Thermodynamics of the Formation and Migration of Defects in Cuprous Oxide

The number of gram atoms of excess oxygen y per mole of cuprous oxide has been measured by means of a quartz‐fiber microbalance. The combined pressure and temperature dependence is log₁₀ y=0.27 log₁₀PO₂—(21.7 kcal) / RT+0.29 with T in °K and PO₂ in mm Hg. This relation has been utilized to determine the extent of the cuprite field in the copper‐oxygen system. The cation vacancy V⁺ is formed in Cu₂O by the reaction ¼O₂→½O⁼+V_Cu⁺. The entropy of formation of the vacancy, computed from $\mathrm{\Delta S=}\frac{1}{2}\mathrm{S(}{\mathrm{O}}^{=}{\mathrm{)+S(V}}^{+}\mathrm{)-}\frac{1}{4}\mathrm{S(}{\mathrm{O}}_2)$ is S(V⁺) = 12.1 cal deg^—1. This positive entropy, caused by the lowering of the vibration frequencies of neighboring copper atoms, is the principal factor responsible for the defect structure of cuprous oxide. The new thermodynamic data lead to a quantitative determination of the diffusion parameters for copper in Cu₂O.