(472c) Thermodynamics From First Principles: Phase Diagramm And Thermodynamic Properties Of Oxygen Adsorbed On Ni(111)

Keil, F. J., Hamburg University of Technology
Lazo, C., Hamburg University of Technology

Abstract The interaction of oxygen (O) with metal surfaces is the basis for a number of important technological processes such as bulk oxidation, corrosion, and heterogeneous catalysis, and has thus been studied in great detail, both from a macroscopic and a microscopic point of view. In the present study we calculated the O/Ni(111) phase diagram and thermodynamic properties, like internal energy, specific heat, free energy, and entropy, based on first principles. Lateral interaction parameters were obtained from density functional theory (DFT, full potential LAPW/APW+lo)) calculations employing techniques from statistical mechanics, i.e. the cluster expansion method. The statistical mechanics problem was numerically solved using Metropolis Monte Carlo simulations in the canonical and grand canonical ensemble, using Kawasaki and Glauber sampling, respectively. To measure continuous order-disorder phase transitions in canonical MC simulations, order parameters are defined according to the symmetry broken during the transition. In order to locate first-order phase transitions, simulations in the grand canonical ensemble were carried out. Scans were performed at constant temperature and chemical potential. For first-order phase transitions simulations in the canonical ensemble can lead to wrong results due to finite size effects and phase coexistence. Additionally, the Wang-Landau Monte Carlo simulations in the canonical ensemble were used to study the nature of the phase transitions at various surface coverages. By shifting the acceptance rule from energy to entropy space, the Wang-Landau algorithm is able to surmount the problem posed by hysteresis at first-order phase transitions and critical slowing-down at continuous phase transitions.

Careful investigations were executed on the proper statistics for obtaining the interaction energy parameters of the lattice-gas Hamiltonian taken from plane-wave DFT calculations.

The experimental phase diagram and thermodynamic properties could quite well be simulated. All phases that have been experimentally determined are present in our simulated phase diagram. Additionally, we found a coexistence region of p(2x2) and (√3x(√3)R30° phases at very low temperatures. Limitations in the accuracy of the calculated temperatures will be discussed.