Nonstoichiometric effect

The elements of Group 5 are in many ways similar to their predecessors in Group 4. They react with most non-metals, giving products which are frequently interstitial and nonstoichiometric, but they require high temperatures to do so. Their general resistance to corrosion is largely due to the formation of surface films of oxides which are particularly effective in the case of tantalum. Unless heated, tantalum is appreciably attacked only by oleum, hydrofluoric acid or, more particularly, a hydrofluoric/nitric acid mixture. Fused alkalis will also attack it. In addition to these reagents, vanadium and niobium are attacked by other hot concentrated mineral acids but are resistant to fused alkali. 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

Studies of the effect of pressure on initial rates limit the possible Hougen-Watson rate expressions to certain classes. Subsequent studies using nonstoichiometric feeds and inerts and product species in the feed mixture further serve to determine the exact form of the reaction rate expression. 

Effect of Oxide Mineralogy on Reductive Dissolution. Oxide/hydrox-ide surface structures and the coordinative environment of metal centers may change substantially throughout the course of a reductive dissolution reaction. Nonstoichiometric and mixed oxidation state surfaces produced during surface redox reactions may exhibit dissolution behavior that is quite different from that observed with more uniform oxide and hydroxide minerals. 

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

A rather similar effect can occur with the formation of nonstoichiometric compounds. For example, copperfl) sulfide may not have the exact ratio of 2 1 expected from the formula. Cu2S. Some of the Cu ions may be absent if they are compensated by an equivalent number of Cu2+ ions. Since both Cu and Cu2 ions are stable, it... 

Mg4Al2(0H)12C03 3 0, is commonly written however, these minerals are generally nonstoichiometric by nature and can include some amounts of alternative elements in their compositions. They function similarly to the zeolites but exist in layered structures and have a different trapping mechanism. In addition to their performance enhancement, the hydrotalcite minerals are compatible with PVC and can be used effectively in clear PVC applications as well as the pigmented formulations. 

Photocatalytic decomposition of water on semiconductors is usually conducted under reduced pressure. There are arguments that 02 production is much less than stoichiometric when water photolysis is carried out under atmospheric pressure.20-34,353 Such nonstoichiometric 02 evolution is often ascribed to the photoadsorption of 02 onto semiconductor particles20,34,353 or the formation of peroxides.363 On the other hand, the electrochemical potential of H2 evolution shifts to the positive direction with increasing ambient pressure according to the Nemst equation. Therefore, pressure effect may be negatively significant for water photolysis by Ti02 photocatalysts, since tne flat band potentials of TiO are close to the potential of NHE.23,243... 

In the Sm2Fei7Xy (X = C and/or N) materials, the Tc value and EMD are dominated by the content of X, which cause expansion of the Fe-Fe interatomic distance and enhance magnetic interactions. The carbide Sm2Fe17Cy consists of nonstoichiometric compounds with a wide range of carbon content and for y < 1 are stable at temperatures ( 1300 K) sufficient for sintering. On the other hand, Sm2Fe17N3, completely decomposes at such temperatures because no nitride with x < 1 is obtained. Similar effects on Tc and EMD are produced when Co substitutes for Fe, e.g. (Sm2Fe1 Co )l7C>,. 

No particular significance is attached to the fact that the molar amounts of glyme and diglyme in these two catalysts are almost the same. Extensive vacuum drying of the cobalt catalyst, which reduced the water level from 4.29 to less than 1.0 mole per mole of the zinc salt (and, presumably, reduced the glyme to a similar or larger extent), gave no appreciable effect on catalytic activity. This result emphasizes the nonstoichiometric nature of the catalytic forms of the hexacyanometalate salt complexes. 

It gradually became clear that the clathrate hydrates distinguished themselves by being both nonstoichiometric and crystalline at the same time, they differed from normal hexagonal ice because they had no effect on polarized light. 

Temperature and pressure effects become important in chemical systems of geological interest. Also, the chemical nature of the system is often not well characterized. Nonstoichiometric compounds and solid solutions are often present, with complex silicates frequently playing an important part. 

A nonideal network may be obtained as in the previous case but using different nonstoichiometric molar ratios or arresting the polymerization at different conversions, to modify the structure. In these cases, the presence of a sol fraction and dangling chains will introduce an additional plasticization effect, surimposed on the new architecture (Vallo et al., 1993). 

The effect of structural parameters on KIc and GIc can be summarized as follows any change in the chemical structure (use of monomers with different molar masses, use of nonstoichiometric formulations, etc.), will produce a variation in Tg this will directly affect the value of fracture resistance. An increase in Tg will lead to an increase in constant temperature, and to a decrease in the fracture resistance. This is why high-Tg epoxy networks exhibit very low values of fracture resistance. 

Nonstoichiometric solid solutions are substances whose composition approximates that of stoichiometric compounds, but which have a range of compositions. The problem of applying thermodynamics to such substances is primarily how to express the composition of the solution. The simplest choice would be to use the mole fractions or atom fractions in terms of the components. In such a case the effects of the formation of the compound from the components would be contained in the values of the activity coefficients or excess chemical potentials. 

To prove the above statement on the determining effect of electric charge of both the CdS colloidal particle surface and the quencher molecule on the adsorption of these molecules from aqueous solution, we have modified the surface of colloidal CdS during its preparation. The most efficient method of such modification consists in changing the surface charge of the colloidal particle via the preparation of nonstoichiometric colloid. In this case, the surface charge is determined by the charge of excessive ion (either S 2 or Cd2+). 

In other nonstoichiometric systems—and in this context, as in others, there is a serious lack of quantitative data—Debye-Huckel effects and incomplete dissociation are much more pronounced. In some as yet unpublished work in my laboratory, Barraclough made extensive measurements on the nicely reversible CaU04 a. system and some relevant information can be extracted from his (p,X)T data. The equilibrium here involves... 

While it is to some extent arbitrary, a classification of this kind provides a means of discussing some of the general features now emerging from studies of metallic oxides. We have stressed the evidence that a nonstoichiometric phase is disordered, but may be related to chemically similar phases of fixed composition where an anomaly of structure is ordered and identifiable by x-ray diffraction methods. Where such ordered phases are found, it is possible that features of them are retained as blocks or domains with short range order in the related berthollide. Efforts should be directed towards order-disorder effects, with a view to reconsidering the status of the nonstoichiometric compound with a very wide composition range. 

Metallic hydrides are usually nonstoichiometric compounds, as expected from their relatively low heats of formation and the mobility of hydrogen. They are ordinarily described, chemically, in terms of any of three models in which hydrogen is considered a small interstitial atom, a proton, or a hydride anion. These models are discussed critically with particular reference to the group V metal hydrides. The interstitial atom model is shown to be useful crystallographically, the protonic model is questioned, and the hydridic model is shown to be the most useful at present. The effect of hydrogen content on the lattice parameter of VHn and the structural and magnetic properties of several hydrides are discussed in terms of these models. 

The optimum ratio for high yields of fS-ketoester is 1.7 (monoethyl malonate acid chloride). A nonstoichiometric reaction for optimum yield is not a serious drawback in this case since the reagent in excess is the inexpensive dilithio monoethyl malonate. Our results show that lowering the ratio also lowers the yield, whereas an increase in the ratio beyond 1.7 has little effect. 

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