Computational study of order-disorder transitions in alloy clusters using the isothermal-isobaric ensemble

Abstract

The low-temperature order-disorder transition for a Pd 7 Ni 6 alloy cluster was considered using the isothermal-isobaric ͑NPT͒ ensemble. The ordered structure consists of a completely segregated arrangement of atoms and the disordered systems present a certain degree of mixing within the cluster. The transition was characterized by monitoring anomalies in the average value of the constant pressure heat capacity, ͗C p ͘, as a function of temperature. The maximum temperature in the ͗C p ͘ versus T graph, T mixing , is used for estimating the equilibrium temperature at which the transition occurs, at a given pressure. It is observed that as the pressure increases, T mixing decreases up to a value of 25 K, where the mixing transition becomes temperatureindependent. The sampling difficulties presented in standard Monte Carlo simulations are circumvented by implementing the J-walking procedure to the NPT ensemble.

Molecular dynamics simulations of the meltinglike transition inLi13Na42andNa13Aguado1, López2 2005Phys. Rev. B472330View full textAdd to dashboardCite

“…In computer simulations of the thermal properties of heterogeneous clusters the quasiergodic hypothesis is thus at question, and several Monte Carlo ͑MC͒-based schemes have been developed in order to efficiently sample the configurational phase space available at each temperature and thus obtain meaningful averages for thermodynamic properties. [5][6][7] In molecular dynamics ͑MD͒ simulations, on the contrary, some of those solid-solid transformations will not be accessible for reasonable time lengths ͑of the order of nanoseconds͒, as different regions of phase space are essentially disconnected at low energies. Time averaging of microscopic properties obtained in an MD run may therefore pertain to the domain of kinetics for low energies, and a different sequence of transformations ͑as compared to MC runs͒ may be observed upon heating the cluster, depending on the dynamically accessible structures.…”

“…5,6 For example, if a segregated structure is the most stable at low temperatures, an entropically driven transition towards a structure with compositional disorder may be observed before melting. 7 The occurrence of solid-solid transformations will depend, however, on the time of observation, because these processes may be governed by very slow kinetics if the transformation pathways connecting different homotops involve the surmounting of large free energy barriers. This happens when the topography of the potential energy surface ͑PES͒ has a multiple-funnel character.…”

Composition-induced structural transitions in mixed rare-gas clusters

“…In particular, there has been a significant amount of work at the level of sophisticated electronic structure calculations, 1,2,3 but these were often limited to small sizes due to the numerical effort involved. On a different scale of chemical complexity, many studies have been carried out using explicit, empirical force fields 4,5,6,7,8,9,10,11,12 in order to investigate the segregation properties of these clusters.…”

Quantum effects in the solid–liquid phase diagram of Ne13 and (para-H2)13

“…1 From a computational point of view, one of the most addressed issues associated with clusters is the so-called cluster phase transitions ͑CPTs͒. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] It has been recognized that CPTs differ from phase transitions in bulk in that these transitions due not occur at a given temperature but rather in a temperature range where isomeric forms of the cluster with considerable difference in energy coexist. For example, the melting transition for Lennard-Jones clusters has been described [2][3][4][5] as an equilibrium between solidlike and liquidlike forms of the system.…”