Surface effects on the order-disorder phase transition ofA3Balloys

The order-disorder phase transition at the surface at $A_{3}B$-type alloys has been studied using the Bragg-Williams theory. A simple electronic theory for the order-disorder transition at the surface is also presented. In agreement with experiments for ${\mathrm{Cu}}_3$Au one finds that the order parameter and in particular the discontinuity in the order parameter at the transition temperature $T_c$ is strongly reduced for the first three atomic surface layers. We study also the effect of surface segregation on the order-disorder transition at the surface. One finds that due to segregation the order parameter can disappear at the surface at temperatures below $T_c$. Also, atomic ordering may suppress segregation.