Spinel coordination site properties

The ionic radius of trivalent boron is 27 pm, much smaller than that of tri-valent Mn + (65 pm). When it is introduced into LiMn204, it prefers trigonal or tetrahedral coordination sites, and the spinel structure is disrupted. After addition of B2O3, the vacancies among particles and the porosity of grains decrease sharply. As a result, electrochemical properties deteriorate the initial reversible capacity is low (<50 mAh/g), and capacity fades very quickly. 

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. 

The inverse spinel structure has one of the cations on the tetrahedral site and the A cation and the other B cation on the octahedral sites. The tetrahedrally coordinated cation shares an ligand with one of the two octahedrally coordinated cations. This feature is responsible for the interesting magnetic properties of the ferrites, a class of ceramic magnets that will be discussed in more detail in Chapter 25. 

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