Spinel stability

The Mg0-Al203 phase diagram was first reported by Rankin and Merwin [5J] in their study of the Ca0-Mg0-Al203 system. They showed no spinel stability beyond stoichiometry. Subsequently, Roy et al. [59] and Alper et al. [7] examined the MgAl204-Al203 system and the Mg0-MgAl204 system, respectively. Their diagrams, reproduced in... 

Li L, King DL, Nie Z, Li XS, Howard C (2010) MgAl204 Spinel-stabilized calcium oxide absorbents with improved durability for high-temperature CO2 capture. Energy Fuels... 

Codoping with Al and F improves the spinel stability not only at room temperature but also at relatively high temperatures. In the case of codoping with Li and Ni, the prepared spinel presents good cycling not only at room temperature and at 60°C but also at 4 C. Even with an increase of the discharge rate from 0.1 C to 4 C, its capacity retention is still 98%. 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

The successful rationalization of these transition-metal inverse spinel structures in terms of the relative LFSE s of tetrahedral and octahedral sites is another attractive vindication of ligand-field theory as applied to structure and thermodynamic properties. Once again, however, we must be very careful not to extrapolate this success. Thus, we have a clear prediction that LSFE contributions favour tetrahedral over octahedral coordination, except for d" with n = 0, 5 or 10. We do not expect to rationalize the relative paucity of tetrahedral nickel(ii) species relative to octahedral ones on this basis, however. Many factors contribute to this, the most obvious and important one being the greater stabilization engendered by the formation of six bonds in octahedral species relative to only four bonds in tetrahedral ones. Compared with that, the differences in LSFE s is small beer. Why , one asks, was our rationalization of spinel structures so successful when we neglected to include consideration of the bond count The answer is that cancellations within the extended lattice of the spinels tend to diminish the importance of this term. 

With respect to non-noble and non-Ru catalysts, transition metal chalcogenides with spinel and pyrite structures have been investigated and shown that these can also be active to oxygen reduction processes. The motivation in the present case is that chalcogen addition might enhance the stability and activity toward the ORR... 

Wendlandt RF, Eggler DH (1980) The origins of potassic magmas 2. Stability of phlogopite in natural spinel Iherzolite and in the system KalSi04-Mg0-H20-C02 at high pressures and high temperatures. Am J Sci 280 421-458... 

Further ternary compounds for which the relative sizes of the ions are an important factor for their stability are the perovskites and the spinels, which are discussed in Sections 17.4 and 17.6. 

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