Crystal field stabilization energy, octahedral

The cations in these compounds are Fe and/or Fe". In iron oxides, Fe " is always in the high spin (unpaired d electrons) state. As Fe with five d electrons has no crystal field stabilization energy (CFSE see Chap. 6), regardless of whether it is octa-hedrally or tetrahedrally coordinated, there is little preference for one or the other type of site. For Fe , on the other hand, CFSE is higher for octahedral than for tetrahedral coordination, so the octahedral coordination is favoured. 

Table 2.2. Electronic configurations and crystal field stabilization energies of transition metal ions in octahedral coordination... 

Crystal field stabilization energy, CFSE. Each electron in a t2g orbital stabilizes a transition metal ion in octahedral coordination by 0.4Ao, whereas every electron in an eg orbital destabilizes it by 0.6Ao. The crystal field stabilization energy, CFSE, represents the algebraic sum of these factors. Cations may have... 

The crystal field parameters of minerals containing Ni2+ ions are summarized in table 5.19. Note that the energy of the first transition, band u, for Ni2+ in octahedral coordination provides a direct measure of the crystal field splitting parameter A . Crystal field stabilization energies for Ni2+ derived from band u, decrease in the order... 

Measurements of absorption spectra of oxides, glasses and hydrates of transition metal ions have enabled crystal field stabilization energies (CFSE s) in tetrahedral and octahedral coordinations to be estimated in oxide structures (see table 2.5). The difference between the octahedral and tetrahedral CFSE is called the octahedral site preference energy (OSPE), and values are summarized in table 6.3. The OSPE s may be regarded as a measure of the affinity of a transition metal ion for an octahedral coordination site in an oxide structure such as spinel. Trivalent cations with high OSPE s are predicted to occupy octahedral sites in spinels and to form normal spinels. Thus, Cr3, Mn3, V3+... 

Crystal chemistry of spinels. A classic example showing that transition metal ions display distinct site preferences in oxides stems from studies of spinel crystal chemistry. The spinel structure contains tetrahedral and octahedral sites normal and inverse forms exist in which divalent and trivalent ions, respectively, fill the tetrahedral sites. The type of spinel formed by a cation is related to its octahedral site preference energy (OSPE), or difference between crystal field stabilization energies in octahedral and tetrahedral coordinations in an oxide structure. Trivalent and divalent cations with large site preference energies (e.g., Cr3 and Ni2+) tend to form normal and inverse spinels, respectively. The type of spinel adopted by cations with zero CFSE (e.g., Fe3+ and Mn2+) is controlled by the preferences of the second cation in the structure. 

Thirdly, cations are not likely to be equally distributed between six-fold, five-fold and four-fold coordination sites in the magma. According to the Maxwell-Boltmann distribution law, the ratio of cations in octahedral and tetrahedral sites, njnv the crystal field stabilization energies of which are E0 and Et, respectively, above a reference level, U0 is... 

Fig. 10. Variation of the da ratio of LaSrB04 (B = Cr, V, Fe) with the octahedral crystal field stabilization energy, ACf, and the optical electronegativity of the B ions in B2Oj compounds.

TABLE 8.5 Electron Configurations and Crystal Field Stabilization Energies for High- and Low-Spin Octahedral Complexes... 

Table 8.5 summarizes the electron configurations possible for 10 electrons in an octahedral crystal field, provides specific examples of transition metals with these configurations, and tabulates the crystal field stabilization energies (CFSE)... 

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