Valence-defect chemical equilibria in nonstoichiometric YBa2Cu3O7-δ

Most of the superconducting oxides are nonstoichiometric. The range of homogeneity varies, but in YBa2Cu3O7-δ it is extensive, 0≤δ≤1, so that the phase can serve as a prototype to illustrate the roles of chemical equilibria among the nominal valence states and defects in predicting the normal state properties. Through models which are essentially nearest-neighbor lattice statistics in the Hamiltonian, one can describe quantitatively the variation of the partial pressure of oxygen with temperature and composition and the variation of the orthorhombic-tetragonal transition. And one can predict the role of localized thermodynamic equilibrium in the transport of electrical charge