Regensburg 2010 – wissenschaftliches Programm

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DY: Fachverband Dynamik und Statistische Physik

DY 3: Complex energy landscapes (addendum to SYEL)

DY 3.2: Vortrag

Montag, 22. März 2010, 14:15–14:30, H42

Phase diagrams of nanoparticles in various thermodynamic ensembles: Lattice-based Monte-Carlo Simulations of Pt-Rh — •Christian Stahl, Johan Pohl, and Karsten Albe — Institut für Materialwissenschaft, TU Darmstadt, Petersenstr. 32, D-64287 Darmstadt

The equilibrium phase diagrams of Pt-Rh nanoparticles, which represent a binary model system with order-disorder transitions, are investigated as a function of particle size. Lattice-based Monte-Carlo simulations in the canonical and semi-grand canonical ensemble are carried out using a refined bond-order simulation (BOS) mixing model. With decreasing particle size, order-disorder transitions shift towards lower temperatures and higher Pt concentration, while phase fields of ordered intermetallic phase broaden and the two-phase areas shrink, which is mainly due to Pt segregation to the surface. Furthermore, we observe that the order-disorder transition becomes of second order for small particles, which is reflected in the size-dependence of Warren-Cowley short range order parameters. Finally, we investigate the phase transition in the microcanonical ensemble using a novel simulation technique for direct sampling of the density of states and hence the entropy of the system. In the region of high energy that is not accessible in the canonical and semi-grand canonical ensemble, we observe decomposition of the components.