Nonstoichiometry oxidative

The chemistry of ceramics plays a role in their behaviour during sintering. Nonstoichiometry of oxides has been found to play a major role in the extent to which a... 

The sulfides have been less thoroughly examined than the oxides but it is clear that a number of stable phases can be produced and nonstoichiometry is again prevalent (p. 679). The most important are the disulfides, which are semiconductors with metallic lustre. TiS2 and ZrS2 have the Cdl2 structure (p. 1211) in which the cations occupy the octahedral sites between alternate layers of hep anions. 

Oxides of the actinides are refractory materials and, in fact, Th02 has the highest mp (3390°C) of any oxide. They have been extensively studied because of their importance as nuclear fuels. However, they are exceedingly complicated because of the prevalence of polymorphism, nonstoichiometry and intermediate phases. The simple stoichiometries quoted in Table 31.5 should therefore be regarded as idealized compositions. 

Metal oxides are not normally stoichiometric, although the nonstoichiometry may be too small to be detected by ordinary methods of... 

Nonstoichiometry is relatively common among mixed metal oxides, in which more than one metal is present. In 1986 it was discovered that certain compounds of this type showed the phenomenon of superconductivity on cooling to about 100 K, their electrical resistance drops to zero (Figure 20.9). A typical formula here is YBa2Cu30 where x varies from 6.5 to 7.2, depending on the method of preparation of the solid. 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley Interscience, New York (1972). 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

Nonstoichiometry of the oxides can be due to a number of reasons, such as hydration,159 incomplete oxidation,158 and the generation of defects at interfaces.157 An important factor affecting the chemical composition of the oxides is the incorporation of electrolyte species into the growing alumina. There have even been suggestions to use this for impurity doping of oxides and modifying their properties.161 Various kinds of anion distributions and mechanisms of anion incorporation and their influence on oxide properties have been reported. The problems attracting attention are ... 

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

Figure 7.1 Schematic representation of the electronic consequences of nonstoichiometry in oxides.

A hopping semiconductor such as an oxide is often best described by Eq. (7.1) rather than by classical band theory. In these materials the conductivity increases with temperature because of the exponential term, which is due to an increase in the successful number of jumps, that is, the mobility, as the temperature rises. Moreover, the concentration of charge carriers increases as the degree of nonstoichiometry increases. 

The conductivity of nickel oxide at 1000°C was found to depend upon the oxygen partial pressure as in the following table. What conclusions can be drawn concerning the nonstoichiometry of NiO under these circumstances ... 

There has been considerable interest in Ce02 as a component of solid oxide fuel cells, especially as an anode material, and also for use in oxygen separation membranes. The material shows a wide nonstoichiometry range, with oxygen vacancies as the... 

CoO and NiO all take the NaCl-type structure and the difference in nonstoichiometry relates to the relative stability of the formal di- and trivalent oxidation states. The stability of the trivalent state and the degree of non-stoichiometry decreases from Fe3+ to Ni2+. Hence the non-stoichiometric nature of Fcj yO is made possible by the relatively high stability of Fe3+ that is reflected in the fact that Fe2C>3 is a stable compound in the Fe-0 system, whereas M2O3 is not in the Ni-O system. This relative stability of the different oxidation states is also reflected in Figure 7.11(c). 

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