defectchemistry

Defect chemistry is the study of point defects and defect equilibria in crystalline solids. It analyzes how nonstoichiometry, defect concentrations, and defect reactions depend on temperature, chemical potentials, and doping. In ceramics and solid-state chemistry, it uses formalisms such as Kröger-Vink notation to describe defect species, their charges, and reactions.

Defects can be intrinsic or extrinsic. End defects include vacancies (missing atoms) and interstitials (extra atoms), and antisite defects where atoms occupy non-native lattice sites. Schottky defects involve paired vacancies; Frenkel defects are vacancy–interstitial pairs. Defects exhibit various charge states, and their formation energies depend on the Fermi level.

Defect concentrations are described by mass-action expressions and defect formation energies. Charge neutrality constrains defect and electronic carrier concentrations. Equilibria shift with temperature and chemical potentials, such as oxygen partial pressure, producing nonstoichiometry by creating vacancies or interstitials in metal oxides.

The defect landscape governs diffusion and transport. Ionic conductivity in oxides arises from mobile vacancies and interstitials; electronic conductivity from defect-associated carriers. Doping or defect engineering tailors properties for solid oxide fuel cells, sensors, catalysis, and electrochemistry.

Representative materials include ceria (CeO2) and zirconia-based electrolytes, where oxygen vacancies enable oxide-ion conduction; perovskites exhibit oxygen nonstoichiometry and mixed valence that influence conductivity and catalytic activity.

Defect chemistry combines thermodynamics with solid-state physics to predict defect populations and behavior under operating conditions. It informs materials design and experimental interpretation and is complemented by computational methods that estimate defect energies and diffusion barriers.