Compounds nonstoichiometric

Apart from inorganic ionic oxides, sulfides, alloys, many minerals, porous solids, and the like also show composition variation. These materials raise a problem How does the structure accommodate the alteration in composition A vast number of different structural ways to account for composition variation are now known. 

If the ratio of cations to anions in a solid reflects the chemical formula, the system is said to be stoichiometric. For example, if the ratio of Na to Cl in NaCl is 1 1, the system is stoichiometric. Nonstoichiometry arises when a cation or anion with a different valency is substituted into an ionic crystal. Anion or cation vacancies must then result in order to maintain charge neutrality. For example, if a Ca were substituted for Na in NaCl, a cation vacancy would be needed to accommodate the extra charge. If was substituted for Cl , an anion vacancy would have to occur. Iron can exist with a valence of -1-2 or - -3. If a trivalent Fe was substituted for Fe in FeO, one Fe vacancy would have to be formed for each pair of Fe + incorporated. Remember, Mother Nature is pretty serious about charge neutrality. 

Some ceramic compoimds have a wide phase field of stability and can be quite nonstoichiometric. Spinel (MgAl204), for example, is shoichiometric at 50 At% MgO and 50 At% AI2O3, but is stable over quite a range of compositions. Here again a Mg vacancy must occur for each additional pair of AP+ above the stoichiometric value. On the other hand, one 0 vacancy must occur for each pair of Mg + substituted for Al above the stoichiometric number. 

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An additional problem is encountered when the isolated solid is non-stoichiometric. For example, precipitating Mn + as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnO ) where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice defects in the crystal structure. ... 

The interest of physicists in the conducting polymers, their properties and applications, has been focused on dry materials 93-94 Most of the discussions center on the conductivity of the polymers and the nature of the carriers. The current knowledge is not clear because the conducting polymers exhibit a number of metallic properties, i.e., temperature-independent behavior of a linear relation between thermopower and temperature, and a free carrier absorption typical of a metal. Nevertheless, the conductivity of these specimens is quite low (about 1 S cm"1), and increases when the temperature rises, as in semiconductors. However, polymers are not semiconductors because in inorganic semiconductors, the dopant substitutes for the host atomic sites. In conducting polymers, the dopants are not substitutional, they are part of a nonstoichiometric compound, the composition of which changes from zero up to 40-50% in... 

The electrochemical reaction drives a transition from a solid to a gel.100 The oxidation depth can be limited at any point. The composition of the nonstoichiometric compound is assumed to be uniform whatever... 

In order to relax 1 mol of compacted polymeric segments, the material has to be subjected to an anodic potential (E) higher than the oxidation potential (E0) of the conducting polymer (the starting oxidation potential of the nonstoichiometric compound in the absence of any conformational control). Since the relaxation-nucleation processes (Fig. 37) are faster the higher the anodic limit of a potential step from the same cathodic potential limit, we assume that the energy involved in this relaxation is proportional to the anodic overpotential (rj)... 

Electrochemically synthesized and then oxidized and reduced conducting polymers, such as polypyrrole, polythiophene, and polyaniline, which are amorphous, are nonstoichiometric compounds ... 

A number of metal oxides are known to form nonstoicbiometric compounds, in which the ratios of atoms that make up the compound cannot be expressed in small whole numbers. In the crystal structure of a nonstoichiometric compound, some of the lattice points where one would have expected to find atoms are vacant. Transition metals most easily form nonstoichiometric compounds because of the number of oxidation states that they can have. For example, a titanium oxide with formula TiO, I( is known, (a) Calculate the average oxidation state of titanium in this compound. 

Uranium dioxide, UQ2, can be further oxidized to give a nonstoichiometric compound U024v, where 0 < x < 0.25. See Exercise 5.77 for a description of nonstoichiometric compounds, (a) What is the average oxidation state of uranium in a compound with composition UO,j-> (b) If we assume that the uranium exists in either the +4 or the +5 oxidation state, what is the fraction of uranium ions in each ... 

This presentation is presented for those who wish to examine the mathematics of both non-stoichiometric intrinsic- defect compounds and the ionization of defects in both stoichiometric and nonstoichiometric compounds as represented by ... 

Nonstoichiometric Compounds Intrinsic defects are stoichiometric defects (i.e., they do not involve any change in overall composition). Defects can also be nonstoichiometric. In the case of extrinsic defects where the host crystal is doped with aliovalent impurities, the solid so formed is a nonstoichiometric compound because the ratio of the atomic components is no longer the simple integer. There is also... 

