Nonstoichiometric Defects

In nonstoichiometric defect reactions, the composition of the crystal changes as a result of the reaction. Said otherwise, a nonstoichiometric reaction is one in which mass is transferred across the boundaries of the crystal. The possible number of nonstoichiometric defect reactions is quite large, and covering even a fraction of them is not feasible here. The best that can be done is to touch on some of their more salient points. 

Another possible nonstoichiometric defect reaction is one in which oxygen is incorporated into the crystal interstitially, i.e.. 

Nonstoichiometric defect reactions, with the selective addition or removal of one of the constituents, naturally lead to the formation of... 

With the notable exception of transition metal oxides that generally exhibit wide deviations from stoichiometry, the concentration of intrinsic or nonstoichiometric defects in most ceramic compounds is so low that their defect concentrations are usually dominated by the presence of impurities. 

It should be noted that both the stoichiometric and nonstoichiometric defect structures may prevail in the same compound, depending on the activities (partial pressures) of the components in the compound. 

Nonstoichiometric defects form by the selective addition or loss of one (or more) of the constituents of the crystal chemistry, and consequently lead to... 

In nonstoichiometric defect reactions, the composition changes. In other words, mass is transferred across the boundaries of the crystal. One common nonstoichiometric reaction is found to occur at low oxygen partial pressures. This is shown in Figure 10.3. In this case, oxygen leaves the crystal, and the reaction is given in Equation 10.6. 

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. ... 

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

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... 

It is interesting to note that cobalt cobaltite, C03O4, is a good catalyst, too, for anodic chlorine evolution. In this case, too, a correlation is observed between the reaction rate and the spinel s defect concentration (amount of nonstoichiometric oxygen). 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

The first sample has the greatest deviation from stoichiometry. Samples JV°1 - JV°4 have been found to be nonstoichiometric oxide -hydroxide type. They have a mixed conductivity - ionic (o,) and electronic (ae). The ionic one is due to the presence of OH" - groups. Namely, they stabilize the defects of chemical nature in such compounds. These defects are determined by the presence of Mn4+ and Mn3+ in the same crystallographic position. 

Oxide compounds are widely used as cathodic materials in the power sources and electrochemical generators. Some literature data indicates that cathodic materials based on nonstoichiometric oxide compounds make it possible to increase the solid-phase reduction process. The kinetics of electrochemical reactions and consequently the current density are the higher, the greater the degree of deviation from stoichiometry, and the lager the number of the defects in the compounds structure [1,2]. 

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. 

These point defect models need to be regarded as a first approximation. Calculations for stoichiometric GaAs suggest that balanced populations of vacancies on both gallium and arsenic sites, VGa and VAs, exist, as well as defect complexes. Calculation for nonstoichiometric materials would undoubtedly throw further light on the most probable defect populations present. 

Even when the composition range of a nonstoichiometric phase remains small, complex defect structures can occur. Both atomistic simulations and quantum mechanical calculations suggest that point defects tend to cluster. In many systems isolated point defects have been replaced by aggregates of point defects with a well-defined structure. These materials therefore contain a population of volume defects. 

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 ... 

There are many ways in which these square antiprism and cuboctahedral defect clusters can be arranged. A nonstoichiometric composition can be achieved by a random distribution of varying numbers of clusters throughout the crystal matrix. This appears to occur in Ca0.94Y0.06F2.06> which contains statistically distributed cuboctahedral clusters. 

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