Nonstoichiometric spinel

This discussion has focused on stoichiometric spinel structure, but nonstoichiometric spinels can exist as well. In the case of spinels that are oxygen deficient there could be significant concentrations of Mn + remaining at full charge. The results of this study suggest that such spinels may be susceptible to cation rearrangement if tliey are energetically metastable. 

Cu/Cr or Cu/M/Cr (M= cadmium, zinc or magnesium ions) cubic nonstoichiometric spinel-type catalysts, containing an excess of bivalent cations,... 

The value of Q is much higher for stoichiometric spinel (4.7 eV), which has an even stronger temperature dependence. Q has the same value for 111 and 110 slip because the CRSS is reported to be the same for stoichiometric spinel [26]. However, in nonstoichiometric spinel (Figure 9.5), the CRSS values are lower for 110 slip and have a lower temperature dependence (lower Q). 

The CRSS values in Figure 9.5 for nonstoichiometric spinel were analyzed by Mitchell et al. [22] using Eq. (1). For slip, they found that the activation... 

The refinement of the structures of the low-temperature aluminum oxides [30] made it possible to put forward a hypothesis concerning the mechanism of the formation of the solid solutions and nonstoichiometric spinels obtained when cations and anions are introduced [32,39]. According to this model, y- has a protospinel structure, slightly different from the spinel one, and is assigned to compounds of the common formula ... 

Some ceramic compoimds have a wide phase field of stability and can be quite nonstoichiometric. Spinel (MgAl204), for example, is shoichiometric at 50 At% MgO and 50 At% AI2O3, but is stable over quite a range of compositions. Here again a Mg vacancy must occur for each additional pair of AP+ above the stoichiometric value. On the other hand, one 0 vacancy must occur for each pair of Mg + substituted for Al above the stoichiometric number. 

It is interesting to note that cobalt cobaltite, C03O4, is a good catalyst, too, for anodic chlorine evolution. In this case, too, a correlation is observed between the reaction rate and the spinel s defect concentration (amount of nonstoichiometric oxygen). 

Since both the oxide reactant and the spinel are ternary (nonstoichiometric) compounds when equilibrated with each other, at a given P and T, the boundaries (A, B)0/spinel and spinel/(B, A)203 are not invariant. They become invariant (and thus provide unique boundary conditions for the reaction) only if an additional intensive thermodynamic variable can be predetermined. This is a consequence of Gibbs phase rule. 

For a-AlOOH - Zn(OH)2 and ti-AlOOH - Zn(OH)2 mixtures, in which the tetrahedral coordination is preferable for zinc ions, spinel is formed only in the case of ri-AlOOH [18], However, spinel as prepared is nonstoichiometric it corresponds to the formula Zno,29Al2.4704 arising from the number of tetrahedral positions in T1-AI2O3. No conditions for conjugation exist in a-AlOOH - Zn(OH)2 mixture, therefore. X-ray diffraction patterns exhibit only the reflections of well crystallized boemite and ZnO. The formation of spinel starts only at high temperatures (above 850°C) according to diffusion mechanisms of solid-phase reactions. 

In [94-98] mechanochemical method was nsed for the synthesis of high disperse stoichiometric and nonstoichiometric Li,Mn 04 spinel starting from different manganese (Mn02, Mh203, MnO) and lithium (LiOH. Li0H-H20, LiiCOi) compounds. 

The kinetic and thermal characteristics of combustion for particles of Al-Mg alloy in air were studied in [21], the CCP contained the significant amount of spinel Al2Mg04. Further experiments on combustion of Al-Mg alloy particles in air were repeated in [43]. The presence of trace amounts of nonstoichiometric oxynitride (AI3O3N) inthe CCP along with an oxide spinel phase was shown in [43]. 

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