ORDERED STRUCTURES AND STRUCTURAL TRANSFORMATIONS ON A K-COVERED AU(110) SURFACE

The various surface phases formed on a K covered Au(110) surface at different coverages, deposition and annealing temperatures were characterized by LEED, AES and TDS. The structural characteristics and thermal stability of these phases were compared with those of other alkali covered fcc(110) transition metal surfaces, where the presence of the K(ad) induces a "missing row" reconstruction similar to that present already on the clean Au(110) surface. At K coverages < 0.3 monolayers (ML) the "missing row" reconstruction of the substrate is stabilized by the presence of K(ad). The K adlayer is adsorbed on a (1 x 2) substrate at coverages between 0.1 and 0.3 ML, while at theta-K less-than-or-equal-to 0.1 a (1 x 3) substrate is more stable. The rather mobile adlayer orders on these substrates at low temperatures only, where additional streaks in the LEED pattern indicate a continuous sequence of ordered structures with increasing coverage. At coverages > 0.3 ML the (1 x 2) coexists with islands of a (mixed layer) c(2 x 2) phase, which completely covers the surface at theta-K = 0.5. The formation of the thermodynamically stable phases is partly connected with kinetic barriers; all structural transformations that require a rearrangement of the substrate are activated while those that merely involve a reordering of the adlayer occur instantaneously even at low temperatures. K deposition at low temperatures (about 100 K) leads to the formation of metastable phases where the K adlayer is adsorbed on a (1 X 2) substrate, independent of coverage. Structural transitions are induced also by loss of K(ad) during annealing to higher temperatures, where stable higher coverage structures transform into lower coverage phases. The close agreement of the structural, thermodynamic and kinetic properties of the alkali induced "missing row" reconstructions on other fcc(110) transition metal surfaces with those of the ordered phases on the K-covered Au(110) surface, where the "missing row" reconstruction is more stable already for the clean surface, provides convincing experimental evidence for a common physical origin of these reconstructions.