Composition and atomic structure of two ordering binary-alloy surfaces: Copper(3)gold(110) and iron(tin)(100)

The composition and atomic structure of the top two layers of the Cu$\sb3$Au(110) surface were studied as a function of temperature to understand segregation in a model ordered alloy. A combination of Auger electron spectroscopy, low-energy electron diffraction, low-energy ion scattering, and molecular statics calculations with many-body potentials, revealed behavior that was different from the previously-investigated Cu$\sb3$Au(100) surface. The pattern of planes consisting of equal parts copper and gold, alternating with planes of pure copper, was found to be disrupted at the (110) surface. The bulk was terminated at a Cu-Au layer, which had a slight depletion of gold (45%) at room temperature. The second layer, nominally a pure copper plane, contained a substantial amount of gold (25%). The top two surface layers each reached 35% gold below the bulk ordering temperature of 390$\sp\circ$C. The differences between the (110) and the (100) surfaces were ascribed to differences in the effective coordination number of first-layer atoms. When annealed below 150$\sp\circ$C, the (110) surface was found to be reconstructed, with every other copper (001) row removed from the surface. The newly-exposed second-layer lattice sites were found to be occupied preferentially by gold. Surface-stress-tensor arguments are used to describe the origin of this reconstruction. Iron crystals doped with tin are known to exhibit segregation of more than one monolayer of tin to the (100) surface. This unusually large coverage raises the question of whether the multilayer enrichment is due to equilibrium segregation or to some kind of metastable precipitation phenomenon. In the latter case, the presence of the equilibrium bulk second phase, FeSn, would be expected to cause a reduction in the surrounding tin coverage. This issue was addressed by the use of chemical-vapor deposition to seed the surface of a tin-doped crystal with FeSn. It was found that the presence of this second phase did not alter the equilibrium surface coverage, which returned quickly to its previous level by surface diffusion from the FeSn crystallites