Oxide ion vacancies, valence electrons, and superconductivity in mixed-metal oxides

High temperature superconductivity in YBa/sub 2/Cu/sub 3/O/sub 7-δ/ is dependent on the mixed oxidation states of copper (Cu/sup 3+//Cu/sup 2+/). Replacement of the d/sup 8/ Cu/sup 3+/-ions with the isoelectronic d/sup 8/ Ni/sup 2+/-ion and Ba/sup 2+/ with La/sup 3+/ for charge compensation has been studied. In the composition series YBa/sub 2-x/La/sub x/Cu/sub 3-x/Ni/sub x/O/sub 7+-δ/; x = 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 → 72K → 29K. Magnetic susceptibility measurements indicate that several phases are present. They are closely related in structure to YBa/sub 2/Cu/sub 3/O/sub 7/ and likely contain some fraction of Ni/sup 3+//Ni/sup 4+/ and more than seven oxygen atoms per unit cell