Nonstoichiometric compounds defect structure

An additional problem is encountered when the isolated solid is non-stoichiometric. For example, precipitating Mn + as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnO ) where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice defects in the crystal structure. ... 

Antisite defects in the pyrochore structure Er2Ti207 were mentioned previously (Section 1.10). These defects also occur in the nonstoichiometric compound Er2.09Ti194O6.952, which is slightly Er203-rich compared to the stoichiometric parent phase. The formation of the antisite pair is now accompanied by the parallel formation of oxygen vacancies ... 

This question must be asked on two grounds. First, because the high concentrations of lattice defects found in some cases (the TiO phase is an extreme example) must raise doubt about the relation of the nonstoichiometric phase to the type structure in terms of which it is described. Secondly, successions of ordered intermediate phases have been discovered during the last few years, in phase diagrams where, previously, nonstoichiometric compounds of broad range had been reported. 

From the thermodynamic standpoint, the conclusion that AG is dominated by a very large positive AH term turns entirely on whether the calculation of the endothermicity of random, defective crystals is valid. In so far as direct thermodynamic measurements or phase equilibrium studies are available for oxide systems (and these, as the most truly ionic of allegedly nonstoichiometric compounds, should be the ones for which the Bertaut type of calculation should be least unreliable), it seems clear that the endothermicity of the random, compared with the ordered, structure has been grossly overestimated. 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

The rules for quasi-chemical reactions are the same as for the normal chemical reactions, namely mass balance and electroneutrahty conditions one extra requirement appears, however, for crystalline solids where the ratio of sites in the crystal structure should be constant and should satisfy to stoichiometric formula. This means that if, for instance, in the AB2 crystal one site for A atom is formed, then automatically two B-sites appear as well, regardless of their occupancy. It should be noted that the point defects and/or the processes of their formation can also classified into two groups, namely stoichiometric and nonstoichiometric. The first type of process does not disturb the stoichiometric ratio of components constituting the crystal, which is a closed thermodynamic system the second type leads to nonstoichiometric compounds by exchanging components between the... 

As already mentioned, the composition of a nonstoichiometric compound varies continuously with change of external conditions (temperature, pressure), and the structure in turn varies continuously with the composition. Continuous change of structure is obtained by adding a defect structure continuously to the structure of stoichiometric composition. 

Thermodynamic data for a system (such as AS, AH, and AG) are obtained by preparing a chemical potential diagram as mentioned in Section A, and these data are useful as information contributing to better understanding of the defect structures of nonstoichiometric compounds. Several methods are available for preparing the chemical potential diagram (40), as described below, taking the U-O system as an example. 

The statistico-thermodynamic calculations developed by Wanger and Schottky (248), Anderson (249) and Libowitz (250 253) are often applied in analyses of the defect structure of nonstoichiometric compounds. In what follows, a description will be given of the statistico-thermodynamic treatment of nonstoichiometric compounds, taking as example the case of UOj... 

Besides vacancies and interstitials, there exists another type of defect structure in nonstoichiometric compound—the heterovalent atoms in a mixed-valence type compound, e.g., Fe(III) ions in FeOi... 

In view of the many types of point defects that may be formed in inorganic compounds and that each type of defect may have varying effective charge, numerous defect reactions may in principle be formulated. In the following, a few simple cases will be treated as examples. First, we will consider defect stmcture situations in stoichiometric compounds (Schottky, Frenkel and intrinsic electronic disorders) and then defect structure situations in nonstoichiometric oxides will be illustrated. Finally, examples of defect reactions involving foreign elements will be considered. 

It should be noted that both the stoichiometric and nonstoichiometric defect structures may prevail in the same compound, depending on the activities (partial pressures) of the components in the compound. 

Various imperfections in solid-state structures include the Schottky and Frenkel defects. The latter can lead to nonstoichiometric compounds. Edge dislocations can make a metal more malleable. Examples include lead, white tin, and the iron of horseshoes. 

The specificity of chemical bonds and defect structure of semiconductors in comparison with metals determines the necessity of further development of the concept of a nonstoichiometric phase. Semiconductor compounds with one-sided wide homogeneity region represent a special group of nonstoichiometric phases. We discussed the specificity of properties of such phases using SnTe as an example. The conducted studies yielded the following results. 

One of the first nonstoichiometric compounds where the defects associated with the nonstoichiometry were identified is wiistite. The nominal composition of wiistite is FeO and the structure is that of rocksalt. Wiistite exists in equilibrium only above 570°C. It exists at 1350°C over the composition range Fe 95O to Feo.840 The stoichiometric composition FeO, which would contain only divalent cations, Fe, is not stable at any temperature at atmospheric pressure. Quenching from above 570° retains wiistite at low temperatures as a... 

Oxide compounds are widely used as cathodic materials in the power sources and electrochemical generators. Some literature data indicates that cathodic materials based on nonstoichiometric oxide compounds make it possible to increase the solid-phase reduction process. The kinetics of electrochemical reactions and consequently the current density are the higher, the greater the degree of deviation from stoichiometry, and the lager the number of the defects in the compounds structure [1,2]. 

In many nonstoichiometric solids high-temperature structures contain disordered defects, but, as in the previous example, these order or form aggregates at lower temperatures. If this aggregation could be suppressed, low-temperature ionic conductivity would be enhanced. An example of this strategy is provided by compounds with the... 

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