Transplutonium oxides

In table 24 are shown the known condensed phase of actinide oxides. The similarity of the transplutonium oxides to known lanthanide type of oxides is obvious. The apparent absence of specific lanthanide-type of oxides beyond An O j (e.g., the complex phases for intermediate oxides of the lanthanides) for the actinides may be real or may merely reflect that comparable in-depth studies for them have not been undertaken. The existence of higher oxides for the actinides, Pa-Np, reflect the differences in their electronic nature, as discussed above. 

Extrapolations for the einsteinium oxide transitions based on the lanthanide oxide and transplutonium oxide systems would suggest the C- to B-type transition for ES2O3 to be on the order of 1400°C, and the B- to A-type transition to be at least 1650°C. Lattice parameters for the three forms of einsteinium oxide are given in table 25. 

The quantity of high-temperature data for the transplutonium oxides is very limited, which is unfortunate "as these are the actinides most like the lanthanides. Some vaporization data from a mixed PuOj g/Am203 system have indicated that the main mode of vaporization/decomposition for americium sesquioxide is via the generation of Am(g) and 02(g) (Ackermann and Chandrasekharaiah 1974). This finding has been supported by other unpublished work (Kleinschmidt 1992, Haire 1992a). The solid monoxide of Am has been considered to be barely stable toward disproportionation in the solid phase and its existence as a gaseous species is reasonable. Based on the tendency toward divalency for higher members in the series, the monoxide would be expected to become the most prevalent vapor species. 

The transplutonium oxides are useful for studying crystal-field effects or interactions, as the dioxides and the C-type sesquioxides have cubic symmetry and both oxide stoichiometries are available for most of the oxides. The magnetic behavior of these transplutonium oxides is compared with their lanthanide counterparts in section 3. The behavior of individual oxides is discussed briefly here and data are provided in table 28. 

Finally, the lanthanide oxides have been studied more thoroughly than the transplutonium oxides and it is possible that future studies of the latter oxides will determine further similarities between the chemistry and the physics of the oxides of these two series. One case may be the so-called intermediate oxides (e.g, oxides with O/M ratios between 1.5 and 2.0). It will require many careful studies in future years to elucidate the behavior of the actinide oxides to the present level of knowledge that is available for the intermediate oxides of the lanthanides. 

Transplutonium(VI) complexes aqua,3,1220 carbonates, 3, 1220 carboxylates chelating, 3, 1220 halogens, 3,1220 monocarboxylates, 3,1220 nitrato, 3,1220 oxides, 3,1220 oxoanions, 3, 1220 Transport cations... 

Tricyclopentadienide complexes of many of the actinides are known (Ac = Th, U, Pu, Am, Cm, Bk, Cf). Indeed, these are the only cyclopentadienide complexes known for the transplutonium elements, where -(-3 is the most stable oxidation state. The transplutonium elements were all prepared by a microchemical procedure which utilized a melt of biscyclopentadienyl beryllium (6) according to ... 

Table 90 Complexes of Transplutonium Actinide(III) /3-Ketoenolates with P-Oxides...

Ross, R. G. Wiggins, J. T. "Preparation of Curium-Americium Oxide Microspheres by Resin-Bead Loading", paper presented at Symposium on Industrial Scale Production-Separation-Recovery of Transplutonium Elements, 2nd Chem. Congr. North American Continent, Las Vegas, NV, 1980. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

Extraction processes (TRUEX, PUREX, Talspeak, DIAMEX, PARC, etc.) generally involve complexation of transplutonium elements by alkyl phosphines, phosphine oxides, phosphoric acids, carbamoyl phosphonates, diamides, and thiophosphinates in aqueous/organic extractions, within derivatized solid supports, or on coated particles. There are excellent reviews of the processes and significant complexes by Mathur et al. and selected chapters in The Chemistry of the Actinide and Transactinide Elements to be published in 2003. " Work on the separation for nuclear waste management in the United States, France, and Russia have been reviewed. " ... 

Phosphine oxides. Few molecular complexes of trivalent transplutonium elements have been reported. Several studies examine the extraction chemistry of Am, Cm, and Bk with a combination of /3-diketones and tri-n-alkyl phosphine oxides and tiialkylphosphates. From these, compounds reported to be of the formula AnF3(R3PO) c (An = Am, Cm R = n-octyl, Bu"0) were isolated, where L = CF3COCHCOR (R = Me, CF3, Bu ). The stoichiometry of the complexes (An P=0) was not always reported. The complex Am(CF3COCHCOCF3)3[OP(OBu )3]2 is reported to be volatile at 175 °C. ... 

Green solutions believed to be Am are prepared by oxidation of Am in concentrated aqueous basic solution by either ozone or the O radical. In contrast to Np, and similar to Pu, Am is unstable and reduces to the hexavalent state within minutes. A review on the chemistry of heptavalent transplutonium elements can be found in the Handbook of the Physics and Chemistry of the Actinides. ... 

Mikheev, N. B. Myasoedov B. F., Lower and Higher Oxidation States of Transplutonium Elements in Solutions and Melts. In Handbook on the Physics and Chemistry of the Actinides Vol. 3 Freeman, A. J. Keller, C. Eds. Elsevier Amsterdam 1985, pp 347-386. 

Dissolution of the calcium fluoride in aluminum nitrate-nitric acid oxidizes the plutonium to the tetravalent hexanitrate complex (3), while the transplutonium nuclides remain in the trivalent state. The only actinides retained by a nitrate-form anion-exchange column are thorium, neptunium, and plutonium. The uranium distribution coeflBcient under these conditions is about ten, but uranium should not be present at this point since hexavalent uranium does not carry on calcium fluoride (4). 

In contrast to the early actinide compounds already discussed, the iodates of the transplutonium actinides that have been prepared contain trivalent oxidation states for the actinide elements. For Am, several higher oxidation states are possible, but attempts to prepare Am(V) or Am(VI) analogs of these iodates... 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

In Table I are summarized some generally useful chemical reactions for the preparation of transplutonium element metal and some compounds. For simplicity, and because of variable oxidation states, the equations are not necessarily balanced. 

---