Actinide carbides

The formation of C-C chemical bonds in a variety of solids, including some refractory dicarbides, has been considered by Li and Hoffman (1989) and Wijeyesekera and Hoffman (1984) based on EHT (extended Huckel theory) calculations. To our knowledge, these works are the only ones where the band analogues of bond populations, the so-called crystal orbital overlap populations (COOPs) have been calculated for refractory compounds. The most noticeable result is that, in spite of the evident crudeness of the nonself-consistent semiempirical EHT method, the calculations allow us to understand the nature of the phase transition from cubic to hexagonal structure which occurs in the ZrC, NbC, MoC. series as the VEC increases. The increase of metal-to-metal bonding when going from cubic (NaCl-type) to hexagonal (WC-type) becomes evident. 

Brooks, Johansson and Skriver (1984) investigated the band structure of UC and ThC by nonrelativistic and relativistic (based on the Dirac formalism) LMTO methods. They analysed the electron density changes in the compounds as compared with free atoms, as well as the influence of pressure on the band structure. Crystal pressures as a function of lattice constants (equations of state) were calculated as well as theoretical values of the lattice constants. The calculated trends in the variations of lattice constants and bulk moduli agree well with the available experimental data. Some of the most important results of these calculations are shown in Figs. 2.20 and 2.21. 

As follows from Fig. 2.20, chemical bond formation in UC is accompanied by an appreciable electron density transfer to the outer region of the atom. Most probably, the electron density near the Wigner-Seitz spheres is still overestimated in the LMTO caleulations (see Section 3.5). Hybridisation effects in the UC band structure ean be seen from the dispersion curves (Fig. 2.21) along the F - direction of the Brillouin zone. For the lattice constant a = 5.01 A, the hybridised C2p-U5/Aj and As bands are clearly seen, which cross the Aj-S/ state band. The bands 

Fermi surface for ThC and UC using the relativistic APW method. The calculated results are in good agreement with the de Haas-van Alphen measurements. The UC Fermi surface appears to consist of three hole pockets in the region of the valence C2p states and six electron pockets in the region of the U5/ states. It was shown that UC is a semimetal and contains almost the same number of holes and electrons. 

As a conclusion to this chapter we present a list of papers devoted to electronic structure calculations of iiia-via subgroup d-metal binary carbides published after 1969. 

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Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

Actin, role in heart excitation and contraction coupling, 5 81 Actinide carbides, 4 689 Actinide carbonate, 25 430-431 Actinide-gallium compounds, 22 355 Actinide oxides, 24 761 Actinide peroxides, 28 410 Actinides, 23 569. See also Actinides and transactinides Actinide series absorption and fluorescence spectra, 2 490... 

The transition elements Nb, Zr, Ti, Ta are able to reduce to metals the actinide carbides (these last obtained by carboreduction of their oxides in vacuum). The actinide... 

Transition metals like Ti, Zr, Nb, and Ta are able to reduce actinide carbides to metals according to the following generic reaction ... 

The free energies of formation of the transition metal carbides are somewhat more negative than the free energies of formation of the actinide carbides. To facilitate separation of the actinide metal from the reaction products and excess transition metal reductant, a transition metal with the lowest possible vapor pressure is chosen as the reductant. Tantalum metal and tantalum carbide have vapor pressures which are low enough (at the necessary reaction temperature) to avoid contamination of the actinide metal by co-evaporation. 

Actinide carbides are prepared by carbothermic reduction of the corresponding dioxides according to the reaction ... 

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 yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

These compounds have a metallic character or are intermetallics. Actinide carbides (mono-, sesqui-, dicarbides) are prepared by carboreduction of the oxides ... 

C. E. Holley, M. H. Rand, E. K. Storms, in Chemical Thermodynamics, Part 6, The Actinide Carbides, International Atomic Energy Agency, Vienna, 1983. 

Ternary systems between two different metal atoms and carbon containing mostly Fe with minor amounts of transition metals are used as structural materials alloys of the transition carbides, mainly WC, containing Co, Fe or Ni as a binder, are used as cutting tools and wear-resistant surfaces and alloys between the various actinide carbides or the transition metals are used as nuclear fuel. 

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