Vacancies defects from

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

The various types of point defect found in pure or almost pure stoichiometric solids are summarized in Figure 1.17. It is not easy to imagine the three-dimensional consequences of the presence of any of these defects from two-dimensional diagrams, but it is important to remember that the real structure of the crystal surrounding a defect can be important. If it is at all possible, try to consult or build crystal models. This will reveal that it is easier to create vacancies at some atom sites than others, and that it is easier to introduce interstitials into the more open parts of the structure. 

Lithium iodide pacemaker batteries use lithum iodide as the electrolyte, separating the lithium anode and the iodine anode. The function of the electrolyte is to transport ions but not electrons. Lithium iodide achieves this by the transport of Li+ ions from the anode to the cathode. This transport is made possible by the presence of Li vacancies that are generated by the intrinsic Schottky defect population present in the solid. Lithium ions jump from vacancy to vacancy during battery operation. 

Fig. 16.2 TEM image of a CNT with indications of topological defects and vacancies. Reproduced with permission from Wiley VCH (2011) [7].

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

In this chapter non-stoichiometric compounds derived from point defects have been reviewed mainly from a thermodynamical point of view, and many examples have been presented for the purpose of understanding the nature of non-stoichiometry. As mentioned above it is not necessary to take the interaction between defects for vacancies) into consideration if the... 

Both mechanisms explain the decrease of the resistance with the formation of a rooted or an isolated hydroxyl group out of an O2" of the lattice. In both cases it is assumed that the bonding to the Sn does not contribute to the concentration of free charge carriers, which implies that not all the surface tin atoms are in oxidation state +4 because otherwise the formation of the Sn—OH bond would need an electron from the conduction band. This assumption is reasonable because tin has two stable oxidation states, +2 and +4, and the most stable surface of tin dioxide, (110), can easily be conditioned to show atoms with both oxidation states. Furthermore it is known that defects like vacancies are an essential factor for the performance of Sn02 gas sensors and it probably is not realistic to base a mechanism on the situation on a perfect surface. Emiroglu et al. (2001) and Harbeck et al. (2003) proved the formation of rooted and isolated hydroxyl group on the Sn02 surface in the presence of water, so the final result is clear even if the exact mechanism still allows for speculation. 

It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiC. ... 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

A general difficulty inherent in these defect-counting and attribution stages arises from the possibility of mutual reactions of complementary defects. Suppose a characteristic color is the result of a trapped electron and that a particular catalytic activity in the same solid results from a trapped hole. If the electron is released upon warming and annihilates the particular trapped hole, then the color and the catalytic activity will seem to be connected. More complex confusion can arise if one species of trapped electron can be annihilated at several kinds of trapped hole. Similar problems can arise with atomic defects, since vacancies and interstitials have an analogous complementary relationship. 

In general, the lattice distortion can be caused both by point and extended defects generated due to incorporation of modifying cations (Zr, Ca) or anions (fluorine, residual lattice hydroxyls). Extended defects differing from the surrounding lattice by the electron density can be detected by using SAXS [6], while point defects (anion vacancies etc) are usually probed by UV-Vis [7]. 

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