A Low Temperature Adiabatic Calorimeter for Condensed Substances. Thermodynamic Properties of Argon

Crystal growth rates in supercooled atomic liquid mixtures

Crystallization is a fundamental process in materials science, providing the primary route for the realization of a wide range of new materials. Crystallization rates are also considered to be useful probes of glass-forming ability^1-3. At the microscopic level, crystallization is described by the classical crystal nucleation and growth theories^4,5, yet in general solid formation is a far more complex process. In particular, the observation of apparently different crystal growth regimes in many binary liquid mixtures greatly challenges our understanding of crystallization^1,6-12. Here, we study by experiments, theory and computer simulations the crystallization of supercooled mixtures of argon and krypton, showing that crystal growth rates in these systems can be reconciled with existing crystal growth models only by explicitly accounting for the non-ideality of the mixtures. Our results highlight the importance of thermodynamic aspects in describing the crystal growth kinetics, providing a substantial step towards a more sophisticated theory of crystal growth.

Thermodynamic Properties of Molecular Crystals Calculated within the Quasi-Harmonic Approximation

A computational study of the possibilities of contemporary theoretical chemistry as regards calculated thermodynamic properties for molecular crystals from first-principles is presented. The study is performed for a testing set of 22 low-temperature crystalline phases whose properties such as densities of phonon states, isobaric heat capacities, and densities are computed as functions of temperature within the quasi-harmonic approximation. Electronic structure and lattice dynamics are treated by plane-wave based calculations with optPBE-vdW functional. Comparison of calculated results with reliable critically assessed experimental data is especially emphasized.

Quantumlike short-time behavior of a classical crystal

We have performed a molecular-dynamics simulation of a face-centered-cubic Lennard-Jones crystal, and studied its relaxation toward equilibrium and its microcanonical equilibrium dynamics through the computation of the normal modes. At low temperature, the weak interaction among normal modes yields a very slow relaxation of the fluctuation of the kinetic energy; this requires a new formulation of the measure of the microcanonical specific heat at constant volume. This specific heat turns out to depend on the time of observation; for times of the order of 20 ps, its values are much nearer to the quantum ones than to the value 3R predicted by the classical Dulong and Petit law. For longer observation times, the classical specific heat progressively approaches 3R over most of the temperature range of the solid crystal, with the exception of the lowest temperature range, where it still drops to values close to zero. The time dependence of the specific heat of the crystal is similar to the behavior found in a supercooled liquid near the glass transition.

The condensation of atmospheric gases on cold surfaces

Data for the deposition of argon, nitrogen and carbon dioxide on cold, metal surfaces are considered in detail. It is concluded that argon and nitrogen deposit when the incident gas pressure equals the sublimation pressure at the respective surface temperature, and growth therefore proceeds without any significant intermediate nucleation barrier. Carbon dioxide, however, requires considerable supersaturation of the gaseous phase and consequently bulk deposition is inhibited by a nucleation barrier. The results are analysed with a view to determining the critical nucleus size and the adsorption energy. The ‘classical’ method of analysis gives unsatisfactory and inconclusive results. In contrast, the ‘atomistic’ approach is found to give a good account of the critical deposition phenomenon. The onset of gross deposition is found to be due entirely to capture of single molecules by stable nuclei, rather than by the formation of critical nuclei as the ‘ classical ’ theory wrongly assumes. The number of molecules in the critical nucleus is found to be about nine and a value of 9.4 kJ mol -1 is obtained for the adsorption energy, suggesting that nucleation occurs on top of a strongly bound adsorbed layer of contaminants or carbon dioxide itself, rather than on bare metal.