Carbides of group VA transition metals

Abstract

Estimates are made of the cohesive energies of the group VA transition metal carbides for various C contents and crystal structures. In the cubic structure MC_x, several factors help to provide optimal cohesion at x < 1, even in the presence of excess C. Notable among these is a drop in the Fermi level when vacancies appear in the C sublattice. A strong C–C repulsion between very close neighbours prevents the hexagonal structure becoming stable when x is large, even though the underlying metal sub lattice is expected to prefer hcp to fcc at these electron concentrations. In M₂ C the C–C repulsive contribution is sufficiently reduced to allow the hexagonal structure to appear. The opportunity for afurther reduction in this repulsive contribution, together with a change in electrostatic energy, provides a strong tendency for ordering in the C sub lattice of M₂ C, leading to high ordering temperatures.

MST/3031