Low-Energy Electron Diffraction Studies of Surface Melting and Freezing of Lead, Bismuth, and Tin Single-Crystal Surfaces

The surface structures of the (111), (100), and (110) faces of lead, the (0001), (011̄2) faces of bismuth, and the (110) face of tin single crystals were monitored up to the melting temperatures and during melting by low-energy electron diffraction. Measurements of the surface Debye–Waller factor from the different lead and bismuth surfaces indicated mean-square displacements of surface atoms perpendicular to the surface plane, 〈u⊥2〉surf, which were much larger than the bulk mean-square displacements at the melting points as predicted by the Lindemann melting model. However, the diffraction features have persisted in all of the crystal faces up to the bulk melting points of these solids. These results indicate that the surfaces remain ordered up to the bulk melting point and that the crystal surface plays an all important role in nucleating or initiating melting. Several experimental melting studies and pertinent melting models have also been discussed. The surface orientation of lead and bismuth crystals were monitored during freezing and growth from the melts. The dominant surface structures which formed upon freezing were dependent on the cooling rate. Slow rates (≤0.5°C/sec) favored the formation of the Pb(111) and Bi(011̄2) surfaces while during rapid cooling (0.5°C/sec) the Pb(100) and Bi(0001) crystal faces have predominated.