Symmetry transformation in Pd quasicrystals upon heating and hydrogenation.

In this work, the structural transformation from a crystalline to quasicrystalline symmetry in palladium (Pd) and palladium-hydrogen (Pd-H) atomic clusters upon thermal annealing and hydrogenation has been addressed by means of atomistic simulations. A structural analysis of the clusters was performed during the heating up to the melting point to identify the temperature for the phase transformation. It has been demonstrated that nanometric pure Pd clusters transform from cuboctahedral to icosahedral structures under heating. This transformation is thermally activated process and the activation barrier depends on the cluster size. The activation energy of the cubo-ico symmetry transformation was measured using the variable heating rate method and was found to increase with the cluster size from 0.05 eV for 55 atomic cluster up to 0.66 eV for 147 atomic cluster. Hydrogenation of the nanometric Pd clusters yields to the modification of the transformation barrier in a non-monotonic form. At low H concentration, the transformation barrier decreases, while by increasing H concentration above a certain threshold, the barrier grows again thus making a minimum around a specific hydrogen concentration. This behaviour was rationalized as a competition between two processes, namely: the structure symmetry breaking at low H concentrations and stabilization of cuboctahedral phase of the clusters at high H concentration. The obtained results provide an estimation of the temperature range at which the symmetry transformation should occur under thermal annealing with experimentally achievable heating rates. [ABSTRACT FROM AUTHOR]