Crystal field splitting lanthanide ions

Lanthanide impurity-ion spectra consist of a series of sharp lines that appear in groups of closely spaced sublevels that correspond to transitions between crystal-field split free-ion levels. The simplest absorption spectra occur at very low temperatures ( 4K), at which only the lowest Stark level is populated, in general. As the temperature is raised, transitions originating from thermally accessible excited levels are possible, thus complicating the spectrum. In fluorescence spectra, transitions arise at low temperature only from the lowest lying sublevel of the excited free-ion level. At higher temperatures, other transitions become possible. 

Crystal field effects are of the order of the free ion interaction thus they cannot be treated as a small perturbation as in the lanthanides. Whereas the crystal field splitting in the oxidation state +3 is comparable to that for the lanthanides, it is significantly increased in the progression... 

Crystal Field Splittings. The key idea is to use fluorescent probe ions that have spectra that are sensitive to the crystal fields they experience within a material (2). Such ions are generally ones with unfilled inner orbitals such as lanthanides (e.g. Pr3+, Eu3+, Er +), actinides (e.g. U), or transition metals (e.g. Cr3+, Re4+, Os4+). 

The f electrons within the 4fn configuration of the lanthanides have only weak interactions with the crystal field because they are shielded by outer 5s and 5p electrons. Consequently the spectra are dominated by transitions between the atomic states of the lanthanide ion. The left side of Figure 1 shows the ionic energy levels of Eu3+ in the gas phase. When the ion enters a crystal lattice, there will be additional crystal field interactions. The interactions cause small crystal field splittings on the order of 200 cm-1 that are superimposed on the atomic transitions and are easily observable. 

The electronic spectra of lanthanide compounds resemble those of the free ions, in contrast to the norm in transition metal chemistry the crystal-field splittings can be treated as a perturbation on the unsplit levels. Complexes thus have much the same colour as the... 

Friedman and Low (6) have shown that the trivalent lanthanides dissolved in the alkaline earth fluorides can be compensated by interstitial fluoride ions at either adjacent or remote sites. If the interstitial is adjacent, the crystal field of the trivalent is axial but if it is remote, the crystal field of the trivalent is cubic. Measurement of the crystal field splitting of radiation-produced divalent lanthanide ions indicate cubic symmetry 16). More recent measurements by Sabisky (20) have shown a small percentage of non-cubic sites. It is thought that the trivalent ions in the cubic symmetry are the species predominantly reduced by radiation. 

All but one of the lanthanide ions show absorption in the Visible or near UV region. The exception is Lu3+ which has a full/shell. In the spectra of lanthanides spin orbit coupling is more important than crystal field splitting. The colours are due to Laporte-forbidden f-f transition i. e., transitions between the J states of 4n configuration since the change in the subsidiary quantum number is zero. The forbitals are... 

After excitation into one of the 5d states the lanthanide ion relaxes by nonradiative transitions to the lowest-energy one from which emission can occur if there is a large energy gap with the ground or excited 4f levels. This condition is fulfilled by Ce + (4T), Yb + (4T ) and generally Eu + (4f ) (Section 2.5). Also Pr + (4f ) exhibits a 5d 4f emission, located mainly in the UV, when the nephelauxetic effect and crystal field splitting lower the first 5d state below the Sq (4f ) level at 47 000 cm L 5d 4f emissions in the VUV are observed for Nd +, Er + and Tm + in fluorides. ... 

Historically, the first extensive developments in crystal-field theory made use of the fact that the crystal-field coupling in lanthanide ions is small. In the operator-equivalent method (Stevens, 1952 Elliot and Stevens, 1953), the coupling of different free-ion levels by the crystal-field interaction is ignored and the crystal-field splitting of each Lj level is treated separately. Traditionally, in this method, the crystal-field Hamiltonian is written as... 

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