Nonstoichiometric compound, any solid chemical compound in which the numbers of atoms of the elements present cannot be expressed as a ratio of small whole numbers; sometimes called berthollide compounds in distinction from daltonides (in which the atomic ratios are those of small integers), nonstoichiometric compounds are best known among the transition elements. Several of them are important as components of solid-state electronic devices, such as rectifiers, thermoelectric generators, photodetectors, thermistors, and magnets useful in high-frequency circuits.
The existence of nonstoichiometric compounds is related to the presence of defects in the lattice structures of crystalline substances, such as the absence of ions from sites that would normally be occupied. For example, a sodium chloride crystal that lacks a sodium ion and a chloride ion is defective, but still stoichiometric because the numbers of sodium ions and chloride ions are the same; if the sodium ion site, however, is filled by a neutral sodium atom, which then gives up its valence electron to fill the chloride ion site, the crystal defect is remedied, but the crystal is now nonstoichiometric because it contains more sodium ions than chloride ions.
Most nonstoichiometric compounds have compositions that are close to those of stoichiometric compounds and can be expressed by such formulas as WO3-x, Co1-xO, or Zn1+xO, in which x is a positive quantity much smaller than 1. In certain cases apparent nonstoichiometry has been shown to result from the existence of homologous series of stoichiometric compounds, such as the series of molybdenum oxides having the formulas MonO3n-1, in which the compounds corresponding to n = 8, 9, 10, 11, 12, and 14 are known; a true nonstoichiometric compound MoO3-x would show a continuous variation of x within some range of values, and no discrete species would be detectable.