In the Zr-C system, the monocarbide is the only intermetallic phase reported, crystallizing in the face-centered cubic NaCl structure (Fm3m, space group 225) (Figure 1). Zr atoms form a close-packed lattice, and the smaller C atoms (rc = 0.48rZr) fill the octahedral interstices.3

The ZrCx phase exists over a wide compositional range and, as further discussed in Section 2.13.3.1, is stable with up to 50% vacancies on the carbon sublattice. Low-temperature ordered phases have been experimentally reported for the Ti-C, V-C, and Nb-C systems, but so far have been suggested only via thermodynamic calculations for the Zr-C system.6 Metallic vacancies comprise at most a few atomic percent.3

The effect of carbon vacancies on unit cell geom­etry has been investigated extensively (Figure 2),

with the relationship between room temperature lat­tice parameter and C/Zr ratio difficult to establish conclusively. Scatter in literature values is a common theme in the study of transition metal carbides because of the difficulty of preparing pure specimens and adequately characterizing them. Oxygen and nitrogen readily substitute for carbon in the lattice, and their presence is correlated with reduced lattice parameter. On the basis of literature values for a range of impurity contents, Mitrokhin et alJ established a quantitative relationship between the lattice parame­ter of such oxycarbonitrides and carbon, as well as the oxygen-nitrogen impurity content:

azrcx (on) = 4.5621 — 0.2080x V 0.3418x

— 0.80y(1 — x) I1]

where x is the C/Zr atomic ratio (0.62 < x < 1) andy is the (O + N)/Zr atomic ratio (y < 0.3).

In general, lattice parameter increases with C/Zr ratio, with evidence for an increase and a decrease as C content increases above approximately ZrC0.8 toward ZrCi.0. Ramqvist8 qualitatively explained the peak in lattice parameter versus C/Zr ratio as being due to competing influences on lattice size: expansion with increasing carbon content due to the increased space required to accommodate interstitials, and con­traction due to the increased bond strength.

The nature of chemical bonding in ZrCx is not fully understood, and electronic structure investiga­tions have sought to establish the relative influences of covalent, metallic, and ionic contributions. Carbon s — and p-orbitals and zirconium d-orbitals participate in bonding and contribute to strong metal-nonmetal bonding and octahedral coordination.9 Other authors10 emphasize the interstitial nature of carbon in the ZrC structure and the donation of electrons from carbon to metal, strengthening Zr-Zr bonds. Lye and Logothetis11 proposed that some charge transfer from carbon to metal occurs and that carbon stabilizes the carbide structure by contributing bonding states. Hollox12 and Storms and Griffin13 suggest that, depending on the carbide, lattice sta­bility decreases with increasing carbon content if antibonding states become filled; this is consistent with observed hardness and melting temperature measurements for ZrCx. The electronic structure of ZrC must be placed in context with the properties of Groups IV, V, and VI transition metal carbides, and the interested reader is referred to the compar­ative reviews seen earlier.