Equilibrium form of Pb-Bi-Ni alloy crystals

Both stable and metastable equilibrium forms of Pb alloy crystals containing 5 at% Bi and either 0.04 or 0.08 at% Ni have been observed. These forms are quite different from those of pure Pb in that several new facets, beyond the {1 1 1} and {1 0 0} facets found on pure Pb, are found to form under conditions close to equilibrium. The appearance of new facets on the alloy dilute in Ni is closely associated with a chemical surface phase transition, in which the surface concentrations of Bi and Ni increase abruptly upon cooling. The results are discussed in the context of a nearest neighbor bond model, which exhibits the chemical surface phase transition, and which predicts a sharp decrease in surface energy at temperatures below that transition. The presence of metastable equilibrium forms is found to be closely tied to the existence of a nucleation barrier which leads to hysteresis in the chemical surface phase transition temperature on {1 1 1} facets, but not on other orientations. The significance of high surface concentrations of Bi and Ni in raising the roughening temperature of certain Pb surface orientations is also addressed.