C. Gueneau, A. Chartier, and L. Van Brutzel

Commissariat a I’Energie Atomique, Gif-sur-Yvette, France

© 2012 Elsevier Ltd. All rights reserved.

2.02.1 Introduction

Owing to the wide range of oxidation states +2, +3, +4, +5, and +6 that can exist for the actinides, the chemistry of the actinide oxides is complex. The main known solid phases with different stoichiome­tries are shown in Table 1.

Actinide oxides mainly form sesquioxides and dioxides. The +3 oxides of actinides have the general formula M2O3, in which ‘M’ (for metal) is any of the actinide elements except thorium, protactinium, ura­nium, and neptunium; they form hexagonal, cubic, and/or monoclinic crystals.

Crystalline compounds with the +4 oxidation state exist for thorium, protactinium, uranium, nep­tunium, plutonium, americium, curium, berkelium, and californium. The dioxides MO2 are all isostruc­tural with the fluorite face-centered cubic (fcc)
structure. Most of these actinide compounds can be prepared in a dry state by igniting the metal itself, or one of its other compounds, in an atmosphere of oxygen. The stability of the dioxides decreases with the atomic number Z. All dioxides are hypostoichio — metric (MO2 _ x). Only uranium dioxide can become hyperstoichiometric (MO2 + x). The thermodynamic properties of the dioxides vary with both temperature and departure from the stoichiometry O/M = 2.

Only uranium, neptunium, and protactinium form oxide phases with oxygen/metal ratio >2. An oxida­tion state greater than +4 can exist in these phases. The +6 state exists for uranium and neptunium in UO3 and NpO3. Intermediate states are found in U4O9 and U3O8 arising from a mix of several oxida­tion states (+4, +5, +6).

Detailed information on the preparation of the binary oxides of the actinide elements can be found in the review by Haire and Eyring.

The absence of features at the Fermi level in the observed XPS spectra indicates that all the dioxides are semiconductors or insulators.2

Systematic investigations of the actinide oxides using first-principles calculations were very useful to explain the existing oxidation states ofthe different oxides in relation with their electronic structure. For example, Petit and coworkers3,4 clearly showed that the degree of oxidation of the actinide oxides is linked to the degree of f-electron localization. In the series from U to Cf, the nature of the f-electrons changes from delocalized in the early actinides to localized in the later actinides. Therefore, in the early actinides, the f-electrons are less bound to the actinide ions which can exist with valencies as high as +5 and +6 for uranium oxides, for example. In the series, the f-electrons become increasingly bound to the actinide ion, and for Cf only the +3 valency occurs. With the same method, Andersson et al.5 studied the oxidation thermodynamics of UO2, NpO2, and PuO2 within fluorite structures. The results show that UO2 exhibits strong negative energy of oxidation, while NpO2 is harder to oxidize and

PuO2 has a positive or slightly negative oxidation energy. As in Petit and coworkers,3’4 the authors showed that the degree of oxidation is related to the position of the 5f electrons relative to the 2p band. For PuO2, the overlap of 5f and 2p states suppresses oxidation. The presence of H2O can turn oxidation of PuO2 into an exothermic process. This explains clearly why hyperstoichiometric PuO2 + x phase is observed only in the presence of H2O or hydrolysis products.6

Solid actinide monoxides ‘MO’ were reported to exist for Th, Pu, and U. According to the experimen­tal characterization of plutonium oxide phases by Larson and Haschke,7 these phases are generally considered as metastable phases or as ternary phases easily stabilized by carbon or/and nitrogen. From first-principles calculations, Petit et a/.3 confirmed that the divalent configuration M2+ is never favored for the actinides except maybe for EsO. On the contrary, the monoxides of actinide MO(g) are stable as vapor species that are found together with other gas species M(g), MO2(g), MO3(g) which fraction depends on oxygen composition and temperature when heating actinide oxides.

In Sections 2.02.2 and 2.02.3, the phase diagrams of the actinide-oxygen systems, the crystal structure data, and the thermal expansion of the different oxide phases will be described. The related thermodynamic data on the compounds and the vaporization behavior of the actinide oxides will be presented in Sections 2.02.4 and 2.02.5. Finally, the transport properties (diffusion and thermal conductivity) and the thermal creep of the actinide oxides will be reviewed in Sections 2.02.6 and 2.02.7.