Lanthanide ions, divalent

In view of this the divalent lanthanide ions are expected to show intense optical absorption in the whole visible region in compounds containing M(d°) ions. Since these compounds are not easy to prepare, our earlier warning is relevant. Only results on well-prepared and characterized compounds may be... 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

As a general rule the c.t. bands shift to lower energies with increasing oxidation state, whereas Rydberg transitions (such as 4f- -5d transitions) shift to higher energies. It may, therefore, be expected that the lowest absorption bands of the tetravalent lanthanide ions will be due c. t. transitions and those of the divalent lanthanide ions to Af- -5d transitions. 

After this discussion of c.t. transitions on Ln i+ ions we now turn to the divalent lanthanides ions. Here the first allowed transitions in the spectra are 4f- -5d transitions as expected. They have been studied in detail. We will here mention some relevant results. 

Organometallic compounds of lanthanide metals other than Sm, Eu, and Yb are very rare until now. But the development of this chemistry became possible after the synthesis of divalent precursors of Tm, Dy, and Nd in the late 1990s, namely of their diiodides (see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry) and by using hgands such as phospholyl or arsolyl, which stabilize divalent lanthanide ions. 

Preparation and Identification of Divalent Lanthanide Ions as Dilute Solutes in Alkaline Earth Halide Solid Solutions... 

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. 

Divalent lanthanide ions have also been obtained in fused barium bromide by Pinch (19), who used ultraviolet radiation for the reduction. In this instance the photolysis produced bromine which was removed from the system by its volatility, and the remaining divalent lanthanide ions were stable. This reduction technique suffers from the same difficulty as the metallic reduction technique in that it is difficult to grow good crystals from this reactive melt. 

In the various solvent-extraction circuits employed in this process, use is made of a solution of D2EHPA in kerosene as the extractant. The selective recovery of the various metals is achieved by careful control of the equiUbrium pH value of the aqueous phases in the multistage extraction and stripping operations. After the leach liquor has passed through two separate circuits, each of which comprises five stages of extraction and four of stripping, the europium product is obtained initially as a solution of europium(III) chloride. Further purification of the product is accomplished by reduction with amalgamated zinc to Eu +, which is by far the most stable of the divalent lanthanide ions with respect to the reduction of water cf. the redox potentials of the Eu /Eu and Sm /Sm + couples, which are —0.43 and —1.55 V respectively ). Sulfuric acid is added to the... 

It would be interesting to compare the formation constant data of the divalent lanthanide ions with the isoelectronic trivalent ones. Unfortunately, there is a paucity of data for the divalent ions. The Eu(II) ion has a 4/7 configuration and the formation constant measurements by Eckardt and Holleck (39) for the EDTA and DCTA complexes show that the Eu(II)-complex has a much lower value for the formation constant than that for either the Eu(III) or the Gd(III) complexes. A lower log K, value for the Eu(II) complex compared to the Gd(III) complex is expected on the basis of the larger size (20) of the Eu(II) ion, and the values for the Eu(II) complexes compare well with those of the alkaline earth ions (log K1EDTA Mg 8.69, Ca 10.59,... 

Fig. 37. Plot of the estimated third ionization potentials (77) against the L-values of the divalent lanthanide ions (Mill). The observed values are shown as crosses and the postulated values for Pm and Er as open squares.

Figure 6. Energy levels and laser transitions for divalent lanthanide ions. Approximate wavelengths of transitions are given in micrometers.

This chemical n-doping procedure can readily be extended to the lanthanide ions Eu and Yb. Europium and ytterbium metals are known to dissolve in liquid ammonia (19). They form solvated divalent cations and solvated electrons in ammonia with the characteristic blue color. Upon immersion of polyacetylene the solvated electrons spontaneously reduce the polyacetylene chains to polycarbanions and the divalent lanthanide ions become countercations to maintain charge neutrality. 

