Hole formation in orthorhombic and tetragonalYBa2Cu3O7−x

While in the ${\mathrm{La}}_{2-y}{\mathrm{Sr}}_y\mathrm{Cu}{\mathrm{O}}_4$ compound holes form because of doping with strontium, it is shown that in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ compound holes form because upon cooling down from high temperature, or increasing the partial pressure of oxygen at constant temperature, oxygen vacancies become occupied with oxygen ions. This process manifests itself as a positive dependence of conductivity on the partial pressure of oxygen. Analysis of the resistivity data of Freitas and Plaskett reveals that upon oxygen absorption at 500°C by the superconducting orthorhombic phase two mobile holes form per absorbed oxygen atom, while upon oxygen absorption at 700°C by the non-superconducting tetragonal phase only one mobile hole forms per oxygen atom. Implications for the valence state of copper and oxygen in the two phases are discussed.