Ion Scattering Study of the W(001)-(5x1)-C Surface

Abstract

There have been several investigations of the surface of carburized W(100), and various models for its structure have been proposed. It is known that the surface exhibits a sequence of LEED patterns with increasing C coverage which can be indexed as p(1×1), c(2×2), c(3√2x√2)-R45° and p(5×1) [1,2,3]. Most authors agree that the c(2×2) and c(3√2 x √2)-R45° patterns are due to ordered C over layers with coverages near 1/2 and 2/3 mono layers, respectively. The p(5×1), accompanied by domains of p(1×5), can be formed by large exposures of ethylene at elevated temperature, and it is stable at temperatures up to 2000K and high pressures of hydrocarbon [1]. Because of this stability and the fact that heavily carburized polycrystalline W forms α-W₂C, OLLIS and BOUDART [1] proposed that the p(5×1) LEED pattern is due to the formation of an over layer of α-W₂C(0001). It was assumed that a coincidence occurs between the thin distorted hexagonal over layer and the W(001) substrate leading to the (5x1) pattern. BENZIGER et al. [2] accepted this structure but concluded from the lower than expected C Auger electron spectroscopy (AES) signal that the C must reside below the first W layer for the W(001)-(5×1)-C surface (Fig. 1, model 1). An alternate explanation offered by RAWLINGS et al. [3] was that the C is present above the surface, but at lower concentration than for a single (0001) layer of α-W₂C. They proposed that the first W layer is reconstructed yielding a single layer of strained W(110) on top of W(001) (Fig. 1, model II). The first layer W atoms form the same coincidence mesh with respect to the W(001) substrate as in the case of the α-W₂C (0001) layer. We have examined the structure of the carburized W(001) surface by incident angle dependent alkali ion scattering and find that the surface is not reconstructed to the quasi-hexagonal structure previously proposed. Instead, the p(5×1) LEED pattern is attributed principally to an ordered C over layer (Fig. 1, model III).