THE RECONSTRUCTION OF RH(001) UPON OXYGEN-ADSORPTION

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density function al theory. We find that oxygen atoms sit at the center of the black sq uares in, a chess-board c(2 x 2) pattern. This structure is unstable a gainst a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, w hile lengthening the distance with respect to the other two. We actual ly find that the surface energy is further lowered by allowing the O a tom to get off the short diagonal of the rhombus thus formed. We predi ct that the latter distortion is associated with an order-disorder tra nsition, occurring below room temperature. The above rhomboid distorti on of the square lattice may be seen as a rotation of the empty white squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our pre dicted reconstruction pattern. (C) 1998 Elsevier Science B.V. All righ ts reserved.