nonstoichiometry

Nonstoichiometry is a property of certain chemical compounds in which the proportions of constituent elements deviate from simple whole-number ratios. Unlike ideal stoichiometric compounds, which have fixed formulas, nonstoichiometric materials exhibit variable composition that cannot be represented by a single integer formula. The deviations are typically caused by lattice defects or by the presence of ions in different oxidation states, allowing the crystal to maintain charge balance while cations or anions are present in non-integer amounts.

In many solids, especially transition metal oxides and chalcogenides, defects such as vacancies (missing atoms), interstitials (extra atoms in interstitial sites), and antisite defects contribute to nonstoichiometry. The most common mechanism in oxides is the creation of anion (oxygen) vacancies or cation vacancies, often accompanied by a change in the average oxidation state of the metal to preserve charge neutrality. This yields formulas like Fe1−xO or CeO2−x over a range of x.

Examples include Fe1−xO (wüstite) showing iron vacancies; UO2±x with oxygen nonstoichiometry; and titanium and ceria systems TiO2−x, CeO2−x used as oxygen storage or ionic conductors.

Factors such as temperature and surrounding oxygen partial pressure determine the extent of nonstoichiometry; higher temperatures and low pO2 promote vacancy formation and larger deviations.

Consequences of nonstoichiometry include altered electrical conductivity, magnetic properties, color, catalytic activity, and ionic transport. This phenomenon is critical in applications such as solid oxide fuel cells, sensors, and catalysts.

Measurement and analysis rely on chemical analysis, thermogravimetric analysis, and structural techniques like X-ray or neutron diffraction to infer defect populations and lattice parameters.