Segregation and chemical ordering in the surface layers of Pt25Co75(111):: a LEED/STM study

Segregation and chemical ordering on Pt25Co75(111) are studied by quantitative low-energy electron diffraction (LEED) analysis and scanning tunnelling microscopy (STM). Although LEED patterns do not show any sign of superstructure, LEED calculations undoubtedly point to a surface that contains about the same amount of both species and reveal a significant short-range chemical ordering (down to the third layer). Pt and Co surface sites are locally arranged with a (1 x 2) unit cell, in the manner of the ordered tetragonal L1(0) phase. More direct evidence is given by STM images that exhibit parallel Pt and Co monoatomic chains a few lattice constants long and an apparent height difference of about 0.2 Angstrom for Pt and Co sites. LEED shows that the Pt sublattice in the top layer actually resides 0.1 Angstrom above the Co one. Otherwise the surface is bulk-like, with weak relaxations of interlayer distances. The use of a chemically ordered model for the LEED analysis, in which sublattice occupancies are optimized, results in a remarkable improvement of the fit with experiment as compared to a totally random distribution; however, most interestingly, it does not modify the average layer concentration profile versus depth (55, 5 and 35 at.% Pt, respectively, for the three outermost layers). The distortions needed for the tetragonal L1(0) phase with respect to the fee L1(2) phase explain why chemical order does not extend over larger domains. Finally, both techniques yield complementary pictures and quite consistent results as to the top layer content and chemical order. (C) 2000 Elsevier Science B.V. All rights reserved.