Crystal field spectra intensities

The most important use of energy level diagrams described in 3.5 is to interpret visible to near-infrared spectra of transition metal compounds and minerals. The diagrams provide qualitative energy separations between split 3d orbitals and convey information about the number and positions of absorption bands in a crystal field spectrum. Two other properties of absorption bands alluded to in 3.3 are their intensities and widths. 

The valence and coordination symmetry of a transition metal ion in a crystal structure govern the relative energies and energy separations of its 3d orbitals and, hence, influence the positions of absorption bands in a crystal field spectrum. The intensities of the absorption bands depend on the valences and spin states of each cation, the centrosymmetric properties of the coordination sites, the covalency of cation-anion bonds, and next-nearest-neighbour interactions with adjacent cations. These factors may produce characteristic spectra for most transition metal ions, particularly when the cation occurs alone in a simple oxide structure. Conversely, it is sometimes possible to identify the valence of a transition metal ion and the symmetry of its coordination site from the absorption spectrum of a mineral. 

From the color (absorption spectrum) of a complex ion, it is sometimes possible to deduce the value of AOJ the crystal field splitting energy. The situation is particularly simple in 22Ti3+, which contains only one 3d electron. Consider, for example, the Ti(H20)63+ ion, which has an intense purple color. This ion absorbs at 510 nm, in the green region. The... 

The intensity of the EPR resonance absorption is a measure of the number of paramagnetic centres present [346], while the type of line observed and the measured g factor are indications of the interactions of the paramagnetic particles and of their distribution within the matrix. Such spectra are much more sensitive to changes in crystal field and atomic orientations than X-ray diffraction and are not dependent upon crystallinity [347]. The nature of the paramagnetic particles may be discerned from the superfine structure of the spectrum. 

Fig. 4.14 Magnetically perturbed quadrupole spectra simulated for powder distributions of the EFG (Vzz > 0) with an applied field B = 4T which is fixed in the laboratory system perpendicular to the y-beam/ The value of the quadrupole splitting is kept constant at AEq = +4 mm s For negative quadrupole splitting (V z < 0), the spectra would be inverted on the velocity scale. Note the difference in relative intensities for the spectrum for ry = 0 and the single-crystal type spectrum given in Fig. 4.13. Similar patterns are obtained for B y...

Figure 1,17 Absorption spectrum of a forsteritic olivine under polarized light. Ordinate axis represents optical density (relative absorption intensity, ///q). From R. G. Burns (1970), Mineralogical Applications of Crystal Field Theory. Reprinted with the permission of Cambridge University Press.

Figure 4.37a represents the time-resolved luminescence spectrum of a hydrozincite under 266 nm laser excitation. A relatively broad band is detected at 430 nm, which is responsible for the well-known blue hydrozindte luminescence. Its spectral position and decay time of approximately 700 ns are typical for Eu luminescence. However, the excitation spectrum of this band consists of one narrow band at 240 nm (Fig. 4.37b), which does not correspond to an Eu " excitation spectrum. Two bands usually characterize the latter with relatively small Stokes shifts of 30-50 nm caused by crystal field splitting of the 4/ 5d-levels. Moreover, the measured Eu concentrations in the hydrozincite samples under investigation are very low (less than 0.5 ppm) and they do not correlate with the intensity of the blue luminescence, i.e. the band at 430 nm. 

The reflectance spectrum of Ni2Si04 spinel contains intense bands with absorption maxima of 9,150 cm-1, 14,780 cm-1 and 22,550 cm-1 at atmospheric pressure (Yagi and Mao, 1977). These led to initial estimates of the crystal field parameters for Ni2+ in silicate spinel of A0 = 9.,150 cm-1 and CFSE = 10,980 cm-1. However, there is also a prominent shoulder in the reflectance spectra around 8,000 cm-1 attributable to trigonal distortion of the octahedral site in the spinel structure (Bums, 1985a). This led to revised estimates for Ni2+ in Ni2Si04 spinel of... 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

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