STRUCTURE AND PHASES OF THE AU(111) SURFACE - X-RAY-SCATTERING MEASUREMENTS

The results of a comprehensive synchrotron x-ray-scattering study of the clean, reconstructed Au(111) surface between 300 and 1250 K are presented. Two surface phases are identified and characterized. For T < 865 K, the reconstruction consists of a series of discommensurations, separating surface regions with the correct face-centered-cubic (fcc) ABC stacking sequence from regions with a faulted AB A stacking sequence. A key feature of this phase is the existence of an equilibrium density of kinks between rotationally equivalent domains. At 865 K, the reconstruction starts to lose long-range order. The first-order transformation to a disordered phase is complete by 880 K. In this phase, the topmost layer has hexagonal symmetry and is on the average isotropically compressed compared with bulk (111) planes. Between 880 and 1250 K the translational correlation length and the average compression of the overlayer evolve only slightly. Interpretation of the x-ray reflectivity, both specular and nonspecular, establishes, however, that the occupation of fcc C sites and faulted A sites is approximately constant throughout the temperature range studied. In addition, the separation between the top two layers is expanded 3% relative to the bulk (111) interlayer spacing and the surface-normal vibrational amplitude in the surface layers is enhanced with respect to the bulk value.