Chemical order driven morphology of a vicinal surface of Fe 3Al(1 1 1): an He diffraction and STM study

Abstract

The morphology and atomic structure of a vicinal surface of Fe 3Al(1 1 1) has been studied by thermal energy helium atom scattering and scanning tunneling microscopy. The second order B2→DO 3 structural phase transition in the chemically ordered alloy induces a morphological transition at its surface: single-height steps, which are evenly distributed above the bulk critical temperature T c (≈820 K), bunch four by four below T c. The latter morphology is related to bulk layer stacking and appears driven by minimization of terrace energy. Terraces are pure Al and exhibit a (√3×√3) R30° surface reconstruction, corresponding to 2/3 Al atoms and 1/3 vacancies in the terminal layer. The first sub-surface layers are enriched in Al by surface segregation. Surface vicinality makes possible the emergence of bulk DO 3 domain boundaries, where four-steps bunches separate in two pairs of steps, which allows following chemical ordering in the near-surface region. The results point out the strong influence of long range chemical order on surface morphology.