Crystal field bands

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

Figure 7.3-left shows diffuse reflection spectra of the different Fe oxides and Table 7.5 lists their transitions. Fig. 7-3 right summarizes the range of the crystal field band... 

Fig. 7.3 Left Diffuse reflectance spectra of Fe oxides in the UV-Vis-near IR range (Sherman et al., 1982, with permission). Right Median and range of the crystal field band positions determined from second-derivative minima the two...

In addition to the above study, the reflectance spectrum of a powdered sample of VOSO HsO has been determined, and the crystal field bands at 13,000 cm.-1 and 16,000 cmv1 were resolved. 

The reflectance spectrum of VO has been measured by RiidorfF, Walter, and Stadler.28 This measurement shows the charge transfer band at 41,700 cm.—1 which is characteristic of vanadyl ion, but the crystal field bands were not resolved. 

Coming to our subject proper, to the intensities of the crystal-field bands, we take our stand on the long established guess or recognition that these are electric dipole transitions assisted by vibrations of the complex. The evidence for this is mainly that as the temperature is varied the magnitude of the intensity follows the famous cotangent law ... 

Intracrystalline Fe2+-Mg2+ distributions in natural and synthetic orthopyroxenes have been determined from intensities of absorption bands in their polarized spectra (Goldman and Rossman, 1977a Steffen et al., 1988). Molar extinction coefficients of crystal field bands centred at 10,500 to 11,000 cm-1 and 4,900 to 5,400 cm-1 originating from Fe2+ ions located in pyroxene M2 sites ( 5.5.4) enabled the iron contents to be estimated from the Beer-Lambert law equation, eq. (3.7). 

Other minerals coloured by a transition metal In addition to many of the gems listed in table 4.1, other examples of minerals that owe their colours to crystal field bands (intra-electronic transitions between 3d orbitals within individual cations) include the following ... 

Figure 4.12 Polarized absorption spectra of a vivianite single crystal measured in three zones of increasing oxidation. (1) Nearly colourless (2) light blue and (3) dark blue. The spectra were measured with light polarized along the b axis corresponding to the Fe2+-Fe3+ vector of edge-shared [FeOg] octahedra. Note the intensification of the Fez+ crystal field bands at 1,200 nm and 800 nm by the Fe2+ — Fe3+ IVCT at 630 nm (from Amthauer and Rossman, 1984).

Perhaps a more fundamental application of crystal field spectral measurements, and the one that heralded the re-discovery of crystal field theory by Orgel in 1952, is the evaluation of thermodynamic data for transition metal ions in minerals. Energy separations between the 3d orbital energy levels may be deduced from the positions of crystal field bands in an optical spectrum, malting it potentially possible to estimate relative crystal field stabilization energies (CFSE s) of the cations in each coordination site of a mineral structure. These data, once obtained, form the basis for discussions of thermodynamic properties of minerals and interpretations of transition metal geochemistry described in later chapters. 

These assignments of the crystal field bands may be used to construct the 3d orbital energy level diagrams illustrated in fig. 5.17 for Fe2+ ions in the Ml and M2 sites of ferrosilite, Fsgg 4. The polarized absorption spectra of this ferrosilite (fig. 5.15b) show that two of the M2 site Fe2+ bands are centred near 10,700 cm-1 and 4,900 cm-1. The lower-level splittings of 2,350 cm-1 and 354 cm-1 listed in eq. (5.11) for enstatite axe assumed to apply to ferrosilite. This information is... 

The polarized spectra of alkali amphiboles such as those of glaucophane illustrated in fig. 4.15, have been studied extensively (Chesnokov, 1961 Littler and Williams, 1965 Bancroft and Bums, 1969 Faye and Nickel, 1970a Smith and Strens, 1976) and are dominated by Fe2+ —> Fe3+ IVCT bands ( 4.7.2). However, Fe2+ crystal field bands occurring at 10,000 cm-1 and 8,600 cm-1 are distinguishable in fig. 4.15, indicating that glaucophane provides CFSE s comparable to those of Fe2+ ions in Ml and M3 octahedral sites of other amphiboles, eq. (5.17). 

The optical spectra of blue tourmalines have attracted considerable attention focused mainly on assignments of Fe2+ —> Fe3+ IVCT bands, positions of crystal field bands for Fe2+ ions expected to be located in two different octahedral sites, and intensification mechanisms of these crystal field bands (e.g., Faye et al., 1968., 1974 Wilkins et al., 1969 Bums, 1972a Smith, 1977, 1978a Mattson and Rossman, 1984,1987b). Curve-resolved spectra yielded two sets of paired bands (Faye et al., 1974). One set at 14,500 cm-1 and 9,500 cm-1 assigned to Fe2+ in the Al or c-site (point group Q mean Al-O = 192.9 pm) yielded A0 and CFSE values of about 11,000 cm-1 and 4,900 cm-1, respectively. The second set of bands at 13,200 cm-1 and 7,900 cm-1 attributed to Fe2+ in the Mg or b-site (point group Cm mean Fe-O = 202.5 pm) provided A0 and CFSE values of approximately 10,000 cm 1 and 4,500 cm-1, respectively. 

Cr +) have been probed by absorption and luminescence spectroscopy. The strongest interaction occurs for the NF complex, where the lowest energy singlet state is very close in energy to a spin allowed crystal field band, giving rise to intense vibronic patterns. ... 

Fig. 2.21. Polarized electronic (optical) absorption spectra of a vivianite crystal with zones (labeled 1, 2, 3) exhibiting three different degrees of oxidation. The arrows identify Fe + crystal-field bands at 8,300 and 11,400 cm , and the Fe + —> Fe intervalence charge-transfer transition at 15,800 cm (after Amthauer and Rossman, 1984 reproduced with the publisher s permission).

Identification of Iron Oxides by Color and Crystal-Field Bands... 

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