Europium metal

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. 

The Eulr2 alloy was prepared by direct combination of the metals. Europium chips, % in. on edge were placed in a V4 in.-diameter cavity of a die. Iridium metal powder, —325 mesh and 99.9% pure, was sprinkled over the rare earth element, and the mixture was compressed at 5000 psi. The compressed pellet was placed in a molybdenum boat, which then was transferred to a quartz sleeve, followed by insertion into a quartz reaction tube. The tube was attached to a glass vacuum line and evacuated. Argon was added to approximately 1 atm and the compressed pellet was heated to 900° C and held at the temperature for 14 hr. The product was air quenched to room temperature. The product was crushed in an agate ball mill, compressed into a pellet again, and reheated in the same manner as before. 

The single-bond radii for the lanthanons (La to Lu, Fig. 11-10) show some interesting features. The magnetic properties26 require that most of the lanthanons have metallic valence 3, but that two of these metals, europium and ytterbium, have metallic valence 2. The interatomic distances reflect these valences dearly, as shown in Figure 11-10. The stability of metallic valence 2, rather than 3, for these two metals may be attributed to the special stability of a half-filled or completely filled 4/ subshell. 

Reaction of metallic europium with cyclopentadiene in liquid ammonia yields [478] a yellow europous-dicyclopentadienyl (eq. 41) complex. Magnetic measurements on this compound (jn = 7.62 Bohr magneton) and its infrared spectrum definitely confirm the divalent state of europium. 

EUROPIUM. [CAS 7440-53-11. Chemical clement symbol Hit. at no 63. al. wt. 151.96, sixth in the Lanthanide Series in the periodic table, nip 822 C. bp 1529 C. density 5.245 g/cm1 20 C). Elemental europium has u body-centered cubic crystal structure at 25"C. The pure metallic europium... 

As a metal, europium is very reactive so that one usually finds it under its trivalent, triply oxidized form (Eu3+ ion) in oxides or salts. A divalent form (Eu2+) also displays some stability. Two minerals that contain many of the lanthanide elements, which are separated by liquid-liquid extraction, are commercially important monazite (found in Australia, Brazil, India, Malaysia, and South Africa) and bastnasite (found in China and the United States). 

To avoid the complication of hydrolysed product or partial decomposition during dehydration, anhydrous diketonate complexes can be prepared by the reaction of diketone with lanthanide 2-propanolate [47] in benzene, with a lanthanide hydride or with metallic europium [48]. 

Fig. 6. A schematic diagram of the reduction apparatus for the production of metallic europium...

There is little doubt that LaL is correctly formulated as La " (1)2, and that a similar constitution appears likely for CeL, and possible for PrL- LaL does not yield solvated electrons in liquid NH3 as do die divalent metals europium and ytterbium (39) the rapid evolution of hydrogen is probably a result of substantial ammonolysis of the La " ion. 

Distillation of solid or liquid europium amalgam in a high vacuum produces a second solid alloy, Hg Eus, which is fairly stable at red heat. Further heating of this alloy does not readily yield metallic europium since the latter is volatile in a high vacuum at the temperature at which the alloy is decomposed. 

Metallic europium is soluble in liquid ammonia with the usual blue color due to a solvated electron in a cavity. It has the broad Eu(II) absorption band in the near ultraviolet. Evaporations of such M(II) phlogistonide solutions allow crystallization of the bronze-colored metals M(NH3) for M = Ca, Sr, Ba, Eu and Yb, as well as Li(NHj)4 (Dye 1984). 

The biggest surprise of their study was the discovery of the divalent character of metallic europium and ytterbium. Earlier studies of the light lanthanide (La, Ce, Pr and Nd) and erbium metals indicated that these lanthanides and yttrium were trivalent and so the expectation was that the remaining lanthanides should also be trivalent. Since divalent salts of europium and ytterbium were known in addition to the corresponding trivalent ones, the divalent nature was readily understood in terms of Hund s rules for stable half-filled and completely filled electronic levels (in these cases, 4f and 4f respectively). 

We now turn our attention to europium and ytterbium, since most of the above discussion was concentrated on the trivalent rare earth metals. Europium metal is the most reactive rare earth metal with respect to handling in air. Within minutes after exposure to moist air a yellow film of Eu(OH)2-H20 develops on the surface... 

In I96U, the blue solution of metallic europium in liquid ammonia was found to react with cyclopentadiene. The sublimation of the crude product at U00 C in vacuo yields an ammonia-free syndwich complex, as the first organometallic compound of a lanthanoide in the oxidation state Ln (183). The ytterbiinn complex has been prepared in a similar manner (15) ... 

Fig. 8. Difference between on one hand the experimental BIS data for the tiivalent lanthanides and the XPS data for the divalent metals europium and ytterbium and on the other the derived values of A u nil- Filled circles correspond to the BIS case and open circles to the XPS case.

The finding that the AEmiv values reproduce the XPS data quite well but that — A n,iii deviates more substantially from the BIS data might lead one to suspect that these two types of experiments are not directly comparable. However, in fig. 7 we have also included the XPS data for the two divalent metals europium and ytterbium and also here we note a most substantial deviation between experiment and A n,iii These deviations are also included in fig. 8 and are in fact even somewhat larger than the deviations from the BIS data. From this we conclude that it is rather the fact that we are dealing with a divalent - trivalent or a trivalent -> divalent transition instead of a trivalent -> tetravalent transformation that causes the difference between the BIS and XPS results and not the experimental technique. We will return to this point in the following section. 

For the divalent metal europium, the BIS experiment involves a transition to a monovalent state. Therefore the complete screening picture implies that we should... 

Nonideal behaviors are also expected for rare earth metals that are alloyed with the two divalent lanthanide metals europium and ytterbium. Cerium and samarium may also be anomalous because these two metals have a tendency to exhibit other valence states rather than the normal 3 state (2" for Sm and 4" for Ce). However, the mischmetal study reported by Palmer et al. (1982) did not indicate any anomalous behavior for cerium in this polycomponent system. 

Although bulk samarium metal is trivalent, divalent surface states have been demonstrated for this metal [76]. The next lanthanide metal, europium, is a divalent metal. It may be possible that californium metal is similar to samarium metal, and that the divalent state for californium metal may only be stable in thin films, very small samples, or in surface layers of atoms, as found for samarium. 

---