Lanthanide elements crystal field effects

In condensed phases, spectra are commonly measured in absorption. Three main types of transitions are observed in the absorption spectra of the actinide ions (1) Laparte-forbidden f to f transitions, (2) orbitally allowed 5/ to 6d transitions, and (3) metal to ligand charge transfer. Of these, study of internal f to f transitions has found wide use in the investigation of actinide chemistry. These band usually in the visible and ultraviolet regions, can be easily identified because of their sharpness, and are sensitive to the metal environment. As discussed earlier, the 5/ orbitals of the actinide elements are more exposed than the lanthanide 4/ orbitals, and therefore, crystal field effects are larger in the 5/ series. The f to f transitions for actinide elements may be up to 10 times more intense and twice as broad as those observed for the lanthanides, due to the action of crystal fields. In addition, extra lines resulting from vibronic states coupled to / / states have been observed. 

Calculated free-ion hyperfine fields for the elements under discussion are summarized in table 2. Hund s rule configuration is assumed. This gives the maximum hyperfine field possible for each charge state since it sets ntj = J. This is appropriate in ordered materials if the free-ion configuration is present and if exchange interaction dominates all other energies. Usually this situation prevails in the heavy lanthanides. Examples will be given later on. Crystal field effects can reduce and this aspect will also be discussed. 

In lanthanide elements, the 5s and 5p shells are on the outside of the 4f shell. The 5s and 5p electrons are shielded, any force field (the crystal field or coordinating field in crystals or complexes) of the surrounding elements in complexes have little effect on the electrons in the 4f shell of the lanthanide elements. Therefore, the absorption spectra of lanthanide compounds are line-like spectra similar to those of free ions. This is different from the absorption spectra of d-block compounds. In d-block compounds, spectra originate from 3d 3d transitions. The nd shell is on the outside of the atoms so no shielding effect exists. Therefore, the 3d electrons are easily affected by crystal or coordinating fields. As a result, d-block elements show different absorption spectra in different compounds. Because of a shift in the spectrum line in the d-block, absorption spectra change from line spectra in free ions to band spectra in compounds. 

The coordination. Transition element ions are in general tetrahedrally or octahedrally coordinated. The ligand field is about twice lower for the fourfold coordination than for the sixfold one (5ee Ligand Field Theory Spectra). The coordination number of lanthanide ions varies from 6 to 12. The nephelauxetic effect and crystal field decrease with increasing coordination number. 

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