Strain-induced local surface chemical ordering observed by STM

PtNi alloys are known to exhibit a tendency towards chemical ordering, which also effects surface segregation. Scanning tunneling microscopy results obtained in the strain field of dislocations on PtxNi1-x(110) surfaces show a (2 x 1) superstructure of alternating Pt and Ni atoms in some regions close to the dislocation core. In other regions, the apparent height of all surface atoms is equal, in agreement with low energy ion scattering results yielding a surface concentration of almost 100% Ni. This indicates that the strain present in the vicinity of dislocations influences both the surface composition and chemical order. The experimental results are compared to simulation calculations of chemical ordering and segregation, using embedded atom method potentials and linear elasticity theory. The simulations indicate that the (2 x 1) superstructure is due to an L1(0) ordered phase in regions where the tetragonal distortion of the L1(0) phase with respect to the cubic substrate can alleviate stress. It is argued that this dislocation-induced ordering can immobilize dislocations.