Chemical order driven morphology of a vicinal surface of Fe3Al(1 1 1): an He diffraction and STM study

The morphology and atomic structure of a vicinal surface of Fe3Al(1 1 1) has been studied by thermal energy helium atom scattering and scanning tunneling microscopy. The second order B2→DO3 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 Tc (≈820 K), bunch four by four below Tc. 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 DO3 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.