Phase changes in Lennard-Jones mixed clusters with composition ArnXe6−n (n=,1,2)

Abstract

We have carried out parallel tempering Monte Carlo calculations on the binary six-atom mixed Lennard-Jones clusters, Ar(n)Xe(6-n) (n=0,1,2). We have looked at the classical configurational heat capacity C(V)(T) as a probe of phase behavior. All three clusters show a feature in the heat capacity in the region of 15-20 K. The Ar(2)Xe(4) cluster exhibits a further peak in the heat capacity near 7 K. We have also investigated dynamical properties of the Ar(2)Xe(4) cluster as a function of temperature using molecular dynamics. We report the interbasin isomerization rate and the bond fluctuation parameter obtained from these calculations. At 7 K, the isomerization rate is on the order of 0.01 ns(-1); at 20 K, the isomerization rate is greater than 10 ns(-1). Furthermore, at 7 K, the bond fluctuation parameter is less than 3%; at 20 K, it is in the range of 10-15% (depending on the sampling time used). Using this information, together with Monte Carlo quenching data, we assign the 15-20 K feature in the heat capacity to a solid-liquid phase change and the 7-K peak to a solid-solid phase change. We believe this is the smallest Lennard-Jones cluster system yet shown to exhibit solid-solid phase change behavior.

Melting of Lennard-Jones rare-gas clusters doped with a single impurity atom

“…Such peak has been associated with a solid-solid transition, and studied in detail for rare gas clusters of 6 25 and 13 [26][27][28][29][30] atoms. It has been suggested [25][26][27][28][29][30] that this bump is due to structural transitions between isomers of the same composition. We reach the same conclusion via an analysis of around 1000 structures, which we sampled, for each replica and each composition in clusters with up to 147 atoms.…”

A path-integral Monte Carlo study of a small cluster: The Ar trimer

“…Changes in quantities such as radial density functions, diffusion coefficients, total internal energy, and bond length distances as the temperature T increases were originally interpreted as indications of a transition from a solidlike state to a liquidlike form for ͑Rg͒ n systems. [3][4][5][6][7][8] Experimental works, on the other hand, have not provided conclusive evidences in support of the possibility of phase coexistence in such microclusters. [9][10][11][12][13][14][15][16][17][18] The onset of this nonrigid dynamics in Rg clusters has also been studied by identifying the solidlike cluster with a conventional molecule displaying a near-rigid behavior and the liquidlike form with a very nonrigid molecule which can explore the different energetically available potential energy minima.…”

First-order corrections to semiclassical Gaussian partition functions for clusters of atoms

“…The imaginary time or Boltzmann operator exp(−β Ĥ) belongs to the most important quantities needed for understanding the equilibrium properties of multi-dimensional systems at finite temperatures. The derivatives of its trace give access to the mean energy and specific heat, which allow for the investigation of thermodynamic properties of atomic clusters [1][2][3][4][5][6][7]. For systems with many degrees of freedom it is, however, still a challenge to evaluate the Boltzmann operator although it is accessible with Monte Carlo methods [8][9][10], since the necessary integrals can become numerically expensive, in particular at low temperatures.…”