AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM2
Formation and Vibrational Entropy-Driven Disordering of Mo(100) and W(100) Surface Alloys

Wednesday, November 2, 2005, 8:40 am, Room 200

Session: Growth and Alloying of Surfaces
Presenter: M.S. Altman, Hong Kong University of Science and Technology
Authors: K.L. Man, Hong Kong University of Science and Technology
Y.J. Feng, Hong Kong University of Science and Technology
C.T. Chan, Hong Kong University of Science and Technology
M.S. Altman, Hong Kong University of Science and Technology
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Atoms that are deposited on a surface of a dissimilar material may either remain on the surface or they may become incorporated in a surface or bulk alloy. Although the T = 0 energetic differences between alloy and overlayer structures can now be understood from first principles in many systems, entropy differences should also be revealed in the formation and stability of alloy versus overlayer structures. However, the entropic contribution to the system free energy is much less well understood than the energetic. The formation and stability of Cu, Ag and Au-induced c(2x2) alloys at the Mo(100) and W(100) surfaces have been investigated with low energy electron microscopy and diffraction. The dependence of the c(2x2) diffraction intensity upon metal deposition flux reveals that alloy formation is governed by atomistic processes that are analogous to those that dictate overlayer island nucleation. A second-order order-disorder transition is also observed that converts the surface from ordered alloy to disordered overlayer structure. Combined with knowledge of energetics obtained from first principles calculations, a comparison of disordering temperatures for alloys of the different metal species and substrates provides information on the vibrational entropic contribution to the system free energy. Effective Debye temperatures for metal adatoms are determined that are substantially lower than bulk values, but exhibit the expected mass dependence. Knowledge of the vibrational properties of metal adatoms should also be relevant to other common surface phenomena such as diffusion and desorption.