RECONSTRUCTION, DISORDERING AND ROUGHENING OF METAL-SURFACES

Until recently, surface reconstruction and roughening have been studie d as separate surface phenomena and have been described theoretically by means of separate models. This approach is reasonable when the deco nstruction transition has a displacive character as occurs for example on Mo(001) and W(001) surfaces. In these cases only in-plane degrees of freedom are involved in the reconstruction process. Conversely, a u nified approach to reconstruction and roughening seems more appropriat e when the reconstructed phase is produced by the low-temperature orde ring of a large concentration of point defects (vacancies, adatoms) or extended defects (steps). The reconstruction in these cases involves off-plane degrees of freedom. Since roughening derives from the prolif eration of thermally excited steps, the interplay between steps and de constructive defects may provide the connection between the two transi tions. A notable example of these latter systems is provided by the (1 10) surface of noble and near-noble metals; the (110) surface of the h eavier metals (Au, Pt) reconstructs at low temperature in the 2 x 1 mi ssing-row structure. In the present article we review some statistical mechanics models able to display both deconstruction and roughening o f the fcc(110) missing-row phase, chosen as a paradigmatic example of the interplay between the two transitions. The properties of the phase between deconstruction and roughening, produced by the different mode ls are analysed, and compared with the results of molecular-dynamics s imulations with continuous potentials. The fingerprint of the differen t phases in scattering measurements are discussed and compared with th e experimental data available.