Vibrational and thermodynamic properties of Pt clusters: an ab initio study

A. MALDONADO; G. F. CABEZA; S.B. RAMOS

Santa Fé

Conferencia; VI San Luis Conference on Surfaces, Interfaces and Catalysis; 2018

Universidad Nacional del Litoral

Nanoclusters represent a new state of matter, with properties being sensitive to their size and geometry, and lying intermediate between those of the isolated atoms and the bulk. In particular, the study and understanding of the structure and properties of transition metal nanoclusters are of current interest due to their important applications in catalysis and magnetic storage. In this work we investigate through ab initio modelling methods, the structural, cohesive, magnetic and vibrational properties of Ptn (n = 2, 4, 13, 19, 55, 79, 85 and 147) clusters, analyzing how these properties evolve towards the typical solid behavior. For most of the clusters we considered the octahedral and icoasahedral geometries as initial configurations. We observe that as the size of the cluster increases, the cohesive energy increases and the magnetic moment reduces, in correlation with an increase in the average interatomic distance. Concerning the vibrational properties, less information is available from the bibliography. For the particular Pt13 cluster case, our study of its vibrational properties for both Oh and Ih symmetric configurations, led us to predict that these geometries are unstable at 0 K; whereas the less symmetric double layer configuration proposed by Wang et al. [1] possess a vibrational spectrum compatible with a stable configuration. Starting from the initial Pt13 Oh cluster and taking into account thermal effects explicitly by performing molecular dynamic runs (MD) at finite temperatures, it is observed that in the first part of the MD simulation, the cluster keeps vibrating around the Oh geometry in a non-magnetic state. Note that the Pt13 Oh cluster has a non-zero magnetic moment at T = 0 K. Then, at a certain moment of the MD run, the cluster experiences a transition towards non-symmetric configurations. For all the studied clusters, the vibrational density of states show a very different behavior compared to the bulk one, with a discrete frequency spectrum, which gives rise to deviations of the Debye model for the specific heat at constant volume and at low temperatures. We predict that the Debye temperatures of the clusters are lower than the bulk one.Referencias1. Wang, L. -L., Johnson, D. D. "Density functional study of structural trends for late-transition-metal 13-atom clusters", Phys. Rev. B, Vol. 75, 235405-10, 2007.