Actinide dioxides

In practice, a mixture of actinide dioxide and graphite powder is first pelletized and then heated to 2275 K in vacuum in a graphite crucible until a drop in the system pressure indicates the end of CO evolution. The resulting actinide carbide is then mixed with tantalum powder, and the mixture is pressed into pellets. The reduction occurs in a tantalum crucible under vacuum. At the reduction temperature, the actinide metal is vaporized and deposited on a tantalum or water-cooled copper condenser. 

The most studied non-stoichiometric system in actinide CaF2-structured compounds is the An-0 system all actinide dioxides (with the exception of Th02) present large departures from stoichiometry. Since uranium and plutonium dioxides (and their solid solutions) are employed as fuels in nuclear reactors, a very large effort has been dedicated to the study of their physical and physico-chemical properties. All these properties are affected by the oxygen composition of the compound. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

Fig. 3. An example of final state mul-tiplet splitting interpretation of the XPS photoemission of actinide dioxides (from Ref. 15)...

All these effects are probably responsible for the discrepancies of reported photoelectron results in actinide oxides. Often, especially for the more radioactive and rare heavy actinides, dioxide samples are prepared for photoemission by growing oxide layers on top of the bulk actinide metal. These samples may then display features of trivalent sesquiox-ides since the underlying metal acts as a reducing medium. 

In this chapter, the discussion will be limited to the actinide dioxides, and, in lesser extent, sesquioxides. Also, no special attention will be given to nonstoichiometry effects, except by indicating when they may be responsible for serious errors in the measurements or in their interpretation. In order to understand, however, the directions of research for the photoelectron spectroscopic technique, a short digression will be made on the nature of the chemical bond in these systems. 

Common features of the 4 f core level spectra of actinide dioxides are the symmetry of the main lines and the appearance of a satellite at about 7 eV in their high binding energy side (Fig. 30). Similar satellites have also been found for UF4, for which compound the intensity is even higher than for the dioxides. It is perhaps interesting to report some analysis of these features, on the basis of final state models. 

It is worthwhile to mention the ample use of screening final states models in understanding core levels as well as valence band spectra of the oxides. The two-hole models, for instance, which have been described here, are certainly of relevance. Interpretational difference exists, for instance, on the attribution of the 10 eV valence band peak (encountered in other actinide dioxides as well), whether due to the non-screened 5f final state, or to a 2p-type characteristics of the ligand, or simply to surface stoichiometry effects. Although resonance experiments seem to exclude the first interpretation, it remains a question as to what extent a resonance behaviour other than expected within an atomic picture is exhibited by a 5 f contribution in the valence band region, and to what extent a possible d contribution may modify it. In fact, it has been shown that, for less localized states (as, e.g., the 3d states in transition metals) the resonant enhancement of the response is less pronounced than expected. 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

BaAn 03 (An = Th Am) all have the perovskite structure and are obtained from the actinide dioxide. In accord with normal redox behaviour, the Pa and U compounds are only obtainable if O2 is rigorously excluded, and the Am compound if O2 is present. Actinide dioxides also yield an extensive series of nonstoichiometric, mixed oxide phases in which a second oxide is incorporated into the fluorite lattice of the An02- The UO2/PUO2 system, for example, is of great importance in the fuel of fast-breeder reactors. 

The calculated crystallographic radii (95) for the LaF3-type actinide trifluorides (Ac — Bk) with coordination number six, and the YF3-type trifluorides (Bk, Cf) reproduced the two-third of the inclined W (Fig. 60(a)). From the crystallographic data for the isostructural tetravalent actinide dioxides (M = Th — Bk) Peterson and Cunningham (96) calculated the sixcoordinated radii for Th to Bk. These values are plotted (Fig. 60(b)) against the L-values of the tetravalent actinides. Like the case of the trivalent actinides here also a linearity within the two tetrads was observed. 

Calculations of the expected XPS spectra for the actinide dioxides uranium through berkelium were reported by Gubanov et al. (10). Results for UO2 are shown in Fig. 3 along with experimental spectra. These calculations, extending about 30 eV below the Fermi level, are based on a one-electron molecular-cluster approach. 

