S-INDUCED TRIANGULAR PITTING ON FE(111)

We report an unusual faceting behavior of the Fe(lll) surface induced by the segregation of S. STM measurements of a Fe(lll) surface covered with segregated S reveal massive triangular pitting on the surface. T his triangular pitting occurs only at a critical S coverage of Auger p eak-to-peak S-(152)/Fe-(651) intensity ratios greater than similar to 1.4 corresponding to more than one ''geometric'' monolayer of S based on one ''geometric'' monolayer coverage for the (1 x 1)-S structure. T he occurrence of triangular pitting is coincident with the transformat ion of the (1 x 1) hexagonal LEED pattern to a new, previously unobser ved (2 root 3 x 1)R30 degrees LEED pattern. The pit depths are as larg e as 50 Angstrom. The pits' edges are as long as 800 Angstrom. The edg es of the triangular pits are oriented along the (110) direction of th e surface. Inside the triangular pits, a step-terrace topography is ob served with a preferential formation of a 14 Angstrom terrace width. T he new surface planes inside the pits correspond to the stepped surfac e notation [5(111)x(100)]. Atomic resolution images obtained from flat areas near the pits reveal surface buckling with two slightly differe nt packing arrangements corresponding to a (2 root x 1)R30 degrees sup erstructure. Reactivity studies with O-2 show that the pitted surface is extremely inert toward oxidation at room temperature, resisting oxi dation to exposures of up to 1600 L of O-2. The passivity of the (2 ro ot 3 x 1)R30 degrees to oxidation may be due to a combination of geome tric and electronic blocking effects of S on O-2 dissociation. A compa rison of our results to previously reported adsorbate-induced faceting on W(111) and Mo(lll) surfaces is briefly discussed. (C) 1997 Elsevie r Science B.V.