Divalent actinides

Halides of divalent actinides are obtained by reaction of the metal with a stoichiometric amount of HgX2 (X = Cl, Br, I) ... 

These data [15] show unambiguously that the chemical behavior of the activity assigned to 26lRf is entirely different from that of trivalent and divalent actinides but is similar to that of Zr and Hf as one would expect for the next member of the Periodic Table following the actinide series. 

The redox chemistry of Am(II), Cm(II) and Cf(II) has been studied by pulse-radiolysis techniques. The relative stability of these divalent actinides is Cf(II)> Am(II)>Cm(II). There is no obvious correlation in this order and the electronic configurations or the ionic radii of these elements (Nash and Sullivan 1986). 

All the solid actinide monoxides which have been reported are now believed to have been oxynitrides, oxycarbides, or hydrides. The highest potential for existence would have the monoxides for the divalent actinide metals einsteinium through nobelium. Only the gaseous monoxides are well-established species. All actinides are known or expected to form gaseous monoxides. 

Truly divalent actinide halides are known only for americium and californium. AnX2 species for Es have been identified by their absorption spectra. For Fm, Md, and No, AnX2 halides should be possible if sufficient amounts of these metals could be obtained. Thl2 is also known, but crystallographic studies of this compound reveal the true formulation to be Th(IV), 21 , and 2e . This compound has some metallic character, including its luster and electrical conductivity. 

Fig. 24. (a) The calculated s, p and d occupation numbers across the actinide series when the 5f states are part of the core. The full line is for trivalent actinides and the dashed line is for divalent actinides, (b) The calculated s, p, d and f occupation numbers across the actinide series when the 5f states are in the energy bands. 

Since the available spectroscopic results for divalent actinides are fragmentary, we adopt a consistent interpretation which accounts for all observations and predicts the energies of other bands that might be accessible to observation. The basic aspects of the required tentative model can be deduced in part from available data for divalent lanthanide spectra. 

Figure 5.14. Compound formation capability in the binary alloys of Sc, Y, light trivalent lanthanides (as exemplified by La), heavy trivalent lanthanides (exemplified by Gd) and of the actinides (exemplified by Th, U and Pu). The different partners of the 3rd group metals are identified by their position in the Periodic Table. Notice that a sharper subdivision between compound-forming and not forming metals will result from a shifting of Be and Mg from their position in the 2nd group towards the 12th group (see 5.12.3). The behaviour of the divalent lanthanides Eu and Yb is shown in Fig. 5.7 where it is compared with that of the alkaline earth metals.

In Fig. 1, we have plotted the oxidation numbers of the actinides and of the lanthanides. We see that for the lanthanides the valence 3 is the most stable valence throughout the series. There are exceptions Ce displays for instance tetravalency in many compounds Eu and Yb display divalency. These exceptions are understood e.g., Eu and Yb are at the half-filling and at the filling of the 4f shell, which are stable electronic configurations. There is a tendency for both to share just the two outer 5 s electrons in bonding, displaying therefore, divalency, and preserve these stable configurations. 

When the f sub-shell is less than half-filled the stabilization of the divalent state (f"" " ) relative to the trivalent state is more rapid for the lanthanides than for the actinides. But the behaviour is reversed in the second half of the series. In part, this must be due to the low binding energy of the 6 d electrons compared to the 5 d. 

For actinides heavier than Cm, a very similar scheme is worked out consisting in a comparison with a) trivalent lanthanides b) surely divalent lanthanides Eu and Yb. In it, Ecoh (trivalent) calculated with the above interpolation scheme, are compared with Ecoh for divalent metals, as obtained by assuming a behaviour across the actinide series, similar to the one found in divalent lanthanides. The divalency of the heavier actinides (and the trivalency of Am and Cm) is concluded. 

Owing to the large variety of surfactants, metal ions, and complex metal ions that have been incorporated into LB films, variations of the stoichiometry given in Eq. (2) are plentiful. Some of these are outlined in the following section. With fatty acid films, metal ions and complex metal ions containing something other than a divalent charge include Ag+, Fe3+, Ti(IV) from the transition metal series, U(III) from the actinide series, and M3+ from the lanthanide series (7). 

The influence of the dose was quite low (see Figure 8.8), and the efficiency of the solvent for the extraction of actinides (III, IV, and VI) and back-extraction of divalent cations was maintained up to 0.6-0.7 MGy. No precipitation was observed after contact of radiolyzed solutions with metallic ions. 

Coprecipitation of actinides with divalent-cation carbonates and sulfates. 

Bulk-Phase Compounds Some of our results in the studies of the bulk-phase compounds have been published (3-7) These studies have shown that oxidation state is preserved for these actinides in either a or fT decay Trivalent einsteinium will transmute to trivalent berkelium which transmutes to trivalent californium It has also been observed that divalent einsteinium yields divalent californium. It is interesting to note in this latter case that it has not yet been possible to synthesize divalent berkelium in the bulk phase Berkelium(II) has not been observed in our aged einsteinium(II) compounds either, but it would be logical to assume it has been produced there. Our inability to observe Bk(II) could be related to weak absorption intensities and/or interference by absorption bands of einsteinium(II) or... 

Examples of actinide complexes formally present in their divalent state are. H. P. Beck and... 

Before continuing, some words must be said with regard to the terms rare earths and f elements used in this chapter. The term rare earths includes the elements Sc, Y and the lanthanides La through Lu. However, this chapter solely deals with divalent or trivalent rare-earth ions which are optically active, i.e., possess a partially filled f-shell. Thus, although the term rare earths is used in this chapter, it should be kept in mind that the elements Sc, Y, La, and Lu are excluded. In some exceptional cases the more general term f elements will be used, as for example when high pressure studies on actinide ions with a partially filled 5f shell are discussed. There are only few studies on 5f elements in non-metallic compounds under pressure, however, it seems interesting to compare the results found for these ions with those for the 4f-elements. 

This study shows that by diluting the salt solution, the selectivity for plutonium and americium is greatly increased whereas in undiluted solutions the actinides tend to migrate through the resin with the other salt constituents. Selectivity between different valencies is a function of the ionic strength, and the selectivity of the resin for the trivalent ion over the divalent ion (such as Pu3+ over Ca2+) is inversely related to the total concentration of the solution (16). Furthermore, the selectivity of the resin for the trivalent ion over the monovalent ion (such as Pu over K or Na"1") is inversely related to the square of the total concentration. 

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