EUROPIUM AND OTHER LANTHANIDES

National Radiological Protection Board Chilton, Didcot, Oxon 0X11 ORQ, England 

Presence in Biological Fluids and Clearance from the Body 

The numerous accounts of the lanthanides in the life sciences testify to the extensive research on these elements and currently there would appear to be no lessening of this research. The recent publication by Evans [1] is an authoritative account of the biochemistry of the lanthanides and the frequency with which publications appear on the lanthanides will no doubt require a further edition to cover this expanding subject. State-of-the-art accounts of the use of the lanthanides as probes in life, chemical, and earth sciences have been compiled by Biinzli and Choppin [2]. In 1987 the manner in which the physical and chemical properties of the lanthanides are exploited in chemistry and life sciences was succinctly summarized by Niimisto as the largest group in the periodic table—comprising one quarter of the stable metallic elements. Many of their properties are similar, thus one bulk element can be replaced by another. However, most of the novel technology uses are based on differences in properties rather than in similarities [3]. 

The lanthanide elements comprise lanthanum and the 14 elements (cerium to lutetium) that follow lanthanum in the periodic table. Frequently, the terms lanthanides and lanthanons are used as synonyms for the rare earth elements (REEs), although strictly the REEs comprise the lanthanides and also yttrium, which together with gadolinium, the subject of Chap. 29, is omitted from this chapter. As in many accounts of the chemistry of the lanthanides, will be used as the collective chemical symbol for the trivalent cations. 

Slight but significant variations (Fig. 1) in the electronic configuration of the lanthanide series result in (1) chemical anomalies that result in Ce and Ce with respective ionic radii of 103.4 and 87 pm (2) chemical anomalies for europium which can exist as the europous cation Eu , and the europic cation Eu, with respective ionic radii of 117 and 95.0 pm. In the near biosphere Eu(II) formation does not occur but in some environments, such as oceans where it is an important aid for understanding oceanic processes [6], oxidation of Ce(III) can occur. 

---

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. 

The a—time curves for the vacuum decomposition at 593—693 K of lanthanum oxalate [1098] are sigmoid. Following a short induction period (E = 164 kJ mole-1), the inflexion point occurred at a 0.15 and the Prout—Tompkins equation [eqn. (9)] was applied (E = 133 kJ mole-1). Young [29] has suggested, however, that a more appropriate analysis is that exponential behaviour [eqn. (8)] is followed by obedience to the contracting volume equation [eqn. (7), n = 3]. Similar kinetic characteristics were found [1098] for several other lanthanide oxalates and the sequence of relative stabilities established was Gd > Sm > Nd > La > Pr > Ce. The behaviour of europium(III) oxalate [1100] is exceptional in that Eu3+ is readily reduced... 

Within the lanthanides the first ones from La to Eu are the so-called light lanthanides, the other are the heavy ones. Together with the heavy lanthanides it may be useful to consider also yttrium the atomic dimensions of this element and some general characteristics of its alloying behaviour are indeed very similar to those of typical heavy lanthanides, such as Dy or Ho. An important subdivision within the lanthanides, or more generally within the rare earth metals, is that between the divalent ones (europium and ytterbium which have been described together with other divalent metals in 5.4) and the trivalent ones (all the others, scandium and yttrium included). 

ETL materials that are used most often are emissive metal complexes, especially aluminium but also beryllium and lanthanides such as europium and terbinm, of ligands such as 8-hydroxyquinoUne, benzoquinolines and phenanthroUne, whilst other effective compounds inclnde extended conjugated compounds, e.g. distyrylarylene derivatives. Some ETL materials are chosen because they are non-emissive to act as combined ET and hole blocking layers. A selection of these ETL materials is illustrated in Figure 3.35. 

Due to the competing non-radiative decay routes for the lanthanide excited state, there is an intrinsic limit to the overall quantum yield in luminescent lanthanide complexes. It has been estimated that these values are 0.50 and 0.75 for europium and terbium, respectively (27). Although quantum yields exceeding these have been reported (31,32), care should be taken in analyzing quantum yield results in the literature, as these are often given for the energy transfer process alone, and not the overall quantum yield, and in other cases it is unclear as to which process(es) the quoted quantum yield refers to. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

A common precursor to LnN are the simple inorganic amides Ln(NH2)x (x = 2, 3) which can be placed between the nitrides and the alkyl substituted amides. Their main use lies in the synthesis of other solid materials like lanthanide hydroxides [21,33], carbides [34] or above-mentioned nitrides. Very recently solutions of europium and ytterbium in liquid ammonia have been rediscovered as synthetic tools (Sect. 7.1). 

---