Theoretical study on the electronic properties and stabilities of low-index surfaces of WC polymorphs

The low-index surfaces of WC polymorphs are calculated using the first-principles method based on density functional theory. It is found that there are large relaxations within the top three layers for all termination surfaces, and charge density falls greatly toward the vacuum. The outermost and second interlayer relaxations for C-terminated surfaces are much larger than those for W-terminated surfaces. The surface energies for all low-index surfaces are large which is due to the breaking of strong W-C bonds. For both WC polymorphs, the C-termination surfaces are thermodynamically more unstable than W-terminated surfaces over the whole range of carbon chemical potentials considered in this paper; the most stable surfaces correspond to the (0 0 1) surface with W termination for α-WC, and the (0 0 1) surface with WC termination for β-WC.