Defect oxides and sulfides

For comparative purposes, table 4.3 lists defect energies (enthalpies) of Schottky and Frenkel processes in halides, oxides, and sulfides. The constant Kq appearing in the table is the preexponential factor (see section 4.7) raised to a power of 1/2. 

In solid state physics, it is well known that many inorganic solids, e.g., the oxides and sulfides, can dissolve metals and nonmetals in excess, and that by this process electron and ion defects in the lattice will be formed. Wagner and co-workers (1) have developed the basic theory of... 

A dead layer usually exists on the surface of phosphor particles, which degrades the phosphors due to crystal defects, residues, oxidation, contamination, etc. A surface treatment with an acid wash is widely used in industry to improve the quality of oxide and sulfide phosphors. Chemical etching on phosphors can also be used to investigate the microstructure properties of the phosphor particles. " ... 

Non-stoichiometric Defects. These often occur in transition-metal compounds, especially oxides and sulfides, because of the ability of the metal to exist in more than one oxidation state. A well-known case is FeO which consists of a cep array of oxide ions with all octahedral holes filled by Fe2 + ions. In reality, however, some of these sites are vacant, while others—sufficient to maintain electroneutrality—contain Fe3 + ions. Thus the actual stoichiometry is commonly about Fe0 950. Another good example is TiO which can readily be obtained with compositions ranging from Ti0 740 to Tij 670 depending on the pressure of oxygen gas used in preparing the material. 

Because the sulfidation rate is much higher than the oxidation rate, it results in less protective layers. A comparison between oxidation and sulfidation is rrrade in Table 15.1. Not only the defect concentration is responsible for this high rate. In fact, there are four reasons for this higher reaction rate. 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Figure 16. Shallow surface crack at the outer radius of a sharp V2T bend on 80,000 psi steel sheet (top). Corresponding subsurface concentration of REM oxysulfides and sulfides in a slab cross section near the surface. The parent ingot was treated with 5 lbs of rare earth silicide per ton of ingot steel (bottom). The bottom picture is from a Baumann print or sulfur print, not sensitive to the oxides and thus eliminating the argument of reoxidation as main cause of surface defects in REM treated steels. Magnification, 2.5X-...

Small semiconductor particles are characterized by a high extent of lattice defects affecting bond lengths (ref. 10) and solubility (ref. 21) and leading to surface states energetically located within the band gap (refs. 22, 23). The surface states trap electrons and therefore suppress the recombination of light induced electron-hole pairs. The holes remain available for photocorrosion, i.e. the oxidation of sulfide ions belonging to the lattice of the semiconductor. 

In contrast to the sulphides of most of the transition metals, sulphides of the refractory metals have quite tight stoichiometry, similar to Cr203, although, in the cases of the refractory-metal sulfides and oxides, the defects appear on the anion sub-lattice. Figure 6.1 compares the rates of oxidation and sulphidation for several of the transition and refractory metals. The low rates of sulphidation of the refractory metals are thought to be due to the low concentrations of defects in the sulphide structures. 

Alloys generally rely upon an oxidation reaction for the formation of a protective scale that will improve the corrosion resistance to sulfidation, carburization, and the other forms of high-temjjerature attack. The properties of high-temperature oxide films, such as their thermodynamic stability, ionic defect structure, and detailed morphology, therefore play a crucial role in determining the oxidation resistance of a metal/alloy in a specific environment. 

The idea of point defects in crystals goes back to Frenkel, who in 1926 proposed the existence of point defects to explain the observed values of ionic conductivity in crystalline solids. In a crystal of composition MX such as a monovalent metal halide or a divalent metal oxide or sulfide, volume ionic conductivity occurs by motion of positive or negative ions in the lattice under the influence of an electric field. If the crystal were perfect, imperfections, such as vacant lattice sites or interstitial atoms, would need to be created for ionic conductivity to occur. A great deal of energy is required to dislodge an ion from its normal lattice position and thus the current in perfect crystals would be very, very small under normal voltages. To get around this difficulty, Frenkel proposed that point defects existed in the lattice prior to the application of the electric field. This, of course, has been substantiated by subsequent work and the concept of point defects in all classes of solids, metals, ionic crystals, covalent crystals, semiconductors, etc., is an important part of the physics and chemistry of crystalline solids, not only with respect to ionic conductivity but also with respect to diffusion, radiation damage, creep, and many other properties. 

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