Defect Chemistry of Oxides

It is a well-known fact that it is not feasible to manufacture crystals that are perfect in all aspects, because of the presence of crystal defects. This effect occurs because of the fact that over absolute zero, the existence of defects in crystals is thermodynamically necessary. 

FIGURE 5.21 Frenkel point defect in ionic crystals. 

Cic is the equilibrium concentration of cation interstitials CVc is the total concentration of cation vacancies 

The Physical Chemistry of Materials Energy and Environmental Applications 

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In an ideal world, crystals would be perfect or stoichiometric with constant composition. But like people crystals are not exempt from imperfections or defects. Crystals with variable composition are termed non-stoichiometric crystals. The defect chemistry of oxides is enormously complex and is extremely vital to their properties. It has involved extensive research in many laboratories and is providing extraordinary insights into structural variations, the stability of structures and the formation of new structures. Here, we first define order-disorder phenomena that are commonly associated with oxides and describe our current understanding of them. The disorder or non-stoichiometry plays a crucial role in oxide applications including catalysis and it is therefore of paramount importance. 

Tq = 800°C. According to Refs.65,65. (Reprinted from K. Sasaki and J. Maier, Low-temperature defect chemistry of oxides. I. General aspects and numerical calculations. J. Appl. Phys 86, 5422-5433. Copyright 1999 with permission from the American Institute ofPhysics.)... 

The preceding sections have focused on Che properties of the bulk oxide. However, computer simulation techniques are also well established tools in the study of the structural and defect chemistry of oxide surfaces, which are often difficult to characterize by experiment alone. 

Nowotny J (1988) Surface segregation of defects in oxide ceramic materials. Solid State Ionics 28-30 1235-1243 Nowotny J (1991) Interface defect chemistry of oxide ceramic materials. Unresolved problems. Solid State Ionics 49 119-128... 

There are a number of excellent reviews and thorough treatments of the defect chemistry of oxides available in the literature, e.g., [25]. The brief overview below serves only to introduce the reader to primary concepts as a basis to the discussion in the rest of this entry. The following discussion is restricted to point defect models as these are predominant for most of the materials discussed. 

Sasaki K, Claus J, Maier J (1999) Defect chemistry of oxides in partially frozen-in states case studies for Zi02(Y203), SrZr03(Y203), and SrTi03. Solid State Ionics 121 51-60... 

Sasaki K, Maier J (1999) Low temperature defect chemistry of oxides II. Analytical relations. J Appl Phys 86(10) 5434—5443... 

In 1972 Per Kofstad published his "Non-stoichiometry, diffusion and electrical conductivity in binary metal oxides". It has been a popular textbook in defect chemistry of oxides worldwide, not least because it contained a comprehensive review of defect stmcture and defect-related properties of all binary metal oxides. It followed Kofstad s equally well-recognized book "High temperature oxidation of metals" from 1967, revised and published under the title "High temperature corrosion" in 1987. 

D. M. Smyth The Defect Chemistry of Metal Oxides, Oxford University Press, Oxford, United Kingdom, 2000. 

Despite the many investigations of the defect chemistry of lithium-oxide-doped nickel oxide, the real nature of the defect structure still remains uncertain. For many years the holes were regarded as being localized on Ni2+ ions to form Ni3+, written Mi - ... 

Driessens F. C. M. (1968). Thermodynamics and defect chemistry of some oxide solid solutions. Parts I and II. Ber. Buns. Phys. Chem., 72 754-772. 

Defect Chemistry of Nonlinear Optical Oxide Crystals... 

The defect chemistry of nonlinear optical oxide crystals can affect many of the materials properties required for device applications. Applications of these crystals,... 

The purpose of this paper is to summarize the current understanding of the defect chemistry of nonlinear optical oxide crystals and specifically the relationship of the defects present to 1) the structure and growth, or processing, of the material and 2) the properties of interest for device applications. The defects in traditional nonlinear optical oxide crystals (i.e. BaTiC>3, LiNbC>3, Sri-xBaxNb206, Ba2NaNb50l5, K3Li2Nb50l5) are reviewed. Our recent work on the defect chemistry of new... 

