Iron octahedral site preference energy

Table 11.8 reveals that most spinels involving Fe (AFe .04) have the inverse structure. The d Fe ion will have a CFSE of zero for both tetrahedral and octahedral coordination, so if there is to be a site preference it will be due to the A(ll) ion. This is cleatly the case for NiFe204. for example, the Nr ion having an octahedral site preference energy of 86 kJ mol. In magnetite. Fe,04, both A and B ions are iron, with some in the +2 oxidation state and others in +3 Fe"Fe" 04. the f" Ft ion, octahedral coordination is more favorable than tetrahedral by about 13 kJ tiu>l, whieh, although only a modest amount, is apparently sufficient to invert the structure. In contrast, the similar oxide Mn]04 has the normal structure. In this instance, the Mn has no CFSE in either octahedral or tetrahedral fields, but d Mn shows a preference of 106 kJ mol for octahedral sites. For Q>,04. another mixed-valence oxide, there is an additional fiictor to take into account—Co is low... 

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. 

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