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Using scanning tunneling microscopy and thermal He diAraction we have studied the morphology of the vicinal Pt(997) surface, close to the hexagonal close-packed (111)face. While oxygen adsorption induces a step-doubling transition, the clean surface is thermally unstable towards faceting. Above T = 0.5T~the surface is found to undergo partial phase separation into large {II I i facets and regions of undisturbed step regions. The faceted phase is stabilized by a reduction of surface stress through reconstruction of the (111)faces. The faceting is found to proceed via a nucleation-and-growth mechanism.The structure of vicinal surfaces, i.e. , regularly stepped surfaces which are generated by a slight miscut with respect to a low-index plane, has been the subject of many recent studies [1][2][3][4]. They constitute ideal model systems to study the relation between surface structure and surface energy. Nominally, vicinal surfaces are composed of terraces of the low-index orientation separated by a superlattice of parallel steps accommodating the misorientation 8. The total surface energy y(8, T) of the vicinal is given byThe first term yo is the surface energy of the low-index terraces while the second term adds the contribution from each of the monatomic steps, with P being the energy per unit length to form an isolated monatomic step of height h. The last term B accounts for interactions between neighboring steps and aII is the unit vector along the step edge. The nominal structure is only stable if the vicinal orientation is a tangent in the Wulff plot (y versus 8) in polar form [5]. Unstable orientations phase separate into a "hill and valley structure" of coexisting regions of stable low-index orientations (facets).Because of the delicate balance between step and terrace energies also stable vicinals can be subject to morphological changes as a function of temperature or impurity concentration. The ordered superlattice of equally spaced steps is stabilized by the repulsive step-step interaction between neighboring steps. At elevated temperatures thermal disorder through excitation of kinks competes with the order established by the repulsive step-step interaction. This thermal kink proliferation eventually results in a roughening transition of the vicinal surface [6]. Changes in temperature might also modify the terrace and step-free energies, and thereby induce an orientational instability. So far only three examples of thermal faceting of clean surfaces are known: vicinal Si surfaces close to (111) and vicinal Pt surfaces close to (100) where the orientational instability is induced by reconstruction [2,7] and vicinal Pb surfaces close to (111) where faceting is driven by surface melting [8]. More frequently observed are faceting transitions induced by impurity adsorption [9]. In this Letter we report measurements of temperature and impurity induced changes in the surface morphology of the vicinal Pt(997) surface. Nominally this vicinal surface is composed of close-packed (111) terraces separated each 20.2 A b...

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