One of the exceptions was the discovery of high ionic conductivity in appropriately doped FaGa03.128 129 As in the other oxide ion conductors, its ionic conductivity depends on both the dopant level as well as on the nature of the dopant. A major difference to ceria and zirconia is the presence of two cations that can be substituted the detailed defect chemistry of such solid solutions is far from being fully understood. Co-doping of Sr on A sites and Mg on B-sites leads to an ionic conductivity of ca. 0.12—0.17 S cm 1 at 800°C,130-133 which is similar to doped ceria but considerably exceeds the value of YSZ (ca. 0.03 S cm 1 at 800°C80 81). The activation energy also varies with composition and can be as low as ca. 0.6 eV.130 131 At about 600-700°C, the... 

To improve the efficiency of photocatalysts, developments in the future must be based on an understanding of the sophisticated factors that determine the photoactivity of the water-splitting reaction (i) molecular reaction mechanisms involved in the oxidation and reduction of water on photocatalyst surfaces, (ii) structure and defect chemistry of photocatalyst surfaces, and (iii) charge transfer mechanisms between... 

The limits of integration are the oxygen partial pressures maintained at the gas phase boundaries. Equation (10.10) has general validity for mixed conductors. To carry the derivation further, one needs to consider the defect chemistry of a specific material system. When electronic conductivity prevails, Eqs. (10.9) and (10.10) can be recast through the use of the Nemst-Einstein equation in a form that includes the oxygen self-diffusion coefficient Dg, which is accessible from ionic conductivity measurements. This is further exemplified for perovskite-type oxides in Section 10.6.4, assuming a vacancy diffusion mechcinism to hold in these materials. 

The topic of mixed conduction in nonstoichiometric oxides was reviewed by Tuller [24], and his comprehensive paper is recommended to the reader interested in more detail concerning the role of multivalent dopants on the defect chemistry of fluorite and fluorite-related oxides, and corresponding transport properties. Equations which express the oxygen flux in solid solutions of, e.g.. 

Additional known examples are various oxides having a perovskite structure, for example, SrTi03 [12] (Fig. 5.6.5). Although the defect chemistry of the perovskites is very well studied, neither a Ga203-based nor a SrTi03-based commercial application is known. 

The foregoing survey was focused on situations where bnlk diffusion processes were rate determining. Such systems are amenable to analysis using an electrochemical approach. Other factors such as transport down pores or cracks, volatilization or melting of the oxide scale may occur and require different analyses but diffusion controlled processes may be mathematically modeled and correlated with the defect chemistry of the corrosion product. These limiting cases provide a guide to understanding the more complex phenomena frequently encountered. 

Levy, M. R., Stanek, C. R., Chroneos, A., Grimes, R. W. 2007. Defect chemistry of doped bixbyite oxides. Solid State Sciences 9 588-593. 

The point defect chemistry of ZnO is different partly because there are two charge states of Zn, namely 2+ (as in ZnO) and 1+. When we heat ZnO in Zn vapor, we form a Zn-rich oxide, Zni+jf). Experimentally it is found that the excess Zn sits on interstitial sites (as you would expect from the crystal structure). We can write the basic defect equation for the singly charged interstitial as... 

Kroger, RA. and Vink, H.J. (1956) Relations between the concentrations of imperfections in crystalline solids, Solid State Phys. 3, 307. The original proposal of the notation that is now universally used to describe charged point defects. This is an invaluable paper when you have time to study it. The official notation is given in the lUPAC Red Book on the Nomenclature of Inorganic Chemistry, Chapter 1-6. Smyth, D.M. The Defect Chemistry of Metal Oxides, Oxford University Efi-ess, Oxford 2000. Clear and at the right level. 

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