High pressure structural transitions in Kr clathrate-like clusters

Abstract

Classical thermodynamic properties of a size-specific Kr clathrate-like cluster, modeled by an ab initio and a semiempirical Kr–water potential, were computed from parallel-tempering Monte Carlo simulations. The temperature and pressure dependence of the cluster’s heat capacity was studied, and phase diagrams were constructed using a multiple-histogram method. By associating the heat capacity (maxima) and the Pearson correlation coefficient (minima/maxima) values with the phase transitions, attempts were made to identify such changes to particular cluster structures. Various isomers were computed by local optimizations of the interaction enthalpy at a set of randomly selected configurations at each temperature–pressure grid point. Their energy distributions and relative abundances were then employed to assign the observed phase transitions of the cluster. It was found that the structural changes at high pressures are related to Kr clathrate-like cages, such as those of the sI, sII, and sH hydrates, as well as a new one at higher pressures, formed by tetragons and pentagons. Such transitions, at low temperatures and as pressure increases, are related to the topology of the intermolecular interactions, that are becoming accessible by the sampling in the MC simulations, through the employed volume model.