An example for a host molecule with a layerlike structure is graphite. Various types of both organic and inorganic inclusion compounds, as well as stoichiometric and nonstoichiometric compounds, are known. 

The carbides and nitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th, and U are considered to be typical interstitial compounds. Their compositions frequently correspond to one of the approximate formulas M2X or MX. As a rule, they are nonstoichiometric compounds with compositions ranging within certain limits. This fact, the limitation to a... 

D. J.M. Bevan, P. Hagenmuller, Nonstoichiometric Compounds Tungsten Bronzes, Vanadium Bronzes and Related Compounds. Pergamon, 1975. 

Compounds are made up of atoms of more than one chemical element. The point defects that can occur in pure compounds parallel those that occur in monatomic materials, but there is an added complication in this case concerning the composition of the material. In this chapter discussion is confined to the situation in which the composition of the crystal is (virtually) fixed. Such solids are called stoichiometric compounds. (The situations that arise when the composition is allowed to vary are considered in Chapter 4 and throughout much of the rest of this book. This latter type of solid is called a nonstoichiometric compound.) The composition problem can be illustrated with respect to a simple compound such as sodium chloride. 

Antisite defects in the pyrochore structure Er2Ti207 were mentioned previously (Section 1.10). These defects also occur in the nonstoichiometric compound Er2.09Ti194O6.952, which is slightly Er203-rich compared to the stoichiometric parent phase. The formation of the antisite pair is now accompanied by the parallel formation of oxygen vacancies ... 

Although the unit cell of a solid with a fixed composition varies with temperature and pressure, at room temperature and atmospheric pressure it is regarded as constant. If the solid has a composition range, as in a solid solution, an alloy, or a nonstoichiometric compound, the unit cell parameters vary as the composition changes. 

Nonstoichiometric compounds such as FeO, Ni, xO, LaCo03+8 LaCo03+x/2 etc. are indexed under the notionally stoichiometric phase, i.e. FeO, NiO, LaCo03. 

Anderson, J. S., in Nonstoichiometric Compounds, American Chemical Society, Washington D.C., 1963, page 1. See also Proc. Chem. Soc. 166 (1964). 

Hydrogen is capable of forming compounds with all elements except the noble gases. In compounds with nonmetals, hydrogen usually behaves like a metal instead of a nonmetal. Therefore, when hydrogen combines with a nonmetal, it usually has a +1 oxidation number. When hydrogen combines with a metal, it usually has a —1 oxidation number. Hydrogen compounds with the transition metals are usually nonstoichiometric. Nonstoichiometric compounds have no definite formula. 

PIO Gas hydrates are nonstoichiometric compounds consisting of hydrogen-bonded water molecules in a cagelike structure, which traps small-diameter gas molecules. (From Tuckerman, 1999)... 

The dioxides (and, in some measure, also the sesquioxides) are nonstoichiometric compounds oxygen-rich An02+x for light actinides An02 x from Pu (or Np) on the nonstoichiometry being related to the relatively close oxygen potentials of An and An or An" " in light actinides and of An " and An in heavy actinides. 

Wadsley, A.D., Nonstoichiometric Compounds, ed. L. Mandelcorn (1964) Academic Press. 

For any heterotype solid solution, or a nonstoichiometric compound, EDX analysis in the AEM on a large number of crystals is required. In a typical laboratory situation 30 to 40 crystals are routinely analyzed for each preparation. This sampling is adequate to establish trends in stoichiometric variations in a heterogeneous material. Fine gradations in compositions of a seemingly phase-pure material by the criterion of bulk diffraction techniques, can also be revealed. For quantitative microanalysis, a ratio method for thin crystals (16) is used, given by the equation ... 

For nonstoichiometric compounds, the general rule is that when there is an excess of cations or a deficiency of anions, the compound is an n-type semiconductor. Conversely, an excess of anions or deficiency of cations creates a / -type semiconductor. There are some compounds that may exhibit either p- or n-type behavior, depending on what kind of ions are in excess. Lead sulfide, PbS, is an example. An excess of Pb + ions creates an n-type semiconductor, whereas an excess of ion creates a /7-type semiconductor. Similarly, many binary oxide ceramics owe their electronic conductivity to deviations from stoichiometric compositions. For example, CU2O is a well-known / -type semiconductor, whereas ZnO with an excess of cations as interstitial atoms is an n-type semiconductor. A partial list of some impurity-controlled compound semiconductors is given in Table 6.9. 

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