Solids with interstitial anions

Note that, in this case, the presence of oxygen helps create a defect, or more specifically an interstitial anion. 

These electrons being from the material s valence band, there is an electron hole conduction. 

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Solids with interstitial anions (see Figure 4.9) In this case, the two steps are written ... 

Ionic binary solids with interstitial A anions... 

Ionic binary solids with interstitial A anions are characterized by the following conditions on concentrations [AJ 0 and [V3] = [VJ = [Bj = 0. Thus, the simplified expression for the distance from stoichiometry for B is, according to equation [2.12],... 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

In order to decide whether the nonstoichiometric phases contain interstitial anions, or vacant cation sites, we compared the pycnometric density with the calculated density for each type of defect. The experimental values agree with the last type of defects, and these solid solutions are represented using Rees s notation by... 

Our binary solid thus has a defect associated with the component j (vacancy or interstitial anion or cation) that we can indicate by S and which is carrying an electric charge We define the coefficient of diffusion of this defect starting from following flux... 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

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. 

If the semiconductor is an ionic solid, then electrical conduction can be electronic and ionic, the latter being due to the existence of defects within the crystal that can undergo movement, especially Frenkel defects (an ion vacancy balanced by an interstitial ion of the same type) and Schottky defects (cation and anion vacancies with ion migration to the surface). This will be discussed further in Chapter 13, as ionic crystals are the sensing components of an important class of ion selective electrodes. 

The influence of substrate structure is briefly as follows. The active entity (H in hydrogenation and dehydrogenation reactions) must be adsorbed, even in the surface layer, in sites which make crystallographic sense, as these proper sites will be those of lowest potential energy on the surface. For example, atoms chemisorbed on metals will be located in interstitial surface sites which fall on the lattice of interstitial sites for the crystal as a whole. Adsorption immediately above surface atoms is improbable, adsorption in surface interstices probable. Diffusion into the solid from the surface is then by a path similar to the subsequent stages in the bulk solid. The oxygen atoms adsorbed on oxides will occupy proper anion lattice sites and will either create or destroy defects. The defects created will be able to diffuse immediately beneath the surface and appear at the site where their properties are required. Molecular reactants of any size cannot usually penetrate the catalyst surface, but atoms or radicals may be separated from them. The molecule itself is chemisorbed with efficiency only if certain structural inter-relations are satisfied. Beeck and coworkers (62) have demonstrated this adequately for the adsorption of ethylene on various metals and on various crystal planes of the same metal. The adsorption of gases... 

In atomic or molecular sohds, common types of point defects are the absence of an atom or molecule from its expected position at a regular lattice site (a vacancy), or the presence of an atom or molecule in a position which is not on the regular lattice (an interstitial). In ionic solids, these point defects occur in two main combinations. These are Schottky defects, in which there are equal numbers of cation and anion vacancies within the crystal, and Frenkel defects, in which there are cation vacancies associated with an equal number of "missing" cations located at non-lattice, interstitial positions. Both are illustrated in Figure 1.1. Point defects are also found in association with altervalent impurities, dislocations, etc., and combinations of vacancies with electrons or positive holes give rise to various types of colour centres (see below). 

Thermal shock produces an essential number of OFF defects in the oxygen sublattice. The concentration of defects grows rapidly during first picoseconds of simulation and than tends to stabilize. In the metastable state obtained about 40% of interstitial sites are populated by displaced anions (see Fig. 1). Kinetics of evolution of defects may be effectively studied by an instant dropping of temperature (quenching) of overheated solid up to temperature about 1000 K. At this temperature another short MD run (about -3-5 ps) was performed to study the relaxation of defect concentration. As it follows from Fig. 2, the defect concentration decreases almost exponentially with a relaxation time of - 2 ps. 

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