Ionic charge compensators

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

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

Mossbauer spectra of calcined samples (Table 1). The Fe3+(3) and Fe3+(4) components are probably located in tetrahedral (framework) positions. The charge distribution around the Fe3+(3) is asymmetric (large QS), thus here the charge compensation is probably provided by Fl+, i.e. indicating the existence of Bronsted sites. The charge symmetry around Fe3+(4) is more symmetric, thus the counterion is probably Na+ or Fe(OFl)+. Fe2+ ions are probably located outside of the framework (due to their larger ionic radius). Thus, in the hydrogen a small part of Fe3+ is reduced to Fe2+, and is probable removed to extra-framework sites. 

It is very frequent in minerals that charge compensation takes place via four elements having two and two the same sum of oxidation munbers, and roughly comparable ionic radii the other way round, such as Na(I) 4-Si(IV) replacing an equal amount of Ca(II) - -A1(III). We already mentioned the substitution... 

Luminescence spectra of hardystonite under 266 nm laser excitation reveal an extremely strong, rather narrow UV band at 355 nm, with a very short decay time of 25 ns (Fig. 4.20b). Usually such bands in minerals are attributed to Ce luminescence. However as another band was already confidently ascribed to this center (Fig. 4.20a) assignment appears problematic. In principle it is possible that several different Ce " centers occur in a structure, which are formed, for example, as a result of substitutions on Ca and Zn positions or because of different types of charge compensations. The first possibihty may be excluded based on the large differences in ionic radii of Ce " (115 ppm) and Zn " " in tetrahedral coordination (74 ppm), while the second possibihty may be taken into consideration. 

It is interesting to note that in magmatic apatites the luminescence of uranium containing centers have not been discovered before or after oxidizing heating. Thus it is reasonable to suppose that uranium is present mainly in the 11" + form. The U with an ionic radius of 0.97 A may be located in the apatite structure instead of Ca with the ionic radius of 0.99 A. The most likely way for achieving the excess charge compensation is the Na" for Ca " structural substitutions. 

This shows that the only effect of using the wrong set of charges is to increase the value of Rq by B ln(k). Fitting the value of i o empirically therefore automatically compensates for an inappropriate choice of ionic charge. The value of k does not even have to be the same for all bonds, only for the bonds that use the same value... 

The diffusion of the charge-compensating counterions through the thin films determines the response time of the systems during redox switching. A more open polymer morphology therefore enhances the ionic mobility and yields a faster response [40]. 

The crystal structure of LiYF4 (YLF) is shown in fig. 7 (Goryunov and Popov, 1992). The space group is I4i/a, the crystal system is tetragonal and the lattice constants are a = 5.171 A and c = 10.748 A. Considering the ionic radii, valency and charge compensation, trivalent... 

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