Surface processes and phase transitions from ab initio atomistic thermodynamics and statistical mechanics Catherine Stampfl a,b, * a School of Physics, The University of Sydney, Sydney 2006, Australia b Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Abstract Knowledge of the surface composition and atomic geometry is a prerequisite for understanding the physical and chemical properties of modern materials such as those used in, for example, heterogeneous catalysis, corrosion resistance, sensors, and fuel cells. To understand the function of surfaces under realistic conditions, it is crucial to take into account the influence of environmental gas at finite (possibly high) temperatures and pressures. Recent experimental and theoretical studies have shown that when transition metal surfaces are exposed to high oxygen pressures, thin oxide-like structures can form which may have little similarity to the bulk oxides, and thus possess unique chemical and physical properties. Given that technological oxidation catalysis typically involves oxygen-rich conditions, such structures may be present, and in fact be the active material for the reaction, as opposed to the traditionally believed pure metal. Using the approach of ab initio atomistic thermodynamics, free energy phase-diagrams for oxygen/transition metal systems in (T , p)-space ranging from ultra- high vacuum to technically relevant pressures, p, and temperatures, T, are discussed. In addition, results of ab initio statistical mechanical schemes, namely, the Lattice-gas Hamiltonian plus Monte Carlo (MC) simulations, are presented for oxygen/transition metal and alkali-atom/metal systems, where for the latter, the recently introduced ‘‘Wang–Landau’’ algorithm is employed, which affords an accurate estimation of the density of (configurational) states, therefore allowing a direct determination of all major thermodynamic functions. # 2005 Published by Elsevier B.V. Keywords: Surface; Phase transition; Atomistic thermodynamics 1. Introduction One of the main goals of theoretical heterogeneous catalysis, and materials and surface science in general, is to achieve an accurate atomistic description of solids and their surfaces that can predict phenomena and properties occurring on macroscopic length and long time scales. Such methods should quantitatively describe measurable properties without relying on experimental parameters, which implies that they have to start ab initio, i.e., from the self-consistent evaluation of the electronic structure [1,2]. Clearly, knowledge of the surface structure and composition are crucial ingredients for such a description, and conse- quently for understanding the associated physical and chemical properties. This is particularly relevant for the long held desire of being able to tailor (and possibly design) functional surfaces for use in a wide range of technological applications such as, heterogeneous catalysis, corrosion resistance, semiconductor and magnetic devices, and fuel cells, to name a few. The present paper will discuss, through various examples, state-of-the-art first-princples-based theoretical methodol- ogy and concepts which are valuable for obtaining a greater understanding and prediction of surface processes and phase transitions at the atomic level, which are of fundamental interest and importance to functional surfaces in general. The paper, largely a review of recent work, is organized as follows: In Section 2, using density-functional theory (DFT), investigations into trends of the initial stage of oxidation at a series of late 4d transition metal (TM) surfaces are described, namely, Ru(0 0 0 1), Rh(1 1 1), Pd(1 1 1), www.elsevier.com/locate/cattod Catalysis Today 105 (2005) 17–35 * Tel.: +49 3084134853; fax: +49 3084134701. E-mail address: stampflc@fhi-berlin.mpg.de. 0920-5861/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.cattod.2005.04.015