Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having an elemental composition whose proportions can not be represented by integers; most often, in such materials, a small number of atoms do not exist or too many atoms are packed into a perfect grid.
Contrary to the preceding definition, modern understanding of non-stoichiometric compounds sees them as homogeneous, and not a mixture of stoichiometric chemical compounds. Because the solids are electrically neutral, they are compensated by changes in the charge of other atoms in the solid, either by altering their oxidation state, or by replacing it with different elemental atoms with different charges. Many metal oxides and sulfides have non-stoichiometric examples; for example, stoichiometric iron (II) oxide, which is rare, has the FeO formula, while the more common material is nonstoichiometric, with the formula Fe 0.95 O. The non-stoichiometric compound exhibits special electrical or chemical properties because defects; for example, when atoms are lost, electrons can move through solids more quickly. Non-stoichiometric compounds have applications in ceramic and superconductive materials and in the design of electrochemical systems (ie batteries).
Video Non-stoichiometric compound
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Nonstoikiometry is pervasive for metal oxides, especially when the metal is not in the highest oxidation state. For example, although wÃÆ'¼stite (ferrous oxide) has the ideal Feo formula (stoichiometry), the stoichiometry is actually closer to Fe 0.95 O. Non-stoichiometry reflects the oxidation of Fe 2 to Fe 3 effectively replaces a small portion of Fe 2 with two thirds of their amount Fe 3 . So for every three lost Fe 2 ions, the crystal contains two Fe 3 ions to balance the charge. The composition of non-stoichiometric compounds usually varies continuously over a narrow range. Thus, the formula for wÃÆ'¼stite is written as Fe 1-x O, where x is a small number (0.05 in the previous example) which represents the deviation from the "ideal" formula. Nonstoichiometry is essential in a three dimensional solid polymer that can tolerate errors. To some extent, entropy pushes all solids into non-stoichiometry. But for practical purposes, this term explains the material in which non-stoichiometry can be measured, usually at least 1% of the ideal composition. Iron sulfur
Monosulfide from transition metals is often nonstoichiometric. The best known is probably the nominal iron (II) sulfide (mineral pyrrhotite) with the composition of Fe 1- x S/O> ). The rare stoichiometric deposits of FeS are known as troilite minerals. Pirhotit is extraordinary because it has many polytypes, ie different crystalline forms in symmetry (monoclinic or hexagonal) and composition (Fe 7 S 8 , Fe 9 S 10 , Fe 11 S 12 and others). These ingredients are always iron deficiency due to a lattice defect, ie iron void. Regardless of the defect, this composition is usually expressed as a large number ratio and relatively high crystal symmetry. This means that iron voids are not randomly scattered over the crystal, but form certain regular configurations. Those vacancies greatly affect the magnetic properties of pyrrhotite: magnets increase with vacancy concentrations and none for stoichiometry of FeS. Palladium hydrides
Palladium hydride is a nonstoichiometric material of the approximate composition of PdH x (0.02 & lt; x & lt; 0.58). This solid conducts hydrogen based on the mobility of hydrogen atoms in solids.
Tungsten oxide
It is sometimes difficult to determine whether a material is not stoichiometric or if the best formula is represented by a large number. Tungsten oxide describes this situation. Starting from the ideal tungsten trioxide material, one can produce a series of related materials that lack a bit of oxygen. These oxygen-deprived species can be described as WO 3- x , but in fact they are stoichiometric species with large unit cells of the formula W n O 3 n -2 , where n = 20, 24, 25, 40. Thus, the last Species can is described with the stoichiometric formula W 40 118 , while the non-stoichiometric description WO 2.95 implies a random distribution of the oxide void.
Other cases
At high temperatures (1000 Â ° C), titanium sulfide presents a series of non-stoichiometric compounds.
The Prussian blue coordination polymer, substantially Fe 7 (CN) 18 , is well known for forming in non-stoichiometric proportions, and the non-stoichiometric phase exhibits beneficial properties vis- ÃÆ' -vis their ability to bind cesium and thallium ions.
Maps Non-stoichiometric compound
Apps
Oxidation of catalysis
Many useful compounds are produced by hydrocarbon reactions with oxygen, a conversion catalyzed by a metal oxide. This process operates through the transfer of "lattice" oxygen to the hydrocarbon substrate, a step which temporarily produces a vacuum (or defect). In the next step, the lost oxygen is replenished by O 2 . Such a catalyst relies on the ability of the metal oxide to form a non-stoichiometric phase. The sequence of analog events describes the types of other atom transfer reactions including hydrogenation and hydrodesulfurization catalyzed by solid catalysts. This consideration also highlights the fact that stoichiometry is determined by the interior of crystals: crystal surfaces often do not follow mass stoichiometry. The complex structure on the surface is described by the term "surface reconstruction".
ion conduction
Atomic migration in solid form is strongly influenced by defects associated with non-stoichiometry. These defective sites provide a pathway for atoms and ions to migrate through a collection of atoms forming crystals. Oxygen sensors and solid state batteries are two applications that depend on the oxide vacuum. One example is the CeO-based sensor 2 in the automotive exhaust system. At low partial pressures O 2 , the sensor allows the introduction of increased air to produce more complete combustion.
Superconductivity
Many non-stoichiometric superconductors. For example, yttrium barium oxide copper, arguably the most famous high-temperature superconductor, is a non-stoichiometric solid with the formula Y x Ba 2 Cu < sub> 3 O 7- x . The critical temperature of a superconductor depends on the exact value of x . The stoichiometric species has x = 0, but this value can be 1.
History
It was primarily through the work of Nikolai Semenovich Kurnakov and his disciples that Berthollet's opposition to Proust law proved to have benefits for many solid compounds. Kurnakov divides the non-stoichiometric compounds into berthollides and daltonides depending on whether their properties exhibit monotonous behavior with respect to composition or not. The term berthollide was accepted by IUPAC in 1960. The names were derived from Claude Louis Berthollet and John Dalton, who in the 19th century advocated the theory of competition of substance compositions. Although Dalton "won" for the most part, it was later recognized that the law of proportion certainly had an important exception.
Further reading
- F. Albert Cotton, Geoffrey Wilkinson, Carlos A. Murillo & amp; Manfred Bochmann, 1999, Advanced Inorganic Chemistry, 6th Edn., Pp.Ã, 202, 271, 316, 777, 888. 897, and 1145, New York, NY, USA: Wiley-Interscience , ISBN. 0471199575, view [3], accessed July 8, 2015.
- Roland Ward, 1963, Nonstoichiometric Compounds , Progress in Chemistry , Vol. 39, Washington, DC, USA: American Chemical Society, ISBN 9780841222076, DOI 10.1021/ba-1964-0039, see [4], accessed July 8, 2015.
- J. S. Anderson, 1963, "Current problem in nonstoichiometry (Ch 1)," in Nonstoichiometric compounds (Roland Ward, Ed.), Pp. 1-22, Progress in Chemistry series, Vol. 39, Washington, DC, USA: American Chemical Society, ISBN 9780841222076, DOI 10.1021/ba-1964-0039.ch001, see [5], accessed July 8, 2015.
See also
- F-Center
References
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