Dynamical stability and vibrational properties of Pt clusters

Abstract

While cohesive and structural properties of Pt clusters have been widely investigated, there is scarce knowledge on their vibrational properties. In this work we investigate, by ab initio methods, the vibrational properties of Ptn (n = 13, 19, 55, 79) clusters. We pay particular attention to the structural stability of the typical octahedral (Oh) and icosahedral (Ih) symmetric Pt13 clusters. Their vibrational density of states (νDOS) reveal that these geometries are magnetically unstable, while these isomers are metastable in their non-magnetic states. By performing finite-temperature molecular dynamic runs, we find that Oh and Ih clusters evolve towards layered lower energy structures. A new lowest energy isomer is thus predicted. For these Pt13 layered clusters, and for Oh bulk type Ptn (n = 19, 55, 79) clusters, the νDOS and heat capacity are calculated and compared to their bulk counterparts. For the Pt13 layered clusters we inspect and interpret, in terms of bond-order analysis, the vibrational modes for the enhanced low- and high-frequency ends of the cluster's νDOS. We identify mostly shear, tangential type displacements for their lowest frequency modes, and mostly longitudinal, intralayer antiphase displacements for the highest frequency modes of the νDOS.