Non-stoichiometric compounds, also known as variable stoichiometry compounds, are substances whose chemical composition does not follow a simple, fixed ratio of atoms. These substances typically contain excess amounts of one or more elements relative to their expected stoichiometry. Synonyms for non-stoichiometric compound include compositionally variable compound, off-stoichiometric compound, and non-stoichiometric mixture. Another term that is sometimes used interchangeably with non-stoichiometric compound is disordered compound. These compounds are of interest to materials scientists and chemists, as they can exhibit unique physical and chemical properties that are not observed in their stoichiometric counterparts. Their non-idealized compositions also make them challenging to synthesize and characterize accurately.