If cupric oxide (CuO) is exposed to reducing atmospheres at...

If cupric oxide $(\mathrm{CuO})$ is exposed to reducing atmospheres at elevated temperatures, some of the $\mathrm{Cu}^{2+}$ ions will become $\mathrm{Cu}^{+}$ (a) Under these conditions, name one crystalline defect that you would expect to form in order to maintain charge neutrality. (b) How many $\mathrm{Cu}^{+}$ ions are required for the creation of each defect? (c) How would you express the chemical formula for this nonstoichiometric material?

If cupric oxide $(\mathrm{CuO})$ is exposed to reducing atmospheres at elevated temperatures, some of the $\mathrm{Cu}^{2+}$ ions will become $\mathrm{Cu}^{+}$
(a) Under these conditions, name one crystalline defect that you would expect to form in order to maintain charge neutrality.
(b) How many $\mathrm{Cu}^{+}$ ions are required for the creation of each defect?
(c) How would you express the chemical formula for this nonstoichiometric material?

Key Concepts

Nonstoichiometric Formula Expression

The chemical formula for a nonstoichiometric material explicitly represents deviations from ideal composition. Such formulas typically include a variable term or a subscript that accounts for the fractional deficiency or excess of ions, effectively capturing the extent of the defect present in the crystal lattice.

Charge Neutrality and Defect Compensation

Charge neutrality in a crystal is maintained by balancing any change in ion charge with the formation of compensating defects. For example, if a cation is reduced to a lower oxidation state, additional vacancies might be generated to offset the extra positive charge. The relationship between the number of ions that change oxidation state and the number of vacancies formed is crucial in determining the defect equilibria within the material.

Vacancy Defects

Vacancy defects are empty lattice sites where an ion is missing. In ionic crystals, vacancies often form as a mechanism to compensate for changes in the oxidation states of other ions. When an ion reduces (or oxidizes) leading to a change in its charge, the creation of vacancies can help to maintain overall charge neutrality in the crystal structure.

Nonstoichiometry in Ionic Compounds

Nonstoichiometry refers to the deviation from the ideal stoichiometric ratio of elements in a compound. In transition metal oxides, nonstoichiometric behavior commonly arises due to redox reactions that alter the oxidation state of metal ions, resulting in a defect structure that is best represented by a formula incorporating a variable amount of vacancies or interstitials.

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