Actinide metal oxides

The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides... 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

The metallothermic reduction of an oxide is a useful preparative method for an actinide metal when macro quantities of the actinide are available. A mixture of the actinide oxide and reductant metal is heated in vacuum at a temperature which allows rapid vaporization of the actinide metal, leaving behind an oxide of the reductant metal and the excess reductant metal, in accord with the following equations ... 

The reductant metal must have the following properties (1) the free energy of formation of the oxide of the reductant has to be more negative than that of the actinide oxide and (2) the vapor pressure of the reductant metal needs to be smaller by several orders of magnitude than that of the actinide metal. This difference in vapor pressure should be at least five orders of magnitude to keep the contamination level of the co-evaporated reductant metal in the product actinide metal below the 10 ppm level. 

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

Methods have been developed (75) to prepare actinide metals directly from actinide oxides or oxycompounds by electrolysis in molten salts (e.g., LiCl/KCl eutectic). Indeed, the purest U, Np, and Pu metals have been obtained (19, 24) by oxidation of the less pure metal into a molten salt and reduction to purer metal (electrorefining. Section III,D). 

In Chap. C, the thermodynamic and structural outlook of the bond, which had been the matter of discussion in Part A of this chapter, is further developed, and the model formalism, which takes advantage of the well known Friedel s model for d-transition metals but is inspired by the results of refined band calculations, is presented for metals and compounds. Also, a hint is given of the problems which are related to the nonstoichiometry of actinide oxides, such as clustering of defects. Actinide oxides present an almost purely ionic picture nevertheless, covalency is present in considerable extent, and is important for the defect structure. 

The choice of the starting compound of the actinide (oxide or carbide) and of the reductant is determined by the vapour pressure of the actinide metal. For a given temperature, the vapour pressures of the actinide metals (Fig. 2) span a ratio of more than 10 . As the vapour pressure of La is similar to those of Ac, Cm, Pu, only the more volatile... 

The compounds with elements of the platinum group can be prepared by direct synthesis, which requires the availabiUty of actinide metals. Such intermetallics can, however, also be obtained by coupled reduction of actinide oxides with hydrogen in the presence of finely divided noble metals ... 

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. 

The most obvious future data needs concern the missing, uncertain, and conflicting data identified above. Additional experimental investigations are needed in the case of Fe(III) and Zr(IV) carbonate complexation, and in the case of the Sn(IV)/Sn(II) and the Se(0)/Se(-II) redox couples. The molecular structure of metal silicate complexes needs clarification in order to remove ambiguities in the speciation scheme of these complexes. A rather challenging topic concerns the supposed transformation of crystalline tetra-valent actinide oxides, AnOz(cr), to solids with an amorphous surface layer as soon as the An4+ ion hydrolyses. The consequences of such... 

In simple oxides, the actinides are most stable in the +4 oxidation state the dioxides, An02, are known for all elements thorium through californium. Although the properties of Th02, U02, and Pu02 are especially important in nuclear technology, complex actinide oxides (oxides with one or more metal ions in addition to an actinide) are also important since they may be found as fission products in nuclear fuels and they are models for possible matrices in which nuclear wastes will be stored. 

The thorium oxide system is dominated by Th02. The dioxide can be synthesized by burning a number of thorium compounds, including hydroxides, oxalates, carbonates, and so on. The Th02 crystalhzes in the cubic fluorite structure. Th02 is very heat resistant as are all of the actinide oxides and melts at 3390 °C, which is the highest for any known metal oxide. 

The stractural chemistry of actinides is very diverse due to the possibility of different oxidation states and the richness of actinide coordination geometries. Whereas actiiudes in lower oxidation states sometimes mimic rare earth elements, actinides in higher oxidation states possess unique coordination chemistry, due to the tendency to form linear actinyl ions. The reviews in this book are written by specialists in their fields and the subjects range from low-valence actinide compoimds to actinide-based metal-orgaiuc frameworks. The active participation of Russian authors provides overviews of some activities undertaken by scientists in the former Soviet Uiuon. Their results are sometimes not well known to western readers because of the relatively closed nature of works in this field during the Cold War years. 

Actinide Oxides in Molten Alkali Metal Nitrates. The chemical behavior of actinide oxides in molten alkali metal nitrates is an area with little available experimental data. Most investigations (1, l j 3, 4), including our own, have shown that molten... 

Chemistry in Molten Alkali Metal Nitrates, The chemical behavior of actinide oxides and fission products reported in the previous sections appear to be based mainly on predicted behavior little experimental data has been provided to support the various claims. This, in part, may be because several of the claims are reported in the patent literature, rather than in technical documents or in journal literature. 

Actinides U metal, Th oxide > 99.99 /, SRM and RM Isotopic CRM are available from nuclear material suppliers (5) As above for lanthanides... 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

The intermetallic compounds are synthesized by heating mixtures of actinide oxides or halides with finely divided noble metal powders in pure hydrogen. Protactinium metal was prepared in a modified version of the van Arkel-de Boer procedure protactinium iodide, formed by reaction between iodine and protactinium carbide, was thermally dissociated on a resistance heated tungsten wire (6,7) ... 

Transformed rare earth and actinide intermetallic compounds are shown to be very active as catalysts for the synthesis of hydrocarbons from CO2 and hydrogen. Transformed LaNis and ThNis the most active of the materials studied they have a turnover number for CH formation of 2.7 and 4.7 X 10 sec at 205°C, respectively, compared with I X 10 sec for commercial silica-supported nickel catalysts. Nickel intermetallics and CeFe2 show high selectivity for CHj formation. ThFcs shows substantial formation of C2H6 (15%) as well as CHi,. The catalysts are transformed extensively during the experiment into transition metal supported on rare earth or actinide oxide. Those mixtures are much more active than supported catalysts formed by conventional wet chemical means. 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

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