Non-stoichiometric compound

A chemical compound whose elemental proportions vary from simple whole-number ratios because of defects or variable occupancy in its crystal structure.

Point defect: A localized disruption of a crystal lattice, such as a vacancy, interstitial atom, or substitutional atom. Missing or substituted atoms alter elemental proportions without destroying the host lattice.
Crystal lattice: A periodic arrangement of points representing the repeating structure of a crystalline solid. Non-stoichiometry arises when atoms or ions occupy this repeating arrangement imperfectly.
Wüstite: An iron oxide mineral, usually written Fe₁₋ₓO, with iron vacancies and variable composition. It is a canonical oxide whose iron deficiency is expressed through lattice defects.
Stoichiometric compound: A compound with elements in a fixed ratio expressible as simple whole numbers. It provides the direct compositional counterpart to a non-stoichiometric compound.
Schottky defect: A crystal defect formed by paired vacancies of oppositely charged ions, preserving electrical neutrality. Paired ion vacancies create composition changes in some ionic solids.
Unit cell: The smallest repeating volume that can generate a crystal structure by translation. Counting occupied sites in a unit cell helps describe variable composition.
Titanium dioxide: A titanium oxide occurring in several crystalline forms and widely used as a pigment and functional material. Oxygen-deficient forms can have altered conductivity and optical behavior.
Solid solution: A single crystalline phase containing multiple chemical species distributed within one structure. Substitutional solid solutions can vary in composition without necessarily being classified as defect-driven non-stoichiometric compounds.
Frenkel defect: A crystal defect in which an ion leaves its lattice site and occupies an interstitial site. This defect changes site occupancy and can accompany variable composition in ionic crystals.
Site occupancy: The fraction of a crystallographic site occupied by a particular atom or ion. Partial occupancy provides a structural description of non-integer elemental proportions.

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