Reconstruction, disordering and roughening of metal surfaces

Until recently, surface reconstruction and roughening have been studied as separate surface phenomena and have been described theoretically by means of separate models. This approach is reasonable when the deconstruction 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 unified approach to reconstruction and roughening seems more appropriate when the reconstructed phase is produced by the low-temperature ordering 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 proliferation of thermally excited steps, the interplay between steps and deconstructive defects may provide the connection between the two transitions. A notable example of these latter systems is provided by the (110) surface of noble and near-noble metals; the (110) surface of the heavier metals (Au, Pt) reconstructs at low temperature in the 2 x 1 missing-row structure. In the present article we review some statistical mechanics models able to display both deconstruction and roughening of 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 models are analysed, and compared with the results of molecular-dynamics simulations with continous potentials. The fingerprint of the different phases in scattering measurements are discussed and compared with the experimental data available.