Regensburg 2004 – wissenschaftliches Programm

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O: Oberflächenphysik

O 6: Struktur und Dynamik reiner Oberflächen

O 6.1: Vortrag

Montag, 8. März 2004, 11:15–11:30, H45

Co Subsurface Segregation in CoAl(111) — •Lutz Hammer, Volker Blum, Christian Schmidt, Ole Wieckhorst, Wolfgang Meier, Stefan Müller, and Klaus Heinz — Lehrstuhl für Festköerperphysik, Universität Erlangen-Nürnberg, Staudtstr. 7, D-91058 Erlangen

The geometrical and chemical structure of the CoAl(111) surface is investigated by quantitative low-energy electron diffraction (LEED) and calculations applying density functional theory (DFT). It is found that the surface assumes an Al-Co-Co-Co layer stacking within the top few layers and the usual alternating B2 stacking below. The topmost layers thus form a unit cell of the bcc-based D03 crystal structure (i.e. a A3B superlattice of bcc(111) atomic planes), despite of the fact that the bulk phase diagram of CoAl shows no D03 phase. Its stability at the surface is due to a slight Co excess of the nominally stoichiometric sample, which is incorporated in the bulk by forming Co antisite defects. These defects are enriched in undercoordinated surface sites, there replacing Al atoms which assume fully coordinated sites in the bulk, in effect lowering the total energy. The process competes with the energetically favourable Al termination of the surface, so that the Co antisites concentrate in the still undercoordinated third layer rather than in the top layer, leading to the observed termination.

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