Non-stoichiometric compound

Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by integers; most often, in such materials, some small percentage of atoms are missing or too many atoms are packed into an otherwise perfect lattice work.

Origin of title phenomena 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.

Contrary to earlier definitions, modern understanding of non-stoichiometric compounds view them as homogenous, and not mixtures of stoichiometric chemical compounds. Since the solids are overall electrically neutral, the defect is compensated by a change in the charge of other atoms in the solid, either by changing their oxidation state, or by replacing them with atoms of different elements with a different charge. Many metal oxides and sulfides have non-stoichiometry examples; for example, stoichiometric iron(II) oxide, which is rare, has the formula FeO, whereas the more common material is nonstoichiometric, with the formula Fe_0.95O. Non-stoichiometric compounds exhibit special electrical or chemical properties because of the defects; for example, when atoms are missing, the other atoms can move through the solid more rapidly. Non-stoichiometric compounds have applications in ceramic and superconductive material and in electrochemical (i.e., battery) system designs.

Occurrence

Iron oxides

Nonstoichiometry is pervasive for metal oxides, especially when the metal is not in its highest oxidation state.^[1]^:642–644 For example, although wüstite (ferrous oxide) has an ideal (stoichiometric) formula FeO, the actual stoichiometry is closer to Fe_0.95O. The non-stoichiometry reflect the ease of oxidation of Fe²⁺ to Fe³⁺ effectively replacing a small portion of Fe²⁺ with two thirds their number of Fe³⁺. Thus for every three "missing" Fe²⁺ ions, the crystal contains two Fe³⁺ ions to balance the charge. The composition of a non-stoichiometric compound usually varies in a continuous manner over a narrow range. Thus, the formula for wüstite is written as Fe_1−xO, where x is a small number (0.05 in the previous example) representing the deviation from the "ideal" formula.^[2] Nonstoichiometry is especially important in solid, three-dimensional polymers that can tolerate mistakes. To some extent, entropy drives all solids to be non-stoichiometric. But for practical purposes, the term describes materials where the non-stoichiometry is measurable, usually at least 1% of the ideal composition.

Iron sulfides

Pyrrhotite, an example of a non-stoichiometric inorganic compound, with formula Fe_(1-x)S (x = 0 to 0.2).

The monosulfides of the transition metals are often nonstoichiometric. Best known perhaps is nominally iron(II) sulfide (the mineral pyrrhotite) with a composition Fe_(1-x)S (x = 0 to 0.2). The rare stoichiometric FeS endmember is known as the mineral troilite. 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). These materials are always iron-deficient owing to the presence of lattice defects, namely iron vacancies. Despite those defects, the composition is usually expressed as a ratio of large numbers and the crystals symmetry is relatively high. This means the iron vacancies are not randomly scattered over the crystal, but form certain regular configurations. Those vacancies strongly affect the magnetic properties of pyrrhotite: the magnetism increases with the concentration of vacancies and is absent for the stoichiometric FeS.^[3]

Palladium hydrides

Palladium hydride is a nonstoichiometric material of the approximate composition PdH_x (0.02 < x < 0.58). This solid conducts hydrogen by virtue of the mobility of the hydrogen atoms within the solid.

Tungsten oxides

It is sometimes difficult to determine if a material is non-stoichiometric or if the formula is best represented by large numbers. The oxides of tungsten illustrate this situation. Starting from the idealized material tungsten trioxide, one can generate a series of related materials that are slightly deficient in oxygen. These oxygen-deficient species can be described as WO_3−x but in fact they are stoichiometric species with large unit cells with the formulas W_nO_(3n-2) where n = 20, 24, 25, 40. Thus, the last species can be described with the stoichiometric formula W₄₀O₁₁₈, whereas the non-stoichiometric description WO_2.95 implies a more random distribution of oxide vacancies.

Other cases

At high temperatures (1000 °C), titanium sulfides present a series of non-stoichiometric compounds.^[1]^:679

The coordination polymer Prussian blue, nominally Fe₇(CN)₁₈, is well known to form in non-stoichiometric proportions, and the non-stoichiometric phases exhibit useful properties vis-à-vis their ability to bind caesium and thallium ions.

Applications

Oxidation catalysis

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). In a subsequent step, the missing oxygen is replenished by O₂. Such catalysts rely on the ability of the metal oxide to form phases that are not stoichiometric.^[4] An analogous sequence of events describes other kinds of atom-transfer reactions including hydrogenation and hydrodesulfurization catalysed by solid catalysts. These considerations also highlight the fact that stoichiometry is determined by the interior of crystals: the surfaces of crystals often do not follow the stoichiometry of the bulk. The complex structures on surfaces are described by the term "surface reconstruction."

Ion conduction

The migration of atoms within a solid is strongly influenced by the defects associated with non-stoichiometry. These defect sites provide pathways for atoms and ions to migrate through the otherwise dense ensemble of atoms that form the crystals. Oxygen sensors and solid state batteries are two applications that rely on oxide vacancies. One example is the CeO₂-based sensor in automotive exhaust systems. At low partial pressures of O₂, the sensor allows the introduction of increased air to effect more thorough combustion.^[4]

Superconductivity

Many superconductors are non-stoichiometric. For example, yttrium barium copper oxide, arguably the most notable high-temperature superconductor, is a non-stoichiometric solid with the formula Y_xBa₂Cu₃O_7−x. The critical temperature of the superconductor depends on the exact value of x. The stoichiometric species has x = 0, but this value can be as great as 1.^[4]

History

Non-stoichiometric compounds are also known as berthollides, as violators of the law of definite proportions, as opposed to the stoichiometric compounds or daltonides. The names come from Claude Louis Berthollet and John Dalton, respectively, who in the 19th century advocated rival theories of the composition of substances. Although Dalton "won" for the most part, it was later recognized that the law of definite proportions had important exceptions.^[5]

References

1 2 N. N. Greenwood & A. Earnshaw, 2012, "Chemistry of the Elements," 2nd Edn., Amsterdam, NH, NLD:Elsevier, ISBN 0080501095, see , accessed 8 July 2015. [Page numbers marked by superscript, inline.]
↑ Lesley E. Smart (2005). Solid State Chemistry: An Introduction, 3rd edition. CRC Press. p. 214. ISBN 0-7487-7516-1.
↑ Hubert Lloyd Barnes (1997). Geochemistry of hydrothermal ore deposits. John Wiley and Sons. pp. 382–390. ISBN 0-471-57144-X.
1 2 3 Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A., 2010, Shriver and Atkins' Inorganic Chemistry 5th Edn., pp. 65, 75, 99f, 268, 271, 277, 287, 356, 409, Oxford, OXF, GBR: Oxford University Press, ISBN 0199236178, see , accessed 8 July 2015.
↑ Henry Marshall Leicester (1971). The Historical Background of Chemistry. Courier Dover Publications. p. 153.

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