Role of Co antisite segregation in the CoAl(111) surface
The geometrical and chemical structure of the CoAl(111) surface is investigated by quantitative low-energy electron diffraction and calculations applying density functional theory. The stacking sequence of the top four atomic planes is Al-Co-Co-Co, followed below by the usual alternating B2 stacking. The topmost layers thus form a unit cell of the well-known bcc-based D0 3 crystal structure [the A 3B superlattice of bcc(111) atomic planes], although the bulk phase diagram of CoAl shows no D0 3 phase. Its occurrence and stability at the surface is due to a slight Co excess of the nominally stoichiometric sample, equivalent to the presence of Co antisite defects in the bulk. These defects are enriched in undercoordinated near-surface sites of the Al sublattice, which lowers the total energy because more Al atoms can then reside in fully coordinated bulk Al sites. However, all three topmost layers are undercoordinated, and the segregation of Co antisite defects competes with a general trend towards a termination of the surface by Al. In the balance, the third layer is the preferred plane for Co antisite defects. ©2005 The American Physical Society.
Hammer, L; Blum, V; Schmidt, C; Wieckhorst, O; Meier, W; Müller, S; Heinz, K
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