Rare-gas solids under pressure: A path-integral Monte Carlo simulation

Rare-gas solids (Ne, Ar, Kr, and Xe) under hydrostatic pressure up to 30 kbar have been studied by path-integral Monte Carlo simulations in the isothermal-isobaric ensemble. Results of these simulations have been compared with available experimental data and with those obtained from a quasiharmonic approximation (QHA). This comparison allows us to quantify the overall anharmonicity of the lattice vibrations and its influence on several structural and thermodynamic properties of rare-gas solids. The vibrational energy increases with pressure, but this increase is slower than that of the elastic energy, which dominates at high pressures. In the PIMC simulations, the vibrational kinetic energy is found to be larger than the corresponding potential energy, and the relative difference between both energies decreases as the applied pressure is raised. The accuracy of the QHA increases for rising pressure.Comment: 9 pages, 6 figure

Topological characterization of crystalline ice structures from coordination sequences

Topological properties of crystalline ice structures are studied by considering ring statistics, coordination sequences, and topological density of different ice phases. The coordination sequences (number of sites at topological distance k from a reference site) have been obtained by direct enumeration until at least 40 coordination spheres for different ice polymorphs. This allows us to study the asymptotic behavior of the mean number of sites in the k-th shell, M_k, for high values of k: M_k ~ a k^2, a being a structure-dependent parameter. Small departures from a strict parabolic dependence have been studied by considering first and second differences of the series {M_k} for each structure. The parameter a ranges from 2.00 for ice VI to 4.27 for ice XII, and is used to define a topological density for these solid phases of water. Correlations between such topological density and the actual volume of ice phases are discussed. Ices Ih and Ic are found to depart from the general trend in this correlation due to the large void space in their structures.Comment: 10 pages, 7 figures, 3 table