Carbides are chemical compounds in which carbon bonds with less electronegative elements. Depending on the difference in electronegativity and the valence state of the constituting elements, they exist as differ­ent bonding types. Accordingly, they are classified as salt-like compounds (in which carbon is present as a pure anion and the other elements are sufficiently electropositive), covalent compounds (SiC and B4C), interstitial compounds (with transition metals of the groups 4, 5, and 6 except chromium), and ‘intermedi­ate’ transition metal carbides.14

In general, carbides display metallic properties, and they are mostly refractory (high melting). Their more specific properties depend on the constituting elements.

2.04.1.1 General Properties of Actinide Carbides

Actinides are known to form three main types of stoichiometric carbides (Table 1): monocarbides of the type AnC, sesquicarbides of the type An2C3,

and dicarbides of the type AnC2 (sometimes called ‘acetylides’). Mono — and dicarbides have been observed for protactinium, thorium, uranium, neptunium, and plutonium. Sesquicarbides have been identified for thorium, uranium, neptunium, plutonium, ameri­cium, and, recently, curium.

Other types of actinide carbides such as CmC3 and Pu3C2 have been observed.

Data for mixed U-Th and U-Pu carbides, briefly summarized and discussed in the last section of this chapter, have mostly been indigenously collected from the few nuclear plants using this kind of fuel.15

2.04.1.2.1 Structure of the matter

In general, actinide carbides are of the ‘salt-like’ type. In these compounds, carbon is present as single anions, ‘C4-’ in the monocarbides; as two atom
units, ‘C2_’ in the acetylides; and as three atom units,‘C3 in the sesquicarbides. This model, useful for a first visual description of these materials, is physically inconsistent with their essentially metallic properties. The An-C bonds are certainly more cova­lent than ionic, as recently confirmed.16 Actinide compounds are characterized by a peculiar elec­tronic structure, where the extended nature of the 5f electron wave functions yields a unique interplay between localized and band electrons. This feature leads, in particular, to properties associated with covalent bonding in these compounds, which show crystal structures normally associated with ionic bonding.5

Monocarbides AnC1±x (An = Th, Pa, U, Np, Pu, Am) crystallize in the NaCl-type space group Fm3m — No. 225 (Table 1). The elementary cell is

Table 1 Synopsis of the known actinide carbides

Compound and lattice Composition and Space group Structure

parameters temperature range ^ — Actinide; ® — C

Other actinide carbides with little information: PaC2, NpC2, probably isostructural to CaC2, Pu3C2, stable between 300 and 800 K, but unknown structure; Cm3C with fcc Fe4N-like lattice with a = 517.2 ± 0.2 pm.

represented by four formula units. The lattice param­eter is dependent on the C/An ratio, and the oxygen and nitrogen impurities. The lattice parameter of pure monocarbides increases with the dissolution of carbon in the ideal face-centered cubic (fcc) lattice in an essentially linear manner.

The sesquicarbides of Th, U, Np, Pu, Am, and Cm have been identified to be body-centered cubic (bcc) of the 143d type, with eight molecules per unit cell
(Table 1). This structure is more complex than that of the mono — and dicarbides, and is often difficult to form. Thus, Th2C3 was observed only under high pressure (2.8—3.5 GPa), and U2C3 is produced by a complex preparation procedure. Both decompose into a mixture of mono — and dicarbides at high tem­peratures. The situation is different in the case of Pu2C3, which is the most stable among the Pu car­bides and forms easily at temperatures ranging from

room temperature to the melting point. Unlike the fcc modifications of mono — and dicarbides, sesquicar — bides can hardly accommodate lattice defects; there­fore, they essentially exist as line compounds.

