Thermodynamic properties of the C60 fullerite at high temperatures: Calculations taking into account the intramolecular degrees of freedom and strong anharmonicity of the lattice vibrations

Compressibility and polymorphism of α-As(4)S(4) realgar under high pressure

The energy-dispersive x-ray diffraction technique has been employed to study the structure and equation of state of realgar As(4)S(4) under pressures up to 8 GPa at room temperature. We have obtained pressure dependences of the unit cell parameters and volume for the monoclinic structure of realgar. An approximation of the equation of state through the Murnaghan equation gives the bulk modulus and its derivative, B(0) = 8.1 ± 0.5 GPa and B(0)(') = 9.0 ± 0.5, respectively. A comparison of the obtained values with the corresponding values for other molecular crystals is drawn and discussed. At a pressure of around 7 GPa, realgar showed a polymorph transition to a new molecular phase with a supposedly orthorhombic structure. Such identification is evidenced by the presence of geometrical correlations between the parameters of the parent monoclinic phase and those of the new phase, and the phase transition is likely to be associated with the removal of a monoclinic distortion in the unit cell.

Mechanical properties of solid C(60) studied with density functional tight binding method augmented by an empirical dispersion term

The bulk modulus of solid fcc C(60) was calculated with a density functional tight binding method, augmented by an empirical van der Waals force. The predicted modulus of 9.1 GPa is in good agreement with the experimental measurements. We found that the geometric structures of C(60) molecules in the solid fcc phase are considerably changed under strong compression, due to variations in the type of hybridization of carbon atoms. We also observed a reduction of the HOMO-LUMO gap under compression which is attributed to the overlap of π orbitals from neighboring C(60) molecules. Finally, we showed that the dispersion energy correction in the adopted scheme plays an essential role for the quantitative description of the weakly bound C(60) solid.

A general perturbation approach for equation of state development: Applications to simple fluids, ab initio potentials, and fullerenes

A new perturbation scheme based on the Barker-Henderson perturbation theory [J. Chem. Phys. 47, 4714 (1967)] is proposed to predict the thermodynamic properties of spherical molecules. Accurate predictions of second virial coefficients and vapor-liquid coexistence properties are obtained for a large variety of potential functions (square well, Yukawa, Sutherland, Lennard-Jones, Buckingham, Girifalco). New Gibbs ensemble Monte Carlo simulations of the generalized exp-m Buckingham potential are reported. An extension of the perturbation approach to mixtures is proposed, and excellent predictions of vapor-liquid equilibria are obtained for Lennard-Jones mixtures. The perturbation scheme can be applied to complex potential functions fitted to ab initio data to predict the properties of real molecules such as neon. The new approach can also be used as an auxiliary tool in molecular simulation studies, to efficiently optimize an intermolecular potential on macroscopic properties or match force fields based on different potential functions.

Thermal Vacancy Effects on the Thermodynamic Properties and Stability of Fullerites

We study the influence of thermal vacancies on equilibrium thermodynamic properties of the high-temperature phase of fullerites taking into account the strong anharmonicity of the lattice vibrations. Treating a crystal with point defects as a quasi-multicomponent system and using the correlative method of the unsymmetrized self-consistent field and the Girifalco interaction potential for the molecular subsystem, we have obtained the vacancy formation parameters for the C(60) fullerite. We also take into account the divacancies. The influence of the lattice defects on the specific heats of fullerites is negligible, since a dominant contribution to them is given by intramolecular degrees of freedom. We have calculated the contributions of vacancies to various thermodynamic properties, the volume thermal expansion coefficient, the isothermal bulk modulus, and the components of the isothermal elastic tensor, depending on the temperature and pressure. Near the estimated triple point, these contributions run to more than 10% and increase still further at a metastable region. We also discuss the influence of defects on the thermodynamic stability of fullerites.