Mechanical and thermodynamic properties of Co2 and N2O molecular cryocrystals under pressure

Molecular crystals are characterized by strong intramolecular forces and much weaker intermolecular forces. High-pressure spectroscopic studies provide useful data for refining the various model potentials which are used to predict of the physical properties of such systems as well as for the formation of various crystalline phases.
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Mechanical and thermodynamic properties of Co2 and N2O molecular cryocrystals under pressure JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 119-125 This paper is available online at http://stdb.hnue.edu.vn MECHANICAL AND THERMODYNAMIC PROPERTIES OF CO2 AND N2 O MOLECULAR CRYOCRYSTALS UNDER PRESSURE Nguyen Quang Hoc1 , Bui Duc Tinh1 and Nguyen Duc Hien2 1 Faculty of Physics, Hanoi National University of Education 2 Mac Dinh Chi Secondary School, Chu Pah District, Gia Lai Province Abstract. The mechanical and thermodynamic properties (such as the nearest neighbor distance, the molar volume, the adiabatic and isothermal compressibilities, the thermal expansion coefficient and the heat capacities at constant volume and at constant pressure) of molecular cryocrystals of many atoms with a face-centered cubic structure such as α-CO2 , α-N2 O, at various temperatures and at pressures up to 10 GPa are investigated using the statistical moment method (SMM) in statistical mechanics and compared with the experimental data. Keywords: Molecular cryocrystal, statistical moment method. 1. Introduction Molecular crystals are characterized by strong intramolecular forces and much weaker intermolecular forces. High-pressure spectroscopic studies provide useful data for refining the various model potentials which are used to predict of the physical properties of such systems as well as for the formation of various crystalline phases. CO2 is an important volatile component of the earth as well as other planets in the solar system. Its high-pressure behavior is therefore of fundamental importance in planetary science. CO2 is one of the model systems involving the π bonding and the hybridization properties of the carbon atom, which are strongly affected by high pressure conditions. Pressure-induced transitions from molecular to nonmolecular CO2 crystals are systematically investigated using first-principle lattice dynamics calculation. Geometrically, likely transition pathways are derived from the dynamical instability of the molecular crystals under high pressures. The phase diagram of CO2 consists of 5 phases. CO2 -I phase or phase α, known as dry ice) has the face-centered cubic P a3 structure. CO2 -II has the P 42 /mnm symmetry. Received August 20, 2014. Accepted October 1, 2014. Contact Nguyen Quang Hoc, e-mail address: hocnq@hnue.edu.vn 119 Nguyen Quang Hoc, Bui Duc Tinh and Nguyen Duc Hien CO2 -III has the orthorhombic Cmca symmetry. CO2 -IV has P bcn symmetry. CO2 -V is the polymeric phase of a tridymite-like structure. In [1], Bonev et al. performed a series of first-principle calculations, including full structural optimizations, phonon spectra and free energies, in order to study the stability and properties of the phases proposed experimentally up to 50 GPa and 1500 K. The DFT calculations were carried out within the Perdew-Burke-Ernzerhof generalized gradient approximation (CGA) [2] using the ABINIT code which implements plane-wave basis sets. Le Sar et al. [3] presented an ab initio method, based on the modified Gordon-Kim (MGK) electron-gas model which worked well in calculating the structure and properties of molecular crystals. A constant pressure Monte Carlo formalism, lattice dynamics and classical perturbation theory are used to calculate the thermal expansion, the pressure-volume relation at room temperature, the temperature dependence of zone center libron frequencies and the pressure dependence of the three vibron modes of vibration in solid CO2 at pressures 0 ≤ p ≤ 16 GPa and temperatures 0 ≤ T ≤ 300 K [4]. Properties of solid N2 O at pressures p ≤ 15Gpa and at T = 0 and 300 K have been calculated using energy optimization and Monte Carlo methods in an (N, p, T ) ensemble with periodic, deformable boundary conditions and lattice dynamics. α-N2 O is consistent with the known low-pressure low-temperature ordered cubic form, space group P a3, up to 4.8 GPa where transition to a new solid occurs [5]. Cryocrystals N2 O and CO2 are ideal systems on which to have a study of the influence of quantum effects on condensed matter. There has been considerable interest in structural and thermodynamic properties of these crystals under temperature and pressure and in line with this general interest and encouraged by the essential success of our calculations, we tried to consider the mechanical and thermodynamic properties of cryocrystals of many atoms with face-centered cubic structure such as α-N2 O, α-CO2 at various temperatures and pressures up to 10 GPa. Heat capacities at constant volume for these crystals are studied by co ...