Cation defect

The homogeneity ranges and defect structures of the hexaborides lead to deviations from stoichiometry through the cation defects (see Table 1). 

On the other hand, oxygen diffusion measurements, by use of as a tracer, indicate that singly charged interstitials, O, are the dominant defects of anions. (Anion defects Of are considered as minority defects compared to the cation defects.) The formation of Of is described by the chemical reaction... 

Reactions of photoelectrons with cation defects as described here have also been found for mercury halides or for oxalates of Fe and Pd. Lead halides, in contrast, possess mobile anions which can trap photoelectron holes. In this case, the absorption of photons leads immediately to a positive image, where the illuminated areas appear light. 

Frequency shift with respect to free CO experiment, 2143 an-1 calculation, 2120 an-1. cNotation Re(0) is zerovalent rhenium, Re(I) is Re+. Vs refers to a cation defect site removal of Mg+ from tiie lattice gives V, for example. 

A polaron is a fermion quasi-particle consisting of an anion (or cation) defect with an associated polarized Gegenion (= counterion) atmosphere or polarization this is an excited state of the system, with energy intermediate between the valence band and the conduction band. Its mobility within the lattice is due to the fact that there is a low energy barrier for the polarization to move from one site to the next. Polarons are the dominant excitations in conducting polymers. 

Ceria-zirconia nanophases were synthesied by a surfactant-assisted method. The refined structural data concerning the crystallite size, lattice parameters, structural microstrain, cationic occupy number and cationic defect concentration are reported. Zirconium addition into the cubic structure of ceria inhibits crystal sintering but leads to structure distortion. Different CO-metal bonds are formed when CO chemisorbs on Pd-loaded CesZr. x02 catalysts. Catalytic tests reveal that the lower zirconium content benefits the CO oxidation. 

The Rietveld refinements show that the crystalline structure contains microstrain that is mainly caused by the crystal distortion related to zirconium replacement. It is also found that cationic occupancy number in the crystalline structure is smaller than their normal value 0.02083 present in an ideal crystal, indicating that the crystalline structure is of cationic deficient. As far as the local environment of the cationic defect is concerned, the lattice oxygen ions around it are not fully bonded, which are mobile and more active under the reaction condition compared to the normal ones. Therefore, the creation of cationic defects in the structure is a possible origin of the unusual reducibility and high mobility of oxygen species from bulk to surface exhibited on the ceria-zircnia solids, that are reported by other groups [5, 6]. 

A population of vacancies on one subset of atoms created by displacing some atoms into normally unoccupied interstitial sites constitute a second arrangement of paired point defects, termed Frenkel defects (Figure 2(b), (c)). Because one species of atom or ion is simply being redistributed in the crystal, charge balance is not an issue. A Frenkel defect in a crystal of formula MX consists of one interstitial cation plus one cation vacancy, or one interstitial anion plus one anion vacancy. Equally, a Frenkel defect in a crystal of formula MX2 can consist of one interstitial cation plus one cation vacancy, or one interstitial anion plus one anion vacancy. As with the other point defects, it is found that the free energy of a crystal is lowered by the presence of Frenkel defects and so a popnlation of these intrinsic defects is to be expected at temperatures above 0 K. The calculation of the number of Frenkel defects in a crystal can proceed along lines parallel to those for Schottky defects. The appropriate chemical equilibrium for cation defects is ... 

Statistical Superstructures Anion defective Cation defective... 

In Mg/Al/Zr/0 pattern the main reflections relative to Mg/Al/0 are still present and only weak and very broad reflections attributable to ZtOz at 20 50-52° and = 60° are observed, while the main reflection of ZrOz (at 20 30° in the reference single oxide) is shifted towards higher angles (20 32-33°) and considerably broadened. In the same region, a broad reflection typically is observed for Mg/Al/0 systems, attributed to extended cationic defects, as due to the introduction of excess positive charges [12,13]. In Mg/Al/Zr/0 the attribution of the broad reflection at 20 32-33° either to Mg -doped Zr02 or Zr -doped Mg/Al mixed oxide is not possible, but nevertheless indications exist of the interaction between cations in this tri-component system. 

This means that for 1 mol Y2Os 2 mol of negative singly charged cation defects Y Zc and 1 mol of positive doubly charged oxygen vacancies V are created and distributed, for example, as Frenkel defects (Figure 2.7). 

Fig. 5.4. Adsorption complexes of rhenium subcarbonyl on dehydroxylated Re(CO)3 OMg 3 (a) and hydroxylated Re(CO)3 HOMg 3 (b) cationic defects of MgO.

The intrinsic activity depends on the chemical and physical properties of the active component. For unsupported catalysts, the most important properties are the composition and structure of the catalyst surface and the presence, or absence, of special sites such as Br0nsted or Lewis acid centers, anion or cation defects, and sites of high coordination. For supported catalysts, the size and morphology of the dispersed phase are of additional importance. If intraparticle transport of reactants occurs with a characteristic time that is short compared to that of the reaction, then the observed and intrinsic rates of reaction will be identical. When the characteristic time for intraparticle mass transport is less than that for reaction, the observed rate of reaction per unit mass of catalyst becomes less than the intrinsic value, and the reaction kinetics are dominated by the effects of intraparticle mass transport. The factors governing intraparticle transport are the diffusivities of the reactants and products and the characteristic distance for diffusion. 

The nature of long-wavelength luminescence that looks rather similarly for all samples can be assigned to cationic defects, more probable, within the particles since this PL is also explicitly x-dependent. The defects can play a role of photoelectron captures, however, a disorder in the mixed phases is a reason of the radiationless relaxation. The research is of importance for understanding features of the ternary semiconductor nanophases and as a tool of PL control in semiconductor-doped glasses. 

Assuming that cationic transport across the growing oxide layer controls the rate of scaling and that thermodynamic equilibrium is established at each interface, the process can be analyzed as follows. The outward cation flux, is equal and opposite to the inward flux of cation defects (here taken to be vacancies). This model is shown in Figure 3.9. 

The class of diampnd-like semiconductors is not restricted to phases with a valence electron concentration of n = 4 per atom. This was shown by one of the authors, ff n > 4, phases are obtained which possess a sphalerite-type structure. At the same time, there are more anions than cations and some of the cation sites in the lattice are therefore unoccupied. In complex systems (with more than two components) phases can be obtained with continuously varying valence electron concentrations and numbers of cation defects. Defect diamond-like phases frequently have structures in which the atoms and defects have a certain ordering. 

The concentration of cation defects is equal to that of oxide ion vacancies [Ca r] =... 

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