Defect model for nonstoichiometry in YBa2Cu3O6+y.

Abstract

A simple defect model is proposed for the variable oxygen content of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$. It is based on the single assumption that ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ is the stoichiometric composition of a highly acceptor-doped version of the prototype compound ${\mathrm{Y}}_{3}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The acceptor centers, ${\mathrm{Ba}}_{\mathrm{Y}}^{\ensuremath{'}}$, are compensated by oxygen vacancies at the stoichiometric composition. The oxidation reaction then involves the filling of extrinsic oxygen vacancies and creation of holes in accord with the conventional treatment of a wide variety of acceptor-doped perovskite and perovskite-related oxides. A standard mass-action treatment leads to the expression (6+y)${\mathrm{y}}^{2}$/(1-y)=3.594\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$${\mathrm{e}}^{0.83\mathrm{}\mathrm{e}\mathrm{V}/\mathrm{k}\mathrm{T}}$P(${\mathrm{O}}_{2}$${)}^{1/2}$, where y is defined by ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{y}}$, and -0.83 eV is the enthalpy of oxidation per added oxygen, in agreement with calorimetric values. This expression is in excellent agreement with published thermogravimetric data over the range 300--900 \ifmmode^\circ\else\textdegree\fi{}C and ${10}^{\mathrm{\ensuremath{-}}15}$-1 atm P(${\mathrm{O}}_{2}$). Correction for intrinsic ionization improves the fit to the experimental data at high temperatures and low oxygen activities, i.e., for low values of y. Linear Arrhenius behavior over four orders of magnitude of the mass-action constant is indicative of ideal behavior. The proposal that ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ is a highly acceptor-doped composition is in agreement with its insensitivity to aliovalent dopants.