Defect Spinels

This was observed to occur at similar temperatures of 800°C. In addition, nanoparticles of hematite prepared via an aqueous precipitation have shown a size and temperature-dependent concentration of spinel defects at the surface. The formation of these defects on hematite was shown to be governed by the thermodynamics of the ly-y phase transition with decreasing particle size [121], Raman spectroscopy has shown that similar defects on the surface of highly efficient anodes prepared by APCVD are important to the performance [13]. 

In2S3 (red) defect spinel, /9-In2Se3 ordered defect wurtzite /9-In2Te3... 

Figure 3. Charge and discharge curves of an Li/Li[Li0jMn, 9]04 cell cycled at voltages between 3.0 and 4.4V at a rate of 0.1 mAcnT2 at 30 °C. To describe the composition of an Li-Mn-O ternary phase a defect-spinel formulation was assumed.

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

Structure types have been established. Similar to Al, the M2X3 crystals (M = Ga, In, Tl X = S, Se, Te) are mostly based on M-defect tetrahedral structures, namely W (Ga2S3, In2Se3) and ZB (Ga2Se3, Ga2Te3, In2Te3). At atmospheric pressure, 283 can be present in three modifications. The low-temperature a-form is a cubic close-packed structure of S atoms, where 70% of the In atoms are randomly distributed on octahedral sites and the rest remain on tetrahedral sites. The P-form is related to the spinel structure, and the y-modification is hexagonal. 

For the spinel, Hercyanite, cissume that the cation changes in valence state. Then, draw a diagram illustrating the probable ion diffusion processes, give the diffusion conditions and the diffusion reactions, including the effect of induced crystal lattice defects. 

In a number of studies a correlation was seen between the amount of nonstoi-chiometric oxygen in the spinel and the spinel s activity. It appears that excess oxygen consolidates the spinel s defect structure with a large number of active sites. Strong anodic polarization leads to ordering of this structure and thus to a decrease in catalytic activity. 

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). 

An antisite defect is an atom on an inappropriate site in a crystal, that is, a site normally occupied by a different chemical species. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom, or a B atom on a site normally occupied by an A atom. Antisite defects are not very important in binary ionic compounds, as the misplacement of an ion is energetically costly, and so unfavorable. In ternary ionic compounds, however, such as spinels, AB204, the transfer of A ions to B sites and vice versa, is not... 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

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