Defect ionization

The effectively neutral atomic defects may trap or donate electrons, thus acquiring a charge, the sign of which will depend on the chemical nature ofthe defects and their surroundings. The following quasi-chemical reactions provide examples ofthe point-defect ionization processes ... 

Figure 3.3 Examples of energy level diagrams for defect ionization, (a) Frenkel defects (b) Schottky defects.

Figure 9.15 Comparison among absorption spectrum of a PPPV film (curve labeled a, dashed bands indicate inhomogeneously broadened So <-5, 0 <-0, and 1 <-0 transitions) photocurrent action spectrum (/p) and fraction of photons absorbed [1 - exp(-oaf)] (right scale). The low-energy peak of Ip is due to defect ionization. The photocurrent was measured on a 6-pm-thick sandwich sample at a field of 10 V cm- and 295 K. (From Gailberger, M. and Bassler, H., Phys. Rev., B44, 8643, 1991. With permission.)...

A number of diamagnetic defects are also beheved to exist in vitreous siUca. Because there is no direct way to study these species, their identification is either done indirecdy, such as by uv absorption, or by employing esr after the material has been made paramagnetic using ionizing or laser irradiation. 

Yes caution—in certain cases electron bombardment may ionize or create defects... 

We will deal with the first two cases, namely 1) defects in the stoichiometric case and 2) ionization of defects in stoichiometric compounds. Since the case of non-stoichiometiic compounds can be very complicated, we will present treatment of the last two in the Appendix at the end of this Chapter. 

We have also shown that the intrinsic defects can become ionized to form species not found in nature. 

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 ... 

Although we win not treat the other types of pairs of defects, it is well to note that similar equations can also be derived for the other intrinsic defects. What we have really shown is that external reactants can cause further changes in the non-stoichiometry of the soUd. Let us now consider ionized defects. It should be clear that an external gaseous factor has a major effect upon defect formation. The equations given above are very complicated and represent more closely what actually happens in the real world of defect formation in crystals. 

In the non-stoichiometric case where ionization of defects is the norm, the mathematics become too complicated so that the equations are not solvable. However, we can use a thermodynamic method to obtain the results we want. We will present here the case of silver bromide whose use in photographic film highlights the use of defect chemistry for practical purposes. 

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