Divalent rare earths

The divalent rare-earth ion Eu has the 4f electronic configuration at the ground states and the 4f 5d electronic configuration at the excited states. The broadband absorption and luminescence of Eu are due to 4f - 4 f 5d transitions. The emission of Eu is very strongly dependent on the host lattice. It can vary from the ultraviolet to the red region of the electromagnetic spectrum. Furthermore, the 4f-5d transition of Eu decays relatively fast, less than a few microseconds [33]. 

Boff and Novae [6] found a divalent rare earth metal complex, (C5Me5)2Sm, to be a good catalyst for the polymerization of MMA. The initiation started with... 

Biltz-Zen trend have been discussed and represented as a function of a charge transfer atomic parameter which correlates with Pauling s electronegativity. This approach has been successfully employed for groups of binary alloys formed by the alkaline earths and the divalent rare earth elements. 

ALLOYS OF THE ALKALINE EARTH METALS AND OF THE DIVALENT RARE EARTH METALS... 

On the other hand it may be noticed that some aspects of the chemistry and alloying behaviour of Ca, Sr and Ba could be conveniently compared with those of the divalent rare earth metals europium and ytterbium. 

Divalent rare earth ions also have an outer electronic configuration of 4f"( including one more electron than for the equivalent trivalent rare earth). However, unlike that of (RE) + ions, the 4f " 5d excited configuration of divalent rare earth ions is not far from the 4f" fundamental configuration. As a result, 4f" 4f " 5d transitions can possibly occur in the optical range for divalent rare earth ions. They lead to intense (parity-allowed transitions) and broad absorption and emission bands. 

A relative ratio between the 4/ and 4/ 5d-configuration levels energies specifies a sharply distinctive position of broad bands in the spectra of trivalent and divalent rare-earth ions, hi the TR + spectra, with the exception of Ce +, broad bands fall into a relatively far UV region and they yield only fine spectra in the visible and adjacent regions. In the spectra broad bands fall into the visible and near-UV regions. Thus in the case of TR + the f-d and /-/ transitions he close to each other and overlap. Three individual cases are distinguished in the TR + liuninescence spectra, namely broad bands due to d-f transitions, line IR spectra and a combination of bands and fines. 

The exceptions to the rule that rare earth hosts are best for rare earth activators are special cases. For example, some anions such as sulfide yield compounds in combination with non-rare earth cations (e.g. Zn) which show higher luminescent efficiency than with rare earths. Additionally, divalent rare earth activators like Eu " " substitute readily for non-rare earth divalent cations. 

For the divalent rare earth ions the situation is slightly complicated, and they may be categorized into three classes according to the nature of their ground states. 

The f d transition in divalent rare earth ions occurs quite early and this broad intense / - -d band almost invariably blots out the weak f—f lines. 

The spectra of divalent rare earths Sm, Eu and Yb both in the solid state and solution have been reported. According to the nature of the ground state, the three types are (a) ions with f1 ground state, (b) ions with ground state from f 1 d configuration, (c) ions in which... 

Fig. 8.32. Spectra of divalent rare-earth ions in CaF2 host crystal showing the f" - f" 1 d absorption bands. The arrows show the position of 4f internal transitions as found in the fluorescence measurements. (Reproduced from J. Chem. Phys. [182] with the kind permission of the American Institute of Physics.)...

In spite of the fact that in many cases the larger alkaline earth metals can be exchanged for divalent rare earth metals, the latter ones form different varieties of three-dimensional or layered triply bonded disilicides with either the a-XhSi2 or the AIB2 structure. These two structure types are clearly dominated by a trigonal prismatic arrangement of the cations and contain silicon in exclusively trigonal planar coordination (Fig. le and If). 

Beryllium, Be = 9, is undoubtedly a divalent rare-earth element, with the oxide RO, as was shown by the author in 1878, 1881, and 1882. Then come the trivalent elements scandium. Sc = 44 yttrium,... 

In 1961 Hayes and Twidell (8) found that if calcium fluoride crystals containing trivalent thulium were irradiated with x-rays, some of the thulium was converted to the divalent state. This discovery was the first of many in the study of dilute solutions of divalent rare earth ions. Most workers prefer to study the alkaline earth fluorides since these materials are stable with respect to air and have more attractive mechanical properties than the alkaline earth chlorides, bromides, and iodides. Enough work has been carried out in these softer materials to show that reactions similar to those in the fluorides do occur. 

Concretely, this chapter will first deal with the molecular chemistry of uncommon divalent rare-earth compounds, excluding those of Eu, Yb and Sm that have now been defined as "common". We will describe the molecular chemistry of the salt-like rare-earth diiodides Ndl2, Dyl2 and... 

About at the same time, the organometallic and coordination chemistry of the classical divalent rare earths Sm°, Fu° and Yb" had started because the diiodide precursors could now be made by solution chemistry. Especially that of Sm° (the most reactive of the "common" divalent rare earths) gained momentum with the milestone discovery by Kagan et al. that Sml2 could be made in THF solution by the simple Grignard-like reaction of Sm with diiodoethane at room temperature (Namy et al., 1977). 

A new breakthrough in the non-classical divalent rare-earth chemistry occurred in 1997, when Bochkarev and Evans in a milestone paper reported that a light-sensitive molecular complex of Tml2, identified as a solvate of composition [Tml2(DME)3] (Figure 1) (DME = 1,2-dimethox-yethane), could be made in relatively mild conditions by the direct reaction of thulium metal with iodine in refluxing DME (Scheme 1). 

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