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

ABEL. S. MALDONADO; SUSANA B. RAMOS; GABRIELA F. CABEZA

Santa Fe

Conferencia; VI San Luis Conference; 2018

INCAPE-UNL

Nanoclustersrepresent a new state of matter, with properties being sensitive to their sizeand geometry, and lying intermediate between those of the isolated atoms andthe bulk. In particular, the study and understanding of the structure andproperties of transition metal nanoclusters are of current interest due totheir important applications in catalysis and magnetic storage.                 In this work we investigatethrough ab initio modelling methods,the structural, cohesive, magnetic and vibrational properties of Ptn(n = 2, 4, 13, 19, 55, 79, 85 y 147) clusters, analyzing how these propertiesevolve towards the typical solid behavior. For most of the clusters weconsidered the octahedral and icoasahedral geometries as initialconfigurations. We observe that as the size of the cluster increases, thecohesive energy increases and the magnetic moment reduces, in correlation withan increase in the average interatomic distance. Concerning the vibrationalproperties, less information is available from the bibliography. For theparticular Pt13 cluster case, our study of its vibrationalproperties for both Oh and Ih symmetric configurations, led us to predict thatthese geometries are unstable at 0 K; whereas the less symmetric double layerconfiguration proposed by Wang et al.1 possess a vibrationalspectrum compatible with a stable configuration. Starting from the initial Pt13Oh cluster and taking into account thermal effects explicitly by performingmolecular dynamic runs (MD) at finite temperatures, it is observed that in thefirst part of the MD simulation, the cluster keeps vibrating around the Ohgeometry in a non-magnetic state. Note that the Pt13 Oh cluster hasa 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. Forall the studied clusters, the vibrational density of states show a verydifferent behavior compared to the bulk one, with a discrete frequencyspectrum, which gives rise to deviations of the Debye model for the specificheat at constant volume and at low temperatures. We predict that the Debyetemperatures of the clusters are lower than the bulk one.  References 1.       Wang, L. -L., Johnson, D.D. ?Density functional study of structural trends forlate-transition-metal 13-atom clusters?, Phys. Rev. B, Vol. 75, 235405-10, 2007.