THE SURFACE STOICHIOMETRY OF VC0.80(100) STUDIED BY LEED

The surface structure of substoichiometric vanadium carbide was studie d by quantitative low-energy electron diffraction on a VC0.80(100) sam ple. A simple 1 x 1 diffractogram was observed on annealing to 1000 de grees C. Optimization in four- to seven-dimensional structural spaces gave two surface structures, close to one another with respect to the agreement between theoretical and experimental spectra but widely sepa rated by the structure and composition of the topmost VC layer. In the primary (secondary) model the top layer is rippled with the C subplan e protruding 0.88 +/- 0.05 Angstrom (0.20 +/- 0.15 Angstrom) from the V subplane and with a C occupancy 0.4 +/- 0.2 (0.2 +/- 0.2). The prima ry model is supported by a previous photo-emission study of VC0.80(100 ), where a strong Is core level signal from the surface C atoms was ob served. Characteristic of VC0.80(100) is the expansion of the first in terplanar spacing of the C sublattice, while for the (100) surfaces of TaC, HfC and VN0.89 a contraction of the first interplanar spacing of the metal sublattice predominates. (C) 1997 Elsevier Science B.V.