A tetragonal form of YBa2Cu3O(y) whose lattice parameters were a = 3.8715 angstrom and c = 11.6506 angstrom was prepared at a high oxygen pressure of 6 GPa and 1050-degrees-C. Using the value of oxygen content determined by oxygen analysis measurement, the tetragonal YBCO was expressed as YBa2Cu3O7.7. If the oxygen is assumed to be divalent as usual, the average valence of Cu becomes 2.8. The onset of the superconducting transition temperature was 92 K, nearly the same as usually obtained with YBCO; however, the volume fraction of superconductivity was a third of the usual YBCO.
