RECONSTRUCTION OF THE PT50NI50(100) SURFACE - A LEED AND STM STUDY

The structure of the Pt50Ni50(100) surface was investigated by low ene rgy electron diffraction (LEED) and scanning tunnelling spectroscopy ( STM). Superstructures corresponding to 12 x 1 and 19 x 1 reconstructio ns were observed in the LEED pattern, and these reconstructions were i dentified with atomic resolution by STM. The reconstructed surface lay er has a quasi-hexagonal atomic mesh with almost (111) atomic density and is divided into terraces and domains the size of which can be more than a thousand angstroms. The domains are corrugated with protrusion s made up by 5 x 1 subunits of the quasi-hexagonal mesh and with valle ys containing depressed atoms or 2 x 1 subunits. The 12 x 1 and 19 x 1 periodicities are determined by the sequence of valleys; depending on the thermal treatment, one or the other dominates but both generally coexist on the terrace. Owing to the complexity of the structure, LEED calculations were performed for a simplified model with surface atoms situated in a planar net and uniform quasi-hexagonal meshes. We find that in both reconstructions the two surface layers are significantly enriched with platinum. The top layer enrichment is consistent with pr evious Auger measurements.