Low-energy electron-diffraction study of the clean (100), (111), and (110) faces of platinum

Abstract

The structures of the (111), (100), and (110) crystal faces of platinum were studied as a function of temperature in the range 300°-1769°C (melting point). The (111) and (100) substrates are stable while the (110) face shows faceting above 6000C. Several surface structures were found to exist on the stable platinum substrates. These have well-defined temperature ranges of stability. There are two types of surface structures, ordered and disordered. Both types appear to be the property of the clean platinum substrates. The ordered structures appear during annealing, after ion bombardment at lower temperatures (<900°C). They exhibit long-range order and their stability ranges overlap on a given substrate. These structures are believed to be due to ordered arrays of vacancies in the substrate plane which, under proper conditions, show remarkable stability. A high, nonequilibrium concentration of defects at the surface may be necessary to induce their formation. The disordered surface structures appear at high temperatures, above the stability range of most of the ordered structures. They are characterized by ringlike diffraction features which develop gradually as a function of increased heating time or temperature. The formation of these structures is irreversible and they can only be removed by ion bombardment. Near the melting point, the ring patterns remain the only diffraction features of the presumably greatly disordered surfaces. The ratio of lattice parameters assigned to the diffraction rings on each substrate indicate that they can be due to domains of (111) surface structures. These hexagonal structures appear on all of the platinum surfaces, are freely rotated in the substrate plane, and show an ∼11% contraction with respect to the interplanar spacing in the ordered (111) face. The disordered close-packed-hexagonal structure seems to be the stable high-temperature surface phase of platinum.