Impurity incorporation reaction

The discussion so far has applied to pure crystals. Most crystals are not pure, however, and their properties, especially electrical and optical, are often dominated by the presence of trace amounts of impurities (see Worked Example 6.3). These impurities cannot be avoided and even if the starting raw materials are exceptionally pure, it is difficult to maintain the purity levels during subsequent high-temperature processing. The next task is thus to consider impurity incorporation reactions — once again, a task that very rapidly gets out of hand, what with literally thousands of... 

Figure 6.5 Bookkeeping technique for impurity incorporation reactions, a) CaCl2 in NaCl leaves a vacancy on cation sublattice, b) An alternate reaction is for the extra Cl ion to go interstitial. This reaction is unlikely, however, given the large size of the Cl ion. (c) AI2O3 in MgO creates a vacancy on the cation sublattice, (c/) MgO in AhOj creates a vacancy on the anion sublattice.

Traditionally, CVD reaction data have been reported in terms of growth rates and their dependence on temperature. The data are often confounded by mass-transfer effects and are not suitable for reactor analysis and design. Moreover, CVD reaction data provide little insight, if any, into impurity incorporation pathways. Therefore, the replacement of traditional macroscopic deposition studies with detailed mechanistic investigations of CVD reactions is an area of considerable interest. A recent, excellent review of CVD mechanistic studies, particularly of Si CVD, is available (98), and the present discussion will be limited to highlighting mechanisms of Si CVD and of GaAs deposition by MOVCD as characteristic examples of the combined gas-phase and surface reaction mechanisms underlying CVD. 

It is possible to obtain such silicates as solids by evaporation to dryness of the organic quaternary ammonium silicate aqueous solutions. In this case, however, the process is very slow and accordingly a problem of impurity incorporation arises. It was reported that silicate solids consisting of the double four-membered ring structure were abruptly separated out after an exothermal reaction on stirring the mixture of (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution and tetraethoxysilane (26). 

The oxidative degradation of polymers involves free-radical chain reactions. For example, degradation of polyolefins such as PE is commonly initiated by hydroperoxide impurities incorporated during synthesis and processing. 

The discussion so far has been with respect to pure crystals. In reality, most crystals are not pure. Impurities can substitute for host ions of electronegativity nearest their own. For example, in NaCl, Ca and O can occupy the cation and anion sites, respectively. In writing a defect incorporation reaction, the following simple bookkeeping operation can be of help ... 

Exactly the opposite occurs, namely the conversion of an ex situ parameter to an in situ one, if foreign components become sufficiently mobile. The corresponding incorporation reaction then becomes reversible. Under such conditions it is natiurally better to speak of solubility equihbria. Important examples are segregation equilibria of impurities at very high temperatures, another refers to the incorporation of protonic defects in oxides by the dissolution of H2O. Materials interesting in this respect are CaO-doped Z1O2 [196] or acceptor-doped perovskites [197], such as the Fe-doped SrTiOs discussed above. (As before we regard the acceptor dopant... 

Recently, some hght was shed on these and other compounds as well as on their structures and properties. Today it may be considered that some of the mistakes made earher could have been avoided by the availability of elements or compounds of higher purity, and thereby have prevented the incorporation of unexpected impurities. In this context it is important to note some experimental principles of solid-state synthesis that should be considered In solid-state synthesis, great attention should be always dedicated to the purity of the starting materials, the purity of the container material (and gas atmosphere) used in the reaction, and the control and adjustment of the reaction conditions in order to finally obtain a high yield product... 

Molinari and Parravano (30) have also noted that the incorporation of a donor impurity (Al, Ga) into ZnO specimens promotes the exchange reaction, while an acceptor impurity (Li) slows it down. 

Various international pharmacopoeias help assure the quality of drugs worldwide. These pharmacopoeias constantly review and revise their monographs. A different impurity profile can be anticipated if a drug s production process is changed this results in the development of new analytical methods that need to be incorporated in the pharmacopoeias. In earlier editions, color reactions were performed for identification and purity evaluation purposes. 

If complexity is suspected from the kinetic behavior, the effect of products and of possible impurities and the occurrence of side reactions should be considered. Later we shall see that medium composition (Sec. 2.9), temperature and pressure (Sec. 2.3) are other important parameters that affect rate. The rate law incorporating these effects is obtained by further experiments of the type indicated in step 3. 

The synthetic route should aim at incorporating the label as late as possible in the sequence. This requires the development of rapid syntheses (generally not more than 3 h for compounds) including HPLC purification and formulation of the radiopharmaceutical for intravenous injection. The large amounts of reagents compared to those of the labelled substrate [20] usually lead to rapid reactions. However, unexpected labelled compounds can also arise from side reactions of reagents in excess or from reactive impurities present in the reaction medium [21]. 

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