Vacancies stoichiometry

On the other side of stoichiometry, we know from experiment that the constitutional defect in NiAl is the vacancy, so we can assume that Cva dominatej he defect populations. In this case we obtain the solution ... 

Above 570°C, a distinct break occurs in the Arrhenius plot for iron, corresponding to the appearance of FeO in the scale. The Arrhenius plot is then non-linear at higher temperatures. This curvature is due to the wide stoichiometry limits of FeO limits which diverge progressively with increasing temperature. Diffraction studies have shown that complex clusters of vacancies exist in Fe, , 0 Such defect clustering is more prevalent in oxides... 

The De Wolff disorder model has been extended to the cation vacancy model for /-Mn02 and -Mn02 by Ruetschi [42]. In this model the occurrence of manganese cation vacancies and the non stoichiometry of electrochemical Mn02 have been taken into account. Furthermore, the vacancy model deals with the explanation of the different water contents of manganese dioxide. Ruetschi makes some simple assumptions ... 

A number of selenium and tellurium compounds of the presently discussed metals show a quite different behavior from the Fe-S system. Iron and selenium form two compounds FeSe with a broad stoichiometry range and FeSe2 with a much narrower composition field. Below 400 the non-stoichiometric Fei xSe exists by creation of iron vacancies and can have compositions lying between FeySes and Fe3Se4. At low temperatures there exist two phases an a (PbO type) and a f) (NiAs type) phase. The crystal sUiicture of the diselenide, FeSe2, is an orthorhombic, C18 (marcasite) type. In the Fe-Te system, the defect NiAs structure is found at a composition close to FeTei.s, as about one-third of the Fe atoms are missing. At compositions around FeTe the behavior is complex, and the f)-phase has the PbO structure (like FeSe) but with additional metal atoms (i.e., FeuTe). 

These five defects are based upon an excess in the number of sites available. This excess we call "6". Note that we are not speaking of the ratio of cations to anions, i.e.- stoichiometry, but of the total number of sites. To see how this is possible, consider the Vacancy-Structure type. 

In theory, the III-V compound semiconductors and their alloys are made from a one to one proportion of elements of the III and V columns of the periodic table. Most of them crystallize in the sphalerite (zinc-blende ZnS) structure. This structure is very similar to that of diamond but in the III-V compounds, the two cfc sublattices are different the anion sublattice contains the group V atoms and the cation sublattice the group III atoms. An excess of one of the constituents in the melt or in the growing atmosphere can induce excess atoms of one type (group V for instance) to occupy sites of the opposite sublattice (cation sublattice). Such atoms are said to be in an antisite configuration. Other possibilities related with deviations from stoichiometry are the existence of vacancies (absence of atoms on atomic sites) on the sublattice of the less abundant constituent and/or of interstitial atoms of the most abundant one. 

The value of the activation energy approaches 50000r near the stoichiometric composition. This diffusion process therefore approximates to the selfdiffusion of metals at stoichiometry where the vacancy concentration on the carbon sub-lattice is small. 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

One vacancy is generated for every two La3+ substituents. If one wishes to include the T1O2, which is a sleeping partner, it is only necessary to maintain the correct stoichiometry. The creation of three new A cation sites (2La a + V a) requires the inclusion of three Ti sites and the corresponding number of anion sites ... 

One anion vacancy is generated for every two Cr3+ substituents. As before, if it is helpful, include the CaO, simply maintain the stoichiometry. The creation of two new B cation sites (2Crxi) requires the inclusion of two Ca sites and two anion sites ... 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

Non-stoichiometry in solid solutions may also be handled by the compound energy model see for example a recent review by Hillert [16]. In this approach the end-member corresponding to vacancies is an empty sub-lattice and it may be argued that the model loses its physical significance. Nevertheless, this model represents a mathematically efficient description that is often incorporated in thermodynamic representations of phase diagrams. 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

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