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Solids with anion vacancies

Note that, in this case, the presence of oxygen cattses a decrease in the concentration of defects caused by the annihilation of anion vacancies and the electrons that are associated with them. [Pg.92]

Actually, the global equation does not express a real charge transfer because the electrons involved are obtained through a vacancy s ionization, these two species being initially in the same phase. [Pg.92]

the only charge transfer to be considered at equihbtium takes place during the first step, which is adsorption. [Pg.93]

Experimentally, this kind of situation is not at all realistic when sensors are concerned, since sensors necessarily use metallic electrodes. [Pg.93]

Electrochemists suggest then the use of a system containing three phases the metal oxide, the gas and the metal, which acts throngh its electrons. [Pg.93]


Solids with anion vacancies (see Figure 4.7) In this case, the two steps are ... [Pg.92]

Ionic binary solids with anion vacancies of A... [Pg.41]

The unique property of solid solutions on the basis of zirconium oxide is oxygen-ionic conductivity and it is due to their crystal structure type. The solid solution of stabilized zirconium oxide has a cubic structure of fluorite type with anionic vacancies that leads to electrical conductivity abrupt increase at temperature increase of > 600° C. [Pg.308]

The model of oxidation will depend on the nature of the main defects in each oxides and their relative layout. We choose two oxides with anion vacancies. BO layer being above the AO one, this implies that B ions are a little soluble in AO to be able to cross through this layer. We assume a solid solution of substitution and we suppose that B diffusion coefficient through AO is very high, which means that B concentration in AO has the same value at each point. [Pg.638]

The origin of the color is as follows. The electron trapped at an anion vacancy in an alkali halide crystal is an analog of a hydrogen atom. The electron can occupy one of a number of orbitals, and transitions between some of these levels absorb light and hence endow the solid with a characteristic color. F centers and related defects are discussed further in Chapter 9. [Pg.11]

EM plays a crucial role in the development of thermodynamic data, especially for defective solids, multi-phase solids and solids with coexisting intergrowth structures. These microstructural details, which are essential to catalytic properties, cannot be revealed readily by other diffraction methods which tend to average structural information. The formation of anion vacancies in catalytic reactions and the resulting extended defects are described here, from which an improved understanding of the formation of CS planes and their role in catalysis can be obtained. These general results are applicable to other CS structures. [Pg.95]

Atoms in the free surface of solids (with no neighbors) have a higher free energy than those in the interior and surface energy can be estimated from the number of surface bonds (Cottrell 1971). We have discussed non-stoichiometric ceramic oxides like titania, FeO and UO2 earlier where matter is transported by the vacancy mechanism. Segregation of impurities at surfaces or interfaces is also important, with equilibrium and non-equilibrium conditions deciding the type of defect complexes that can occur. Simple oxides like MgO can have simple anion or cation vacancies when surface and Mg + are removed from the surface,... [Pg.155]

For a 1 1 solid MX, a Schottky defect consists of a pair of vacant sites, a cation vacancy, and an anion vacancy. This is presented in Figure 5.1 (a) for an alkali halide type structure the number of cation vacancies and anion vacancies have to be equal to preserve electrical neutrality. A Schottky defect for an MX2 type structure will consist of the vacancy caused by the ion together with two X anion vacancies, thereby balancing the electrical charges. Schottky defects are more common in 1 1 stoichiometry and examples of crystals that contain them include rock salt (NaCl), wurtzite (ZnS), and CsCl. [Pg.201]

Colour centres are formed if a crystal of NaCl is heated in sodium vapour sodium is taken into the crystal, and the formula becomes Nai+/fl. The sodium atoms occupy cation sites, creating an equivalent number of anion vacancies they subsequently ionize to form a sodium cation with an electron trapped at the anion vacancy. The solid so formed is a non-stoichiometric compound because the ratio of the atomic components is no longer the simple integer that we have come to expect for well-characterized compounds. A careful analysis of many substances, particularly inorganic solids, demonstrates that it is common for the atomic ratios to be non-integral. Uranium dioxide, for instance, can range in composition from UOi 05 to UO2.25, certainly not the perfect UO2 that we might expect Many other examples exist, some of which we discuss in some detail. [Pg.246]

The NaCl structure is also found in compounds like TiO, VO and NbO, possessing a high percentage of cation and anion vacancies. Ternary oxides of the type MggMn 08 crystallize in this structure with of the cation sites vacant. Solid solutions such as Li,j )Mg Cl (0 x 1) crystallize in the rocksalt structure stoichiometric MgCl may indeed be considered as having a defect rocksalt structure with 50% of ordered cation vacancies. [Pg.20]

When divalent cation impurities (e.g. Cd, Sr ) are present in an ionic solid of the type MX consisting of monovalent ions, the negatively charged cation vacancies (created by the divalent ions) are bound to the impurity ions at low temperatures. Similarly, the oppositely charged cation and anion vacancies tend to form neutral pairs. Such neutral vacancy pairs are of importance in diffusion, but do not participate in electrical conduction. The interaction energy of vacancy pairs or impurity-vacancy pairs decreases with the increase in distance between the two oppositely charged units. [Pg.232]

Equation (1.204) shows an equilibrium between the solid and gas phases for metal M, assuming that the molecular species of metal gas is M(g). Towards the right-hand side, the reaction gives an excess metal atom (neutral), Mm, with a vacancy at an anion site, Vx. Equation (1.205) shows an equilibrium between the solid and gas phases for anion X, assuming that the molecular species of anion gas is X2(g). Towards the right-hand side, the reaction gives an excess anion (neutral), Xx, with a vacancy at a cation site, Vm- In place of these two reactions, we can choose the following reactions ... [Pg.86]

Irradiation of all kinds of solids (metals, semiconductors, insulators) is known to produce pairs of the point Frenkel defects - vacancies, v, and interstitial atoms, i, which are most often spatially well-correlated [1-9]. In many ionic crystals these Frenkel defects form the so-called F and H centres (anion vacancy with trapped electron and interstitial halide atom X° forming the chemical bonding in a form of quasimolecule X2 with some of the nearest regular anions, X-) - Fig. 3.1. In metals the analog of the latter is called the dumbbell interstitial. [Pg.139]


See other pages where Solids with anion vacancies is mentioned: [Pg.174]    [Pg.174]    [Pg.22]    [Pg.684]    [Pg.22]    [Pg.506]    [Pg.461]    [Pg.425]    [Pg.431]    [Pg.202]    [Pg.278]    [Pg.363]    [Pg.193]    [Pg.73]    [Pg.12]    [Pg.31]    [Pg.42]    [Pg.208]    [Pg.40]    [Pg.40]    [Pg.98]    [Pg.102]    [Pg.219]    [Pg.81]    [Pg.231]    [Pg.176]    [Pg.281]    [Pg.249]    [Pg.236]    [Pg.238]    [Pg.447]    [Pg.20]    [Pg.93]    [Pg.7]    [Pg.103]    [Pg.104]    [Pg.104]    [Pg.315]   


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Anion vacancy

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