Charge compensation

Rutile pigments, prepared by dissolving chromophoric oxides in an oxidation state different from +4 in the mtile crystal lattice, have been described (25,26). To maintain the proper charge balance of the lattice, additional charge-compensating cations of different metal oxides also have to be dissolved in the mtile stmcture. Examples of such combinations are Ni " + Sb " in 1 2 ratio as NiO + Sb202, + Sb " in 1 1 ratio as Cr202 + Sb O, and Cr " +... 

Many pigments having such substitutions have been commerciali2ed. The most important one is the Ti—Ni—Sb yeHow pigment having nickel oxide [12035-36-8], NiO, as the chromophoric component and Sb " as the charge-compensating cation. 

These conjugated polymers can be chemically and electrochemically reduced and reoxidized in a reversible manner. In all cases the charges on the polymer backbone must be compensated by ions from the reaction medium which are then incorporated into the polymer lattice. The rate of the doping process is dependent on the mobiHty of these charge compensating ions into and out of the polymer matrix. 

Electrogenerated conducting polymer films incorporate ions from the electrolyte medium for charge compensation (182). Electrochemical cycling in an electrolyte solution results in sequential doping and undoping of the polymer film. In the case of a -doped polymer, oxidation of the film results in the... 

A fraction x of the Mn4+ ions is missing in the manganese sublattice. For charge compensation, each Mn4+ vacancy is coordinated by four protons in the form of OH anions at the sites of the O ions. 

It was reabzed early on that because of their high electron transport rates, the charging rates of conducting polymer films would be controlled predominantly by the rate at which charge-compensating ions [Eq.(l)] could be extracted from, or ejected into, the bathing electrolyte solution.160,161 However, these and some other studies employing chronoam-... 

The new phases were discovered by the combination of exploratory synthesis and a phase compatibility study. As commonly practised, the new studies were initially made through the chemical modification of a known phase. Inclusion of salt in some cases is incidental, and the formation of mixed-framework structures can be considered a result of phase segregation (for the lack of a better term) between chemically dissimilar covalent oxide lattices and space-filling, charge-compensating salts. Limited-phase compatibility studies were performed around the region where thermodynamically stable phases were discovered. Thus far, we have enjoyed much success in isolating new salt-inclusion solids via exploratory synthesis. 

Vo) in the crystal. (Vo) can catch electrons to form F and centers. (Pb) is also able to attract electrons while (Vb)" can trap holes to give rise to color centers. They vdll make a contribution to the X-ray irradiation-induced absorption. Of course, the charge balance of the crystal is kept by charge compensation among these defects. Regretfully, the detailed characterization of these defects is too difficult to cover here and further experiments need to be performed. 

PMo 12-polymer composite film catalyst [9]. This demonstrates that PM012 catalyst was not in a crystal state but in an amorphous-like state, indicating that PM012 catalyst was molecularly dispersed on the PS support via chemical interaction. As attempted in this work, it is believed that heteropolyanions (PMoi204o ) were strongly immobilized on the cationic sites of the PS bead as charge-compensating components. 

Tlius, the charge compensation mechanism represents the single most important mechanism which operates within the defect ionic solid. 

A considerable body of scientific work has been accomplished in the past to define and characterize point defects. One major reason is that sometimes, the energy of a point defect can be calculated. In others, the charge-compensation within the solid becomes apparent. In many cases, if one deliberately adds an Impurity to a compound to modify its physical properties, the charge-compensation, intrinsic to the defect formed, can be predicted. We are now ready to describe these defects in terms of their energy and to present equations describing their equilibria. One way to do this is to use a "Plane-Net". This is simply a two-dimensional representation which uses symbols to replace the spherical images that we used above to represent the atoms (ions) in the structure. 

The alkali halides cire noted for their propensity to form color-centers. It has been found that the peak of the band changes as the size of the cation in the alkali halides increases. There appears to be an inverse relation between the size of the cation (actually, the polarizability of the cation) and the peak energy of the absorption band. These are the two types of electronic defects that are found in ciystcds, namely positive "holes" and negative "electrons", and their presence in the structure is related to the fact that the lattice tends to become charge-compensated, depending upon the type of defect present. 

They are usually joined along the 110 plane of the lattice of the face-centered salt crystal, although we have not shown them this way (The 100 plane is illustrated in the diagram). Note that each vacancy has captured an electron in response to the charge-compensation mechanism which is operative for all defect reactions. In this case, it is the anion which is affected whereas in the "F-center", it was the cation which was affected (see equation 3.2.8. given above). These associated, negatively-charged, vacancies have quite different absorption properties than that of the F-center. 

In the last equation, we have the instance where charge-compensation has occurred due to inclusion of a monovalent cation. A vacancy does not form in this case. All of these equations are cases of impurity substitutions in the normal lattice.. 

We have shown that defects occur in pairs. The reeison for this lies in the charge-compensation principle which occurs in aU ionic solids. 

Consider the crystal, AgBr. Both cation and anion are monovalent, i.e.- Ag+ and Br-. The addition of a divalent cation such as Cd2+ should introduce vacancies, VAg, into the crystal, because of the charge-compensation mechanism. To maintain electro-neutrality, we prefer to define the system as ... 

While this maybe true for the reaction in 4.7.2., l.e. - Mi2+ <. Xj, what of the case for BaSiOs where diffusion was limited to one direction It is not reasonable to assume that the solid would build up a cheirge as the Mj2+ ions are diffusing (and the Si04= ions are not) and we must search for compensating species elsewhere. It turns out that charge compensation occurs by diffusion of chained vacancies in the lattice. 

In one case, the diffusion of A + is much faster than b3+ and in the other the opposite is true. Note that charge-compensation of migrating species is maintained in aU cases. 

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