Ab initio determination of structural stability in fcc-based transition-metal alloys

Analysis of Au-Pd driving forces via the effective site energy model: LRO, antisites and enthalpy of permutation

Recently we have developed a new energetic model based on the determination of the energies on each site of random solid solutions after relaxation as a function of both the local composition and the nominal concentration. It allows to determine the main thermodynamics driving forces of disordered alloys. Here, we extend the effective site energy model to ordered alloys and illustrate the results for the AucPd1-csystem. As a first step, we show the ability of this energetic model to reproduce the hierarchy of ordered phases. Then, we derive general mean-field analytic formulae for ordered systems and get the phase diagram. We determine the relative role of the cohesive effect, the chemical effect and the size effect and find that the chemical effect differs significantly between the disordered state and the ordered state. Finally, we link the energy formation of antisite to the permutation enthalpy and give the driving forces for the formation of antisite.

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.