ORIENTATIONAL INSTABILITY OF VICINAL PT SURFACES CLOSE TO (111)

Using scanning tunneling microscopy and thermal He diffraction we have studied the morphology of the vicinal Pt(997) surface, close to the h exagonal close-packed (111) face. While oxygen adsorption induces a st ep-doubling transition, the clean surface is thermally unstable toward s faceting. Above T almost-equal-to 0.5T(m) the surface is found to un dergo partial phase separation into large {111} facets and regions of undisturbed step regions. The faceted phase is stabilized by a reducti on of surface stress through reconstruction of the (111) faces. The fa ceting is found to proceed via a nucleation-and-growth mechanism.