Oxygen deficient oxides

In fluorite-structure oxygen-deficient oxides MO2-X there is general agreement that the oxygen vacancy is the point-defect responsible for non-stoichiometry. Unfortunately, no direct observation is available of a basic cluster species for MO2-1 such as the Willis cluster in UO2+X. 

Figure 4.47 Schematic representation of temperature dependence of (a) oxygen vacancy and (b) oxygen diffusivity in oxygen-deficient oxide ceramics. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Consider an oxygen-deficient oxide MO2-X containing a low concentration of solute A, due to the addition of the soluble oxide AO. Oxygen diffusion occurs by a vacancy mechanism. Assume that all oxygen vacancies are doubly ionized. 

Let us further consider an oxygen-deficient oxide The equilibrium with... 

FIGURE 4.10. Idealized phase diagram of the M-W-O system [4.45] (A) monotungstates and polytungstates (B) oxide and bronzes of ideal composition (C) oxygen-deficient oxides (D) reduced polytungstates (E) bronzes with cation excess (F) bronzes with cation deficiency (G) oxygen-deficient oxides with cation contamination. 

CuO is an example of a group of oxygen-deficient oxides, MOi c, where the nonstoichiometry is accommodated by oxygen vacancies. We write the equilibrium equation in the usual way. 

A two-step process, involving the production of fine metallic particles followed by their oxidation, has been used to synthesize oxide powders with sizes smaller than a few tens of nanometers (102,103). In the process, a metal (e.g., Ti) is evaporated into an inert atmosphere (e.g.. He) with pressure of —100 Pa. The particles that condense in the inert atmosphere are transported by convective gas flow to a cold shroud where they adhere. Oxygen gas at a pressure of —5 kPa is then admitted to the chamber to produce oxidation of the metal particles. The particles are finally scraped off the cold shroud and collected. Starting with Ti, this process produces a highly oxygen deficient oxide TiOi,7 with the ratile stmcture, but subsequent heating at —300°C produces a nearly stoichiometric composition, TiOi.gs. 

Ce02 is reduced at high temperatures and low oxygen pressures to form non-stoichiometric oxygen-deficient oxides, which after cooling organize themselves into highly ordered superstructures, often with complex stoichiometry. 

Ejfects of dissolution of aliovalent oxides on the oxygen-deficient oxide MO2-X... 

The examples in the previous section show the effects of additions of higher and lower valent oxide to the p-conducting, metal-deficient Mi-yO. Let us also briefly consider the effects of doping an oxygen-deficient oxide MO2-X with higher and lower valent oxides, respectively. The predominating defects in MO2-X are oxygen vacancies compensated by defect electrons. The oxide it thus an n-type electronic conductor. 

If these oxygen vacancies and the compensating electronic valence defects are the predominating defects in the oxygen deficient oxide, the principle of electroneutrality requires that... 

A general tendency similar to that of oxygen deficient oxides applies to metal deficient oxides in the oxide Mi-yO the metal vacancies are doubly charged at very small deviations from stoichiometry and tend to become singly charged with increasing nonstoichiometry. 

Oxygen-deficient oxides doped with lower valent cations... 

Figure 4-2, Brouwer plot of the concentrations of defects as a function of oxygen partial pressure in an oxygen deficient oxide predominantly containing doubly charged oxygen vacancies,...

Let us first examine the effects of water vapour on the properties of an undoped, oxygen-deficient oxide. We will use M2O3 in the example, but most of the treatment applies to any oxide. The predominant defects are electrons and oxygen vacancies and the electroneutrality condition in dry environments is then (from the preceding chapter)... 

Sketch a Brouwer diagram (double logarithntic diagram of defect concentrations vs P02) for an oxygen deficient oxide doped with a substitutional higher-valent dopant. 

Oxygen vacancy diffusion in oxygen-deficient oxides. 

Figure 5-13. The dijfusion coefficient for oxygen diffusion by the vacancy mechanism in an oxygen deficient oxide in which oxygen vacancies are the predominant native point defects. At high temperatures the oxide exhibits intrinsic behaviour and at reduced temperatures extrinsic behaviour (i.e. the oxygen vacancy concentration is determined by the concentration of lower valent cations).

For nonstoichiometric oxides the concentration of electronic defects is determined by the deviation from stoichiometry, the presence of native charged point defects, aliovalent impurities and/or dopants. The concentration of electronic defects can be evaluated from proper defect structure models and equilibria. Various defect structure situations have been described in previous chapters and at this stage only one example - dealing with oxygen deficient oxides with doubly charged oxygen vacancies as the prevalent point defects - will be described to illustrate the electrical conductivity in nonstoichiometric oxides. 

Figure 6-5. Schematic presentation of dijferent isotherms of the n-conductivity at temperatures from Tj to T5 for an oxygen deficient oxide where the predominant defects are doubly charged oxygen vacancies and electrons.

We now apply our derived expression for the example case of an oxygen-deficient oxide MOi-x. This contains oxygen vacancies compensated by electrons, and we have earlier shown that the defect concentrations are given by n = 2[Vq ] = 4K. ) Po[ - Since the mobility of... 

The reaction in Equation 10.9 will result in an oxygen-deficient oxide, whereas the reaction in Equation 10.13 will result in an oxygen-rich oxide. 

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