8,322 research outputs found
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
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