Optical absorption spectroscopy in geosciences: Part II: Quantitative aspects of crystal fields
Michael Andrut, Manfred Wildner, Czesław Z. Rudowicz, 2004. "Optical absorption spectroscopy in geosciences: Part II: Quantitative aspects of crystal fields", Spectroscopic methods in mineralogy, Anton Beran, Eugen Libowitzky
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In Part I (Chapter 3 in this volume - Wilder et al., 2004) we described the basic principles of crystal field theory (CFT) based on group theory and symmetry. The usefulness of CFT resides in the fact that it can predict the type and number of electronic transitions and their relative energies for transition metal ions in crystals. Hence CFT enables interpretation of the optical absorption spectra. The crystal (or ligand) field induced on the central ion depends on the type and positions of the ligands (i.e., bond angles and distances R) and on the point symmetry of the resulting coordination polyhedron. The number of exited crystal field (CF) states and the type of the ground state arising from a given free-ion dN configuration depends solely upon molecular symmetry, i.e. the site symmetry in case of crystals, and is independent of any model used to describe the metal-ligand bonds. Although the exact energies cannot be calculated ab initio, it is possible to extract empirical parameters from experimental electronic absorption spectra which describe the interaction between metal and ligand. For a given dNX6 complex with an ideal octahedral coordination (symmetry Oh), the cubic CF splitting parameter 10Dq, together with Racah parameters B and C, provide basis for a reasonably complete description of the electronic spectra (Lever, 1984). In most crystals the site symmetry is, however, lower than Oh. This requires introduction of additional, so-called distortion parameters to describe the lower symmetry CF components
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Spectroscopic methods provide information about the local structure of minerals. The methods do not depend on long-range periodicity or crystallinity. The geometric arrangement of atoms in a mineral phase is only one aspect of its constitution. Its vibrational characteristic, electronic structure and magnetic properties are of greatest importance when we consider the behaviour of minerals in dynamic processes. The characterisation of the structural and physico-chemical properties of a mineral requires the application of several complementary spectroscopic techniques. However, it is one of the main aims of this School to demonstrate that different spectroscopic methods work on the same basic principles. Spectroscopic techniques represent an extremely rapidly evolving area of mineralogy and many recent research efforts are similar to those in materials science, solid state physics and chemistry. Applications to different materials of geoscientific relevance have expanded by the development of microspectroscopic techniques and by in situ measurements at low- to high-temperature and high-pressure conditions.