Neptunium dioxide, Np03, is the most stable of the neptunium oxides. It crystallizes with the fluorite structure of all the actinide dioxides, with a crystalline density of 11.14 g/cm . It can be formed from the thermal decomposition of other neptunium compounds, such as the hydroxide, the nitrate, or the oxalate, in the temperature range of 600 to 1000°C. High-fired NpOj can be dissolved in hot concentrated nitric acid containing small amounts of fluoride. 

Americium oxides. Keller [K2, K3] reports three stoichiometric binary oxides of americium AmO, Amj O3, Am02. The dioxide Am02 is the most stable of the americium oxides. It crystallizes with the cubic fluorite structure of aU the actinide dioxides. It can be formed as a dark brown powder, stable up to 1000°C, by heating trivalent americium nitrate, hydroxide, or oxalate in oxygen to 700 to 800 C. Americium dioxide is readily soluble in mineral acids. Hydrogen reduction of the dioxide yields Am2 O3. 

PEN/GR1] Peng, S., Grimvall, G., Heat capacity of actinide dioxides, J. Nucl. Mater., 210, (1994), 115-122. Cited on page 116. 

The compounds LisM Og = Np, Pu) have been obtained by heating the actinide dioxide with lithium oxide in oxygen at ca. 400°C(Np), and the analogous BaaM NpOe (M = Li or Na) by heating Np03-H20 with the alkali metal peroxide in oxygen above 400 "C. The [NpOg] " anion appears to be octahedral. 

Solid rare-earth dioxides have been reported for Ce, Pr and Tb. They have a fee structure like the actinide dioxides. In addition, a compound of stoichiometry Nd02 is known [58]. It is not a true dioxide but a Nd(III) peroxide, Nd202(02). It is reported to be thermally stable, decomposing to NdOi.s at 420 °C in argon. 

The sesquioxide is formed by reduction of the dioxide in hydrogen or CO/COj atmospheres at elevated temperatures. Some care must be used to assure that reduction is complete (e.g., the O/M ratio reached is 1.50). The dioxide of Bk (black/brown) is readily obtained by decomposition of a variety of berkelium salts (e.g., nitrate, oxalate, etc.) in air or oxygen-containing atmospheres. In fact, precautions must be used to avoid the uptake of oxygen by the sesquioxide, even at room temperature. Heating lower oxides of Bk to 500°C in air is sufficient to produce the stoichiometric dioxide. The dioxide crystallizes in the fluorite structure (see table 25) and is isostructural with the earlier actinide dioxides. 

All of the actinides from Th through Cf form dioxides but several of these have not been studied thermodynamically, due in part to their instability and to limited availability (e.g., it is very difficult to prepare multi-milligrams of Cf02 even though such quantities of the isotope are available). Plots of enthalpy of solution for the f elements have been established (Morss 1986) which permit estimating values for the other actinide dioxides. Although binary oxides above the dioxide stoichiometry are known for some of the actinides (Pa, U, Np), little thermodynamic data are available for these oxides. 

The thermodynamic behavior of protactinium oxides is not as well established as for the Th02 system. The free energy of formation of Pa02(s) has been derived from carbothermic reductions. The entropy of formation of the dioxide is comparable to the other actinide dioxides (see table 27 data from Ackermann and Chandrasekharaiah (1974)]. A partial study of the reduction of Pa205 has also provided data for the protactinium oxide system (Kleinschmidt and Ward 1986). 

A qualitative picture can be formed by examining fig. 17, where the (IV)/(III) couples are shown for the lanthanides and actinides, and by assigning Pr and Tb as the reference lanthanides for formation of a dioxide. It would be expected that all the actinides before Es would form dioxides (which they do) curium and californium dioxides would only be marginally stable, and should be more difficult to prepare than the other actinides dioxides (which is also correct). Further, Bk02 should be more stable than Tb02, which is seen experimentally. For reference, the order of stability of the transplutonium and lanthanide dioxides has been determined to be Bk > Ce > Am > Cm > Pr > Tb > Cf this order is in agreement with that predicted from the curves in fig. 17. 

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