temperature of the sample doped with oxygen at 400°C(starting material:YBa2Cu3O6.1.(decreases linearly with decreasing the oxygen content from 6+x=6.9(92 K) to 6.38(30 K) and decreased sharply around 6+x=6.35(5 K). No distinct superconducting phase corresponding to further ordered orthorhombic phase has been observed in this case.