Molecular dynamics simulation study of the melting and structural evolution of bimetallic Pd-Pt nanowires

Thermal characteristics of Pd-Pt metal nanowires with diameters ranging from 2.3 to 3.5 nm and of several compositions were studied by molecular dynamics simulations utilizing the quantum Sutton-Chen potential function. Monte Carlo simulations employing bond order simulation model were used to generate the initial wire configurations that consisted of surface segregated structures. Melting temperatures were estimated based on variations in thermodynamic properties such as potential energy and specific heat capacity. We find that the melting transition temperatures for the nanowires are much lower than those of bulk alloys of the same composition and at least 100-200 K higher than those of nanoclusters of the same diameter. Density distributions along the nanowire cross section and axis as well as components of shell-based diffusion coefficients and velocity autocorrelation functions were used to investigate the melting mechanism in these nanowires. Our findings indicate a surface-initiated melting process characterized by predominantly larger cross-sectional movement. This two-dimensional surface melting mechanism in nanowires differs from that in nanoclusters in which atomic movement is more isotropic in all three dimensions. Differences in the surface melting mechanism result in structural transformations from fcc-hcp type and lead to simulated phase boundaries for nanowires that are different from bulk alloys as well as from same-diameter nanoclusters. A composition and temperature dependent fcc-hcp transformation occurs prior to the melting transition in both nanowires and nanoclusters. Hcp phase occurs over a wider temperature range at Pd-rich compositions and a narrower range at low Pd compositions with the fcc-hcp and hcp-liquid transition temperatures showing a minimum at 25% Pt composition. In contrast, the nanoclusters exhibit a near-linear dependence of melting temperature on Pd composition with the hcp phase existing over a much narrower range of temperatures, closer to the melting transition. Thermal stability of the solid phases of these nanowires was investigated by simulating two alternative starting configurations such as a hypothetical hcp and an annealed-solid structure for two compositions. The size and composition dependence of nanowire melting temperatures are consistent with those predicted by available melting theories.