Surface structural transitions during tin segregation to the surface of a (100) iron crystal

Abstract

A study of the segregation of Sn to the (100) surface of an Fe-1.25 at% Sn crystal has been carried out by means of AES, LEED, and UPS using synchrotron radiation. The room temperature LEED patterns transformed gradually with increasing Sn coverage from p(1 × 1) for the clean surface to c(2 × 2) at a half-monolayer, then to p(1 × 1) at one monolayer, and finally to p(2 × 2) at the maximum observed coverage of 1.2 monolayers. The latter was the ultimate, presumably equilibrium, coverage achieved at 500 °C; above that temperature the equilibrium coverage decreased linearly up to 650 °C, above which evaporation occurred. The final LEED pattern transition was accompanied by a rapid increase in the Sn 4d binding energy toward the value associated with pure Sn and also a broadening of the Sn4d 5 2 peak. At the ultimate maximum coverage of 1.2 monolayers the LEED pattern transformed gradually and reversibly from p(2 × 2) to c(2 × 2) upon heating above about 300 ° C. A further gradual and reversible transformation to p(1 × 1) occurred upon heating above about 570 °C and was presumably associated with the reduction in equilibrium coverage above 500 °C. These results differ in several respects from a contemporaneous study of a similar alloy, and they raise interesting questions about the nature of the equilibrium surface in this intriguing alloy system.