Crystal nephelauxetic effect

By far, the most studied quantum cutting system to date is for praseodymium, where the 4f2 1 So state lies below the lowest 5d state for many fluorides and some oxides. For most Pr3+ doped systems, the nephelauxetic effect and the 5d crystal field splitting are large enough to push the lowest 5d level below the 1 So state. However, for those systems in which the So... 

The energy levels of lanthanide free ions in crystals and solutions determined from the absorption spectra differ somewhat from those of gaseous Ln3+ ions and depend on the environment [33-35]. In the presence of ligand fields, the whole 4f" structure appears to shorten as compared to gaseous ions. The mean J level in Pr3+ shifts by 5 percent [33] and by one percent in Er3+ [36]. The red shift observed in the absorption spectra of lanthanides has been termed as the nephelauxetic effect. The nephelauxetic effect has been discussed in detail [34],... 

The phenomenon of the nephelauxetic effect cannot be interpreted in the framework of an electrostatic model which considers exclusively first-order crystal field effects. The observed band shifts could be due to second-order crystal field effects. However, the theoretical values of the second-order crystal field effects are of the order of 10 cm-1 or so [40,41] and cannot account for the experimentally observed band shifts, which in some cases are as high as 100 to 1000 cm 1. 

The possible explanation [233] for the environmental sensitivity of the transitions (nonhypersensitive) may be due to an increase in 7) parameters which results in lowering of the excited states of the ions since the surrounding crystal field is influenced by the symmetric part of the crystal field. The observed changes in 7 and the small nephelauxetic effect show that the nonsymmetric part of the crystal field affects 7 values. The modifier ions distort the oxygen bonded to the lanthanide and thus lowers the site symmetry and increases the f-f transition probability. 

For the same ligand, the nephelauxetic effect and the crystal field depend on the following factors ... 

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

After excitation into one of the 5d states die lanthanide ion relaxes by nomadiative transitions to the lowest-energy one from which emission can occm if there is a large energy gap with the ground or excited 4f levels. This condition is fulfilled by Ce + (4f ), Yb + (4f ) 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 held splitting lower the first 5d state below the So (4f ) level at 47 000 cm 5d 4f emissions in the VUV are observed for Nd +, Er + and Tm + in fluorides. ... 

Ce + The energy of the emitting 5d level lowers with increasing nephelauxetic effect and crystal field splitting cryst (Figme 5). The positions of the shorter-energy 4f 5d absorption band and the emission band published... 

Crystal field strengths, nephelauxetic effects, and experimentally based molecular orbital schemes (in the f range) of low-symmetric [Sm(rj5-Cp)(ri3-Tp )(ri2-Tp )], have been determined through absorption and luminescence spectra, carried out at room and low 1 o... 

The energy of the emitting level depends on the nephelauxetic effect and the crystal Geld at the luminescent-ion site. 

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