Vacancies anions and

In crystals of more complex formula, such as titanium dioxide, TiC>2, a Schottky defect will consist of two anion vacancies and one cation vacancy. This is because it is necessary to counterbalance the loss of one Ti4+ ion from the crystal by the absence of two O2- ions in order to maintain composition and electroneutrality. This ratio of two anion vacancies per one cation vacancy will hold in all ionic compounds of formula MX2. In crystals like A1203, two Al3+ vacancies must be balanced by three O2- vacancies. Thus, in crystals with a formula M2X3, a Schottky defect will consist of two vacancies on the cation sublattice and three vacancies on the anion sublattice. These vacancies are not considered to be clustered together but are distributed... 

As the partial pressure of X2 falls away from that at the stoichiometric point, reduction will occur. Anion vacancies and electrons would be expected to be more favored than cation vacancies and holes. The relevant equation is... 

A similar relation holds for anionic vacancies and for diffusion through interstitial migration (replace [Vm] and TTym. nd so on). 

Figure 1.12. Principle of crystallographic shear (CS) in Re03 (a) formation of anion vacancies and (b) elimination of the vacancies by crystal shear and collapse, from corner-sharing octahedra to edge-sharing octahedra forming extended CS plane defects.

Dynamic ETEM experiments on CS defects have shown mat mey consume anion vacancies and grow (figure 3.7). These correlation studies indicate mat CS planes are secondary or detrimental to catalytic reactivity. They eliminate anion vacancies by accommodating the supersaturation of the vacancies in the reacting oxide catalyst and me catalyst reactivity (selectivity) begins to decrease with the onset of CS formation, i.e. CS planes are the consequence of catalyst reduction reactions rather than the origins of catalytic reactivity (Gai 1981,1992, 1993, Gai etfl/ 1982). 

The increase of the catalytic activity in the room-temperature oxidation of carbon monoxide, which results from the increase of the temperature of preparation of NiO from 200° to 250°C., is related to the difference in the reactivity of oxygen adsorbed on both surfaces. The interaction between adsorbed oxygen and carbon monoxide has roughly the same velocity on both oxides. But on NiO (200) this interaction yields only C02(ads)> whereas on NiO(250) the same interaction produces C03"(ads) on the most active sites (anionic vacancies) and C02(g) on the less... 

The BaFBr Eu2 + phosphors are gaining increasing industrial importance because of their storage effect. On irradiation with X rays (or UV light) some of the Eu2 + ions are ionized to Eu3+ with loss of electrons to the conduction band. These electrons are captured by F+ centers (anion vacancies) and can be liberated from these traps by the action of light (stimulation). The electrons return to the Eu3 + ions via the conduction band, converting them to Eu2+ [5.413]-[5.417],... 

Ideas about the tunneling mechanism of the recombination of donor acceptor pairs in crystals seem to be first used in ref. 51 to explain the low-temperature of photo-bleaching (i.e. decay on illumination) of F-centres in single crystals of KBr. F-centres are electrons located in anion vacancies and are generated simultaneously with hole centres (centres of the Br3 type which are called H-centres) via radiolysis of alkali halide crystals. 

A cation vacancy may be paired with a nearby cation interstitial. This is called a Frenkel pair. An example is the formation of Zn+2 vacancies and Zn+2 interstitials in ZnO. This is illustrated in Figure 5.2B. In principle, paired anion vacancies and interstitials are possible, but this is less likely because of the larger size of the anions. 

Abstract. An embedded-cluster approximation is adopted for simulating the heterolytic dissociation of hydrogen at two intrinsic defects on the (001) surface of magnesium oxide the isolated anion vacancy, and the tub divacancy. The dissociation process is shown to be critically dependent on the structure of the electrostatic field at the surface both as concerns energetics and final configuration. 

The reason that our development so far has been restricted to cases of cation interstitial (or anion vacancy) and electron transport, as illustrated in Fig. 13, is that these are the species which have their sources at the metal—oxide interface. To consider the other possibilities of cation... 

THE CASE OF OXIDE GROWTH BY DIFFUSION OF ANION VACANCIES AND ELECTRONS... 

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). 

F -centers (i.e., two electrons trapped in the same anion vacancy) and M-centers (two electrons occupying two adjacent anion vacancies, i.e., two adjacent F-centers) can also be present. Symons [78] observed by electron spin resonance spectroscopy that in solid alkali halide crystals doped with metal the F-centers are the most stable, while F -centers and M-centers may be formed at higher concentrations of trapped electrons. Durham and Greenwood [79] proposed that the dissolved metal dissociated into metal cations and nearly free electrons scattered in the conduction band. 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

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