Nonstoichiometry in oxides

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

However, structural chemistry in oxides with large departures from stoichiometry cannot be solely explained by point defects. Defects cannot remain isolated and interactions between them begin to occur. Our discussion therefore begins with a general description of the fundamental knowledge about nonstoichiometry in oxides. Understanding such disorder and the complex defect... 

Determination of nonstoichiometry in oxides is a key point in the search for new materials for electrochemical applications. In recent decades, owing to their current and potential applications (electrodes in fuel cells, insertion electrodes, membranes of oxygen separation, gas sensors, catalytic materials, etc.), various methods of precise characterization of MfECs have been proposed, either the measurement of the defect concentrations and the stoichiometric ratio as functions of the oxide composition, of the surroxmding oxygen pressure and of temperature, or the transport properties. There are different methods to determine the electrical properties of MIECs and, more specifically, the ionic and electronic contributions. The most appropriate method depends on different parameters, i.e., the total electrical conductivity of the studied oxides, the ionic and electronic transport numbers, the... 

Karen, P. (2006). Nonstoichiometry in oxides and its control. Journal of Solid State Chemistry, Vol.179, pp.3167-3183... 

Structure, Defects and Nonstoichiometry in Oxides An Electron Microscopic View, L. Eyring. In T. Sprensen (Ed.), Nonstoichiometric Oxides, Academic Press, pp. 337-398,1981, Chapter 7. 

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... 

Nonstoichiometric compounds are mixed-valence compounds with nonintegral electron/atom ratios. Electronic properties of these compounds depend crucially on the nature and magnitude of nonstoichiometry. Electronic conduction in many such compounds occurs by hopping between the cations of different valencies (e.g. Pr " " and Pr" " in Pri2022)- Nonstoichiometry with a wide range of compositions is more common in oxides, sulphides, and related materials where the bonding is not completely ionic. In ionic nonstoichiometric compounds, structural rearrangements... 

Partial pressure of oxygen controls the nature of defects and nonstoichiometry in metal oxides. The defects responsible for nonstoichiometry and the corresponding oxidation or reduction of cations can be described in terms of quasichemical defect reactions. Let us consider the example of transition metal monoxides, M, 0 (M = Mn, Fe, Co, Ni), which exhibit metal-deficient nonstoichiometry. For the formation of metal vacancies in M, 0, the following equations can be written ... 

It is convenient to discuss the thermodynamics of nonstoichiometry of oxides in terms of the relative partial molar-free energy of oxygen, AGq, which is defined (Sorensen, 1981) as... 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

Many metal oxides and sulfides exhibit semiconductor properties by virtue of their nonstoichiometry. In Section 5.3, we distinguished positive (p-type) and negative (n-type) semiconductors. 

Crystal Self-Diffusion in Nonstoichiometric Materials. Nonstoichiometry of semiconductor oxides can be induced by the material s environment. For example, materials such as FeO (illustrated in Fig. 8.14), NiO, and CoO can be made metal-deficient (or O-rich) in oxidizing environments and Ti02 and Zr02 can be made O-deficient under reducing conditions. These induced stoichiometric variations cause large changes in point-defect concentrations and therefore affect diffusivities and electrical conductivities. 

Partial oxidation or reduction leading to nonstoichiometry in solids such as TiOo, Fe304, or Mn02 may be expected to shift the IEP(s) toward that characteristic of the oxidation state produced (72). 

Several isovalent ions form solid solutions with KTP (Table II), showing that this structure is relatively tolerant, with respect to isovalent impurities, as are the traditional nonlinear optical oxide crystal structures. But due to the relatively limited range of nonstoichiometry in KTP, aliovalent impurities, such as divalent Ba, Sr and Ca introduced through ion exchange in nitrate melts, which substitute on the K site, are incorporated at concentrations less than one mole percent.(36) Typical impurity concentrations present in flux and hydrothermally grown KTP are shown in Table ID. 

Superstructures, Ordered Defects Nonstoichiometry in Metal Oxides of Perovskite Related Structuresf... 

Anion vacancy nonstoichiometry in perovskite oxides is more common than that involving vacancies... 

Anion-deficient nonstoichiometry in AB03 x perovskites is not accommodated by the CS mechanism. The reason probably is that the constant A/B ratio required by the composition of perovskites. prevents formation of CS planes. Defect-ordering in AB03 oxides involves a conservative mechanism in the sense that the vacancies are assimilated into the structure resulting in large supercells of the basic perovskite structure. The type of superstructure formed depends however on the identity of the B-cation. 

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