Homonuclear IVCT transitions

Such Fe-Fe bonding provides the mechanism for electron delocalization in mixed-valence Fe2+-Fe3+ minerals (Sherman, 1987a). Electron delocalization requires that the adjacent Fe atoms be ferromagnetically coupled, however. 

The MO calculations also provide an explanation of the intensity-enhancement of Fe2+ CF bands by the Fe2+ — Fe3+ IVCT transitions observed, for example, in optical spectra of vivianite (fig. 4.12) and babingtonite (Amthauer and Rossman, 1984). The strong coupling between the Fe2+ and Fe3+ ions removes the centre of symmetry of the 3d orbitals at the Fe2+ site, making the Fe2+ CF transitions Laporte-allowed ( 3.7.1) and, hence, gready intensified (Sherman, 1987a). 

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Other possible examples of homonuclear IVCT transitions include Mn2+ — Mn3+ and Cr2+ — Cr3 IVCT transitions. 

On the other hand, the effect of pressure on IVCT transitions dominates the temperature variations (table 9.3). Intensities increase considerably at high pressures but decrease with rising temperature. In general, energies of homonuclear IVCT transitions (e.g., Fe2+ —> Fe3+ Ti3+ —> Ti4 ) are relatively... 

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