Hexagonal V and Nb carbides

Phase diagrams of the (V, Nb)-C systems show that, along with monocarbides, hcp-subcarbides of M2C composition are also formed with the carbon atoms located in octahedral interstitials of the metallic sublattice. 

X-ray emission spectra of these phases have been studied by Kindrat (1982), quantum-chemical calculations of V2C and Nb2C have been carried out by Ivanovsky, Gubanov and Kurmaev (1985) using the cluster MO LCAO method, and 16-atom [MCjMi2] (M = V, Nb) clusters have been considered. The results obtained are given in Fig. 2.8. The valence band of the hcp-carbides has the typical three-band structure formed by the C2s, C2p, and Ms, d, and Md, s functions (the band of the quasi-core C2s states is not shown in Fig. 2.8). Table 2.2 gives 

As the metal content in carbides decreases (MC M2C), the effective charges on atoms become lower. As in the cubic phases, the maximal overlap population in MjC is due to Md-C2p interactions, whose contributions to the formation of M-C bonds are reduced when going from NbjC to V2C. 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

The results reveal a general similarity of the energy spectra for all B2 carbides, and therefore we consider only VC as an example (Fig. 2.9). The VC valence band consists of two subbands, the lower one containing comparable contributions from the V3d and C2p states. The intensive peak near the Fermi level is of predominantly metal d-type with an admixture of the C2p states. The Vs- and Vp-orbitals give relatively small contributions to the carbide valence band and to the band of quasi-core C2s states. 

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