Actinide dicarbides AnC2_x have been observed in a larger variety of allotropes (Table 1). At inter­mediate temperatures, generally between 1700 and 2050 K, Th, U, Pu, and probably, Pa and Np, form tetragonal dicarbides of the type CaC2 (I4mmm — Group 139). Th also forms a monoclinic C2/c (No. 15) substoichiometric dicarbide that is stable from room temperature to 1713 K. The high-temperature form of actinide dicarbides has been observed to be fcc of the type KCN, which belongs to the same symmetry group as NaCl, Fm 3m. Such structure, clearly estab­lished for g-ThC2, was observed with more difficulty by high-temperature X-ray diffraction (XRD) for p-UC2 and p-PuC2. The lattice transition between tetragonal and cubic fcc dicarbide (a! p for U and Pu, p! g for Th) is diffusionless of the martensitic type. It occurs very rapidly despite its important enthalpy change, mostly due to the lattice strain contribution. For this reason, the high-temperature cubic modification is impossible to quench to room temperature, hence the difficulty in investigating its properties. fcc allotropies of mono — and dicarbides are mostly miscible at high temperature, and for uranium and thorium, they can be considered as a single high-temperature cubic phase with a wide nonstoichiometry range. In fact, this solid solution can easily accommodate interstitial excess carbon atoms and lattice vacancies. The first ensure the existence of a broad hypostoichiometry range of the dicarbides, where most of the excess carbons form C2 dumbbells in the (^,0,0), (0,^,0), and (0,0,^) positions as in the KCN lattice (see Table 1). The second are responsible for the existence of hypostoichiometric monocarbides An1_x, extending to the pure metal for thorium but only to a narrow UC1_x domain for uranium. The situation is different for Pu carbides due to the high stability of Pu2C3 up to its melting point and to the fact that fcc plutonium monocarbide exists only in a vacancy-rich hypostoichiometric form, with 0.74 < C/Pu < 0.94. This originality, common to other Pu compounds, is certainly related to the pecu­liar behavior of the six 5f electrons of plutonium, which exhibit behavior on the limit between valence and conduction, and can follow one or the other (or both) in different compounds.

The electronic (band) structure of actinide car­bides has been studied rather extensively, both exper­imentally (by low-temperature calorimetry and X-ray
photoelectron spectroscopy, XPS) and theoretically (by tight-binding methods and, more recently, by density functional theory techniques). These com­pounds are, in general, good electronic and thermal conductors, with a nonzero density of electronic states at the Fermi level (Figure 1).

However, the actual filling of the levels largely depends on the peculiar behavior of the 5f electrons,

(c) Energy (eV) ef
which tend to be more localized or more itiner­ant according to the actinide and the compound involved. Thus, Pu carbides have much higher elec­trical resistivity than Th and U carbides. Similarly, mono — and dicarbides are better electronic conduc­tors than sesquicarbides are. Magnetic transitions have been observed at low temperatures in sesqui — carbides, and Np and Pu monocarbides.

The electronic structure dependence on defect and impurity concentrations has been studied in a number of cases. For example, in ThC1_x, the density of states (DOS) increases with increasing carbon vacancy concentration. Auskern and Aronson17 showed by thermoelectric power and Hall coefficient measurements that a two-band conductivity model can be applied for ThC1_x: the bands overlap more and the number of carriers increases with decreasing C/Th ratio. The valence bands have mainly a carbon 2p and a thorium 6dg character, while the Th-6de character dominates the conduction bands. Also, the increase of the DOS at the Fermi level with vacancy concentration is due to the 6d thorium electronic states. In stoichiometric ThC, the 6d Eg states are hybridized with the 2p states of carbon and are split between low-energy bonding and high-energy antibonding states. In hypostoichiometric ThC1_x, the 6d Eg dangling bonds contribute to an increase of the DOS in the vicinity of the Fermi level.18

For uranium carbides, it was shown that, following the general rules of Hill19 that imply that U—U dis­tance is <3.54 A, these compounds exhibit a metallic electronic structure due to the overlaps of f-orbitals. This rule applies to uranium monocarbide for which the U—U distance is 3.50 A, as shown by experimental measurements as well as by ab initio calculations.20, For hyperstoichiometric uranium carbides, the metal­lic character persists and the C—C bonds are covalent as in graphite. In an X-ray and ultraviolet photoelec­tron spectroscopy (XPS and UPS) study of sputtered UCx thin films (0 < x < 12), Eckle et a/.22 showed that the U-4f core levels do not change strongly with increasing carbon content, and demonstrated the pre­dominantly itinerant character of U-5f electrons. Similarly, valence region spectra show three types of carbon species for different UCx films, which are differentiated by their C-2p signals. A strong hybri­dization between C-2p and U-5f states is detected in UC, while the C-2p signal in UC2 appears only weakly hybridized, and for higher carbon contents, a p-band characteristic of graphite appears.

Calculated charge distribution maps for stoichio­metric fcc ThC and tetragonal p-ThC223 are shown in Figure 2, giving an idea of the covalent or ionic nature of the different bonds in these structures.

The analysis by Shein et a/.23 revealed that bonding in ThC2 polymorphs is of a mixed covalent-ionic-metallic character. That is, the cova­lent bonding is formed due to the hybridization effects of C-C states (for C2 dumbbells) and C2-Th states. In addition, ionic bonds emerge between the thorium atoms and C2 dumbbells owing to the charge transfer Th! C2, with about 1.95 electrons redistrib­uted between the Th atoms and C2 dumbbells. The metallic Th-Th bonds are formed by near-Fermi delocalized d and f states. Similar charge distributions have been calculated for uranium carbides.24