Crystal field theory, spectral bands

Analysis of the valence-band spectrum of NiO helped to understand the electronic structure of transition-metal compounds. It is to be noted that th.e crystal-field theory cannot explain the features over the entire valence-band region of NiO. It therefore becomes necessary to explicitly take into account the ligand(02p)-metal (Ni3d) hybridization and the intra-atomic Coulomb interaction, 11, in order to satisfactorily explain the spectral features. This has been done by approximating bulk NiO by a cluster (NiOg) ". The ground-state wave function Tg of this cluster is given by,... 

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. 

Having discussed the theoretical principles of crystal field theory, theories of intensity of the hypersensitive transitions, it is logical to examine the absorption spectral features of lanthanide ions in aqueous solutions. It is probably worthwhile looking into the historical development of the spectra of lanthanides. The most prominent era in the development is the 1930 s. Prandtl and Scheiner [118] presented a complete collection of absorption spectra of trivalent lanthanides in solution. The covered region is 7000-2000 A. An apparent symmetry in the region of absorption with band structure shifting toward the ultraviolet in approaching the centre of the series from both ends was observed. 

---