OXIDE ION VACANCIES, VALENCE ELECTRONS, AND SUPERCONDUCTIVITY IN MIXED-METAL OXIDES. .

High temperature superconductivity in YBa//2Cu//3O//7// minus // delta is dependent on the mixed oxidation states of copper (Cu**3** plus /Cu**2** plus ). Replacement of the d**8 Cu**3** plus -ion with the isoelectronic d**8 Ni**2** plus -ion and Ba**2** plus with La**3** plus for charge compensation has been studied. In the composition series YBa//2// minus //xLa//xCu//3// minus //xNi//xO//7// plus or minus // delta ; x equals 0. 0, 0. 1, 0. 2, 0. 3, 0. 4, 0. 5, 0. 7, and 1. 0 a systematic change from orthorhombic to tetragonal symmetry was observed along with a decrease in the resistive transition temperature from 92K yields 72K yields 29K. Magnetic susceptibility measurements indicate that several phases are present. They are closely related in structure to YBa//2Cu//3O//7 and likely contain some fraction of Ni**3** plus /Ni**4** plus and more than seven oxygen atoms per unit cell.