As a fundamental class of compounds in the fields of synthetic solid-state (and also molecular) chemistry, cyanamides and carbodiimides have gained increasing attention within the past decade. Because of their 2-fold anionic charge, both cyanamide and carbodiimide structural units allow the realization of nitrogen-based pseudo-oxide chemistry since NCN is able to replace in a wide variety of novel materials. A number of alkali metal, alkaline-earth metal, main-group metal,divalent transition-metal, trivalent rare-earth metal,and also triva-lent transition-metal cyanamides/carbodiimides were obtained following different synthetic routes. The only carbodiimide containing divalent lanthanide ions was reported by DiSalvo et al., who found that EuNCN is isostructural to the already known a-SrNCN. ... 

ENDOR measurements of the transferred hyperfine interaction parameters A, (all in MHz) for the nearest fluorine neighbours of two divalent lanthanide ions substituting for Ca in CaFj. The measurements on Eu are by Baker and Hurrell (1963), and on Tm by Bessent and Hayes (1965). Hyperfine constants for the two europium isotopes are given in table 3 for Tm the ENDOR measurements of Bessent and Hayes give g = ( + )3.443(2), A = (-)l 101.376(4)MHz, for the single stable isotope of mass 169 I =2). 

Fig. 9.6. Melting slope dT/dP and ionization energy I for divalent lanthanide ions against atomic number, from Jayaraman (1965) and Johansson and Rosengren (1975> respectively. To be noted is the remarkable resemblance of the two sets of data. The smooth curve is the interpolated binding energy difference between divalent and trivalent metallic states.

The luminescence of the divalent lanthanide ions Eu, Tm ", Sm, Yb " has been reviewed by Rubio O [85]. Generally, the d f emission transitions occur at lower... 

Fig. 1.6 Position of the 4f state of divalent lanthanide ions in CaGa2S4. m is the numbca- of 4f" electron, and E is the Fermi energy level. Reprinted with the permission from Ref. [25]. Copyright 2005 American Chemical Society...

As seen from Table 5.3, the volume of a unit ceU of CaCl2 doped with Tm " is slightly increased (by approximately 0.68 %), whereas the unit cells of SrCl2 and BaCl2 undergo a compression by 1.21 and 2.41 %, respectively. Such behavior can be explained by a simple comparison of the ionic radii of aU ions involved. However, inspection of the database of the ionic radii given by Shannon [47] shows that the ionic radii of Tm " ions with eightfold and ninefold coordination numbers (CN) are absent. Due to the consideration of the chemical similarity across the whole lanthanide series, we can propose a linear ionic-radius dependence of divalent lanthanide ions on their atomic number Z and coordination number N to bypass the problem we encountered above as follows ... 

Figure 6. Energies of the lowest levels of the 4f 5d configuration for the divalent lanthanide ions as free ions (open circles) and in CaF (filled circles and open squares) (Refs, 12 and 15).

In fact Sm and Yb compounds are the only ones obeying the Luttinger criterion for Ef in the gap. The condition is that the lanthanide ions can exist in a divalent state 4f and 4f for Sm and Yb, respectively, with even number of electrons. In the intermediate-valent state due to hybridization, part of the 4f electrons obtain 5d character, but the sum of f and d electrons remains even. For other divalent lanthanide ions such as Eu or Tm with 4f or 4f states, respectively, the 4f count is odd. For Tm, however, an additional condition, namely anti ferromagnetic order has been found, which doubles the chemical unit cell and one arrives nevertheless at an even f and d electron count (more about this below, section 4.3). 

The colors of the divalent lanthanide ions are orange-yellow to brown-red for Sm, colorless or yellow for Eu +, brick red for green or yellow for Yb. ... 

Since the lanthanides that exhibit the divalent state in aqueous solution, Sm, Eu ", and Yb, are all readily oxidized to the trivalent state, attempts to record their absorption spectra have usually proceeded from the rapid dissolution of a soluble anhydrous compound. The absorption spectra shown in fig. 24.16 for Sm ", Eu, and Yb were adapted from results published by Butement and Terrey (1937), Butement (1948), and Christensen et al. (1973). Production of other divalent lanthanide ions by pulse radiolysis, and the observation of their spectra is discussed in section 5.2. 

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