Non-stoichiometric compound

The type of equilibrium defects in non-stoichiometric compounds can vary with attendant variation in bulk properties of the material.

[1] Non-stoichiometric compounds also exhibit special electrical or chemical properties because of the defects; for example, when atoms are missing, electrons can move through the solid more rapidly.

[not verified in body] Non-stoichiometric compounds have applications in ceramic and superconductive material and in electrochemical (i.e., battery) system designs.

Pyrrhotite is remarkable in that it has numerous polytypes, i.e. crystalline forms differing in symmetry (monoclinic or hexagonal) and composition (Fe7S8, Fe9S10, Fe11S12 and others).

[citation needed] Many useful compounds are produced by the reactions of hydrocarbons with oxygen, a conversion that is catalyzed by metal oxides.

The process operates via the transfer of "lattice" oxygen to the hydrocarbon substrate, a step that temporarily generates a vacancy (or defect).

[6] An analogous sequence of events describes other kinds of atom-transfer reactions including hydrogenation and hydrodesulfurization catalysed by solid catalysts.

For example, yttrium barium copper oxide, arguably the most notable high-temperature superconductor, is a non-stoichiometric solid with the formula YxBa2Cu3O7−x.

[6] It was mainly through the work of Nikolai Semenovich Kurnakov and his students that Berthollet's opposition to Proust's law was shown to have merit for many solid compounds.

Kurnakov divided non-stoichiometric compounds into berthollides and daltonides depending on whether their properties showed monotonic behavior with respect to composition or not.

[7] The names come from Claude Louis Berthollet and John Dalton, respectively, who in the 19th century advocated rival theories of the composition of substances.

Origin of title phenomenon in crystallographic defects . Shown is a two-dimensional slice through a primitive cubic crystal system showing the regular square array of atoms on one face (open circles, o), and with these, places where atoms are missing from a regular site to create vacancies , displaced to an adjacent acceptable space to create a Frenkel pair , or substituted by a smaller or larger atom not usually seen (closed circles, • ), in each case resulting in a material that is moved toward being measurably non-stoichiometric.
Pyrrhotite, an example of a non-stoichiometric inorganic compound , with formula Fe 1− x S ( x = 0 to 0.2).