Does hydrogen pre - melt palladium clusters ?

We use molecular dynamics simulations to study the influence of an ambient atmosphere on the melting process in the icosahedral palladium clusters Pdl3, Pd55 and Pd147. Novel methods to capture the character of melting in finite systems are presented. The signature of melting in clusters heated by inert carrier gas collisions closely resembles that in systems heated in a Nos6-Hoover thermostat. Exposure to hydrogen instead of an inert carrier gas, however, significantly lowers the melting temperature. Recent studies of finite-sized systems have addressed the intriguing question of how the physical properties of atomic clusters evolve with their increasing size [ 1 ]. Equally interesting is to follow the signature and changing nature of bulk phenomena, such as phase transitions, as the size of the system decreases. Melting is a well-defined first-order phase transition in the bulk, which involves the loss of long-range order, a stepwise increase in the free energy and a singularity in the specific heat. The large portion of surface atoms in small systems is expected to change the nature of the transition, and the absence of long-range order to smear out the transition region over a finite temperature interval [2]. Early experiments on small gold aggregates reported a drastic decrease in the melting temperature with decreasing particle size [3]. Later experiments on metal particles have also observed structural instabilities [4] and surface pre-melting [5]. Recently, several authors investigated the atomistic origin and nature of this "phase transition" in homonuclear clusters [6-10]. In this Letter, we focus our interest on the effect of an ambient atmosphere on structural transitions in small Pd aggregates. We will study how the general thermodynamical behavior and melting transition are affected by the presence of hydrogen which modifies the metal-metal interaction. The key question is whether the presence of adsorbates could drive a "chemical" melting transition. To address these issues, we have to revisit two basic topics in finite systems, namely the temperature control and the characterization of melting in both a thermodynamical and structural fashion. The systems we investigate are the icosahedral clusters Pdl3, Pd55 and Pd147. We describe the rehybridiza-tion of Pd orbitals in clusters exposed to hydrogen using the many-body alloy (MBA) potential [1 1,12]. This potential consists of a many-body binding term, based on the second-moment of the electronic density of states, and pairwise repulsive Born-Mayer interactions. Our parameterization, based on ab initio local density functional results for bulk …