The ${\mathrm{VC}}_{0.80}$(111) surface forms a metastable 1×1 structure upon argon-ion bombardment and annealing at a temperature not exceeding 1000 K. We study this structure by low-energy electron diffraction and are able to determine the relaxation and composition of a four-layer slab inside the surface. The slab terminates with a vanadium layer at vacuum and is, to a good approximation, stoichiometric; the error bars of the carbon concentrations in layers two and four are estimated with great care. The interlayer spacing of the vanadium-carbon double layer next to vacuum is contracted 10% with respect to the (111) spacing of the bulk. We regard such a strong contraction, not observed on (111) surfaces of metals, as related to a compensating charge giving rise to a metastabilization of the polar (111) surface of vanadium carbide.

• Received 8 July 1991

DOI:https://doi.org/10.1103/PhysRevB.45.4445