Stoichiometric defect

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... 

Due to participation in oxidation-reduction reactions the reducing or inflammable gases affect the stoichiometricity of oxide and, consequently the concentration of stoichiometric defects which usually control the dope electric conductivity of adsorbent [26, 67, 85, 86, 90]... 

M. Razeghi, LP-MOCVD Growth, Characterization, and Application of InP Material T.A. Kennedy and P.J. Lin-Chung, Stoichiometric Defects in InP... 

Defects fall into two main categories intrinsic defects which are integral to the crystal in question—they do not change the overall composition and because of this are also known as stoichiometric defects and extrinsic defects which are created when a foreign atom is inserted into the lattice. 

CQ is the concentration of available sites for the crystallization of barium sulfate. This is equal to the sum of the initial concentration of already precipitated sulfate in the medium (Cg0) and of the potentially precipitable sulfate, equal to the concentration of that of the two reactants (Ba + or S0 2-) n stoichiometric defect. It was checked that the addition of EDTA did not change the rate of reaction. 

At a specified point in the tank, 100 cm of hydrochloric acid 1.2N(Cg0 = 0.83 moles. m- after complete mixing in the tank in the absence of reaction) were injected by means of a cylinder obturated at its lower end by an elastic membrane which becomes inflated and bursts out when submitted to the pressure of the liquid pushed by a piston. The acid was thus injected without any preferential direction. This locally released acid B triggers reactions f2j and 3]. If the local micromixing state is perfect, the acid is totally and instantaneously neutralized, as it is in stoichiometric defect with respect to A. The first reaction being very fast as compared to the second one, the precipitate S does not appear. Conversely, if mixing of the acid is not instantaneously... 

What determines whether nonstoichiometric behavior can readily be observed is largely the form of the dissociation pressure-composition isotherms in the equilibrium diagram. If the free energy rises too steeply with excess of either component, it may not be practicable to detect stoichiometric changes which, in principle, are taking place. The explicit relation between the stoichiometric defect (= x for the composition MOx+a.) and the free energy is complicated and awkward to work... 

Figure 1. Reduced pressure, p(x)/p(0) for phase ABX + x as dependent on stoichiometric defect...

Association effects clearly vary widely in magnitude from one substance to another. As with electrolytes in solution, the nonionic terms in the potential function for interaction may be critically important. Bloem s detailed analysis of equilibria in the Pb1 (rS system shows that although the total stoichiometric defect... 

The second type of stoichiometric defect involves the movement of an ion on to an interstitial site. An interstitial site is a gap in the lattice, which is not a normal lattice site. The silver halides most commonly possess this type of defect, where the silver cation moves on to the interstitial site. The cation moves into a site in the centre of a cube of cations and anions, as shown in Figure 6.3. 

Non-stoichiometric defects can occur by oxidation and reduction of the cations, by introduction of interstitials, or by loss of the anions. 

Stoichiometric Defects. There are some defects that leave the stoichiometry unaffected. One type is the Schottky defect in ionic crystals. A Schottky defect consists of vacant cation and anion sites in numbers proportional to the stoichiometry thus, there are equal numbers of Na+ and Cl- vacancies in NaCl and 2C1- vacancies per Ca2+ vacancy in CaCl2. A Schottky defect in NaCl is illustrated in Fig. 2-14(a). 

Non-stoichiometric Defects. These often occur in transition-metal compounds, especially oxides and sulfides, because of the ability of the metal to exist in more than one oxidation state. A well-known case is FeO which consists of a cep array of oxide ions with all octahedral holes filled by Fe2 + ions. In reality, however, some of these sites are vacant, while others—sufficient to maintain electroneutrality—contain Fe3 + ions. Thus the actual stoichiometry is commonly about Fe0 950. Another good example is TiO which can readily be obtained with compositions ranging from Ti0 740 to Tij 670 depending on the pressure of oxygen gas used in preparing the material. 

A stoichiometric defect reaction by definition is one where the chemistry of the crystal does not change as a result of the reaction. Said otherwise, a... 

Stoichiometric reaction is one in which no mass is transferred across the crystal boundaries. The three most common stoichiometric defects are Schottky defects, Frenkel defects, and antistructure disorder or misplaced atoms. 

The stoichiometric defects are Schottky defects for which the formation enthalpy is Ahs-... 

In compound crystals, balanced-defect reactions must conserve mass, charge neutrality, and the ratio of the regular lattice sites. In pure compounds, the point defects that form can be classified as either stoichiometric or nonstoichiometric. By definition, stoichiometric defects do not result in a change in chemistry of the crystal. Examples are Schottky (simultaneous formation of vacancies on the cation and anion sublattices) and Frenkel (vacancy-interstitial pair). 

What stoichiometric defect do you think such a structure would favor Why ... 

In both cases the first reaction (Equations 4.33a and 4.34a) is instantaneous (e.g., a neutrahzation) while the second reaction has characteristic reaction time comparable to the characteristic mixing time. The reactant A2 is added in an overall stoichiometric defect to the stream of Aj in consecutive reactions or to the stream of solution containing A and A in parallel reactions. If the aggregates of A2 are rapidly mixed forming homogenous solution with a rate much faster than the rate of the second reaction t. ), species A2 will be almost totally consumed by... 

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