Defects oxygen interstitial

Another source of departure from stoichiometry occurs when cations are reduced, as for example in tire reduction of zinc oxide to yield an oxygen-defective oxide. The zinc atoms which are formed in tlris process dissolve in the lattice, Zn+ ions entering interstitial sites and the coiTesponding number of electrons being released from these dissolved atoms in much the same manner as was found when phosphorus was dissolved in the Group IV semiconductors. The Kroger-Viirk representation of dris reduction is... 

An iron deficiency could be accommodated by a defect structure in two ways either iron vacancies, giving the formula Fe] /D, or alternatively, there could be an excess of oxygen in interstitial positions, with the formula FeOi+ f. A comparison of the theoretical and measured densities of the crystal distinguishes between the alternatives. The easiest method of measuring the density of a crystal is the flotation method. Liquids of differing densities which dissolve in each other, are mixed together until a mixture is found that will just suspend the crystal so that it neither floats nor sinks. The density of that liquid mixture must then be the same as that of the crystal, and it can be found by weighing an accurately measured volume. 

The defective structure in nanocrystalline ceria based catalysts proved to have strong effect on the OSC. Mamontov et al. (2000) reported the neutron diffraction studies of the atomic structures of nanocrystalline powder of ceria and ceria-zirconia solid solution. They found that the concentration of vacancy-interstitial oxygen defects has a direct correlation with the OSC. This effect is stronger than the correlation of surface area with OSC. Zirconia reduces ceria and preserves oxygen defects to retard the degradation of ceria-zirconia in OSC. Yan et al. observed the strong correlation between OSC and the lattice strain in nanosized ceria-zirconia, which could be measured via XRD (Si et al., 2004 Figure 11). 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

Figure 10.6. TempcTature dependence of oxygen defect concentrations upon heating. Filled circles are oxygen interstitials, and open circles arc oxygen vacancies. The lower set of data points at 20°C were obtained upon completion of the thermal cycle. [ 13]...

As with oxygen ion-conducting electrolytes, proton conduction in these electrolytes occurs only within a limited range of hydrogen partial pressures. In addition, as they are oxides, oxygen defects can occur. Figure 13.6 shows the predominant defects in indium-doped calcium zirconate, which were calculated based on an extrapolation of conductivity measurements [79]. Hydrogen conduction occurs by interstitials H ... 

Because the activity of electrons is equal to unity due to the disorder of electrons no matter how large the deviation from stoichiometry, the / 02-dependence of the minority defects in the oxide is generally not affected either by the properties of majority defects or by their electrical charge (Millot and Niu 1997). If oxygen vacancies and interstitial oxygen defects contribute to diffusion in an oxide, such as Fe304, one could expect experimental results to be described by a relation that combines Equations (123-125) in the form... 

According to Niu and Millot (1999), at low/ 02, where the majority defects are iron interstitials, the prevailing oxygen defects are free oxygen vacancies. At high P02, where the majority defects are cation vacancies, the observed slope of 1/6 supports the presence of clusters of iron vacancies with oxygen vacancies formed as a result of Coulombic attractive forces. The main two points here are (a) oxygen diffusivities in select oxides (but not all) may follow a complex pattern as a function of Po2 md (b) information of this type is necessary in order to assess the relation between rates and defect structures. 

A number of other -alumina related phases have been prepared. In some of these the spinel blocks have an increased thickness, the so-called P, P" and P " phases, while in others, the Na or A1 components have been replaced with similar species. Related structures, such as BaMgAlnOiy doped with Eu +, are widely used as phosphors. Crystal-structure studies on such materials show that the defects present depend sensitively upon both temperature and the constituents of the phase. Large replacement ions, lanthanide or alkali metals, tend to occupy the interlayer regions as interstitial defects, but surprisingly, some also enter the spinel blocks as substitutional defects, in association with oxide ion vacancies. Smaller ions occupy the spinel blocks as substitutional point defects. The delicate balance between oxygen interlayer interstitials and spinel block cation vacancies varies with composition. These defect interactions can often be successfully explored by using simulation techniques. Ordering occurs at lower temperatures see Ionic Conductors). 

It should be noted that recently in nano-scale powders of Ce02 Frenkel type defects have been found. Prom a neutron diffraction study interstitial oxygen defects have been proofed to exist depending on the fomation conditions of the ceria material [13]. The interstitial oxygen was revealed on the octahedral sites of the structure which is the center of the anion cube. These interstitials are charge compensated by oxygen vacancies in the structure. 

There are mainly four defects that can have an important role on several properties of an oxide oxygen vacancy, metal vacancy, interstitial oxygen, and a Frenkel defect (interstitial chromium coupled to a chromium vacancy in the case of chromium oxide). Some properties depend on the experimental method of synthesis and so can it be for defects. Some charged defects induce a polaron made of electrons and holes and a polarization field. The local structural deformation extends to 3 A around the defect. For chromium oxide, a neutral chromium... 

Fig. 2 Schematic view of the energy levels in ITO showing the components of the valence band and conduction band regions of the oxide, the presence of oxygen defect sites and interstitial tin dopant sites that contribute to electron density near the conduction band edge, rendering the oxide conductive at room temperature (after Ref 25).

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