Computational study of order-disorder transitions in alloy clusters using the isothermal-isobaric ensemble - art. no. 033203

The low-temperature order-disorder transition for a Pd7Ni6 alloy cluster wa s considered using the isothermal-isobaric (NPT) ensemble. The ordered stru cture consists of a completely segregated arrangement of atoms and the diso rdered systems present a certain degree of mixing within the cluster. The t ransition was characterized by monitoring anomalies in the average value of the constant pressure heat capacity, [C-p], as a function of temperature. The maximum temperature in the [C-p] versus T graph, T-mixing, is used for estimating the equilibrium temperature at which the transition occurs, at a given pressure. It is observed that as the pressure increases, T-mixing de creases up to a value of 25 K, where the mixing transition becomes temperat ure-independent. The sampling difficulties presented in standard Monte Carl o simulations are circumvented by implementing the J-walking procedure to t he NPT ensemble.