Trivalent Lanthanide Ions

Fig. 10 The product function /moi T vs. kT/X for lanthanide trivalent ions left - less than half-filled shells, right - more than half filled shells (X < 0) numbers correspond to the/ configuration...

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

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. 

Many of the trivalent lanthanide ions exhibit long-lived excited states. These excited states cannot be populated directly, since the lanthanide(III) ions themselves have very low absorption coefficients, due to the fact that the f-f transitions are formally forbidden by the LaPorte rule. In addition, a number of transitions are also forbidden by the spin crossover rule. Typically, these extinction coefficients are of the order of 1 M-1 cm-1 (20). 

Carnall, W. T. Fields, P. R. Rajnak, K. Electronic energy levels of the trivalent lanthanide aquo ions. I-IV. J. Chem. Phys. 1968, 49(10), 4424-4455. 

Although some of the lanthanides form compounds with oxidation states other than + 3, the vast majority of stable species involve the trivalent state. Due to the nature of the weak bonding f electrons, complexes in solution are normally quite labile, and as described below, the preparation and the isolation of pure enantiomers containing a central lanthanide ion and achiral ligands are extremely rare. The coordination number of lanthanide(III) ions is somewhat variable and depends on the size and charge of the coordinated ligands, and the size of the lanthanide ion that varies slightly... 

Mandelic acid is known to bind with trivalent ions with greater efficiency, and log K values ranging from 2.3 to 2.8 have been reported for the various trivalent ions of the lanthanide series. 

Electronic Energy Levels of the Trivalent Lanthanide Aquo Ions. II. Gd3+, W.T. Camall, P.R. Fields, andK. Rajnak. J. Chem. Phys. 49, 4443-4446 (1968). 

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

Although the conductivity of the trivalent-ion / ""-aluminas is too low for solid electrolyte applications (e g. batteries, sensors), they have potential use in optics, phosphors, and lasers because they can serve as single crystal or powder hosts for the optically active lanthanide ions. For example, Eu +-/3""-alumina emits red luminescence when excited by UV rays. A Nd +-/3""-alumina single crystal shows luminescent... 

The zinc reduction of Eu + to Eu +, followed by its precipitation as the sulfate, is a traditional step in the separation of europium from other lanthanides. In general, the solubilities of the inorganic compounds of the Ln + ions resemble those of the corresponding compounds of the alkaline earth metals (insoluble sulfate, carbonate, hydroxide, oxalate). Both europium and the Sm + and Yb + ions can also be prepared by other methods (e.g. electrolysis), although these solutions of the latter two metals tend to be short-lived and oxygen-sensitive in particular. Eu + is the only divalent aqua ion with any real stability in solution. Several divalent lanthanides can, however, be stabilized by the use of nonaqueous solvents such as HMPA and THE, in which they have characteristic colors, quite distinct from those for the isoelectronic trivalent ions on account of the decreased term separations. 

Mixed oxides of Ce and other lanthanides (La, Nd) were studied by Kubsch et al. [133], who found segregation of these trivalent ions to the surface in samples calcined at 1253 K a high Eb peak in the 01s feature was observed, being ascribed to carbonates formed in the trivalent ion-rich surface. A tendency to formation of such mixed oxides was detected in (Ce,Tb)Ox supported on lanthana-promoted alumina after calcination the La XP spectrum displayed two components, ascribed to La species on the AI2O3 surface and dissolved into the Ce,Tb oxide respectively [146]. A high Eb Ols peak observed there was ascribed to both alumina 0 ions and carbonate species on La-rich zones while the (Ce,Tb) oxide gave lower Eb values. 

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. 

Because of the instability of the radiation produced divalents, their use in lasers was not desirable, and work on finding methods to prepare materials containing stable divalent lanthanides was stimulated. Reduction techniques were developed for the fused state from which crystals had to be grown and also for the solid state in which the as-grown crystals contained trivalent ions. 

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