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Equation of state and melting temperature for N2, CO, CO2 and N2O molecular cryocrystals under pressure

The equation of state, the absolute stability temperature of crystalline state and the melting temperature for N2, CO, CO2 and N2O molecular cryocrystals under pressure are determined using the statistical moment method and are compared with the experimental data and other calculations. | JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci. 2014 Vol. 59 No. 7 pp. 67-75 This paper is available online at http stdb.hnue.edu.vn EQUATION OF STATE AND MELTING TEMPERATURE FOR N2 CO CO2 AND N2 O MOLECULAR CRYOCRYSTALS UNDER PRESSURE Nguyen Quang Hoc1 Dinh Quang Vinh1 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 equation of state the absolute stability temperature of crystalline state and the melting temperature for N2 CO CO2 and N2 O molecular cryocrystals under pressure are determined using the statistical moment method and are compared with the experimental data and other calculations. Keywords Molecular cryocrystal statistical moment method limiting temperature absolute stability. 1. Introduction Molecular crystals are characterized by their 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 the physical properties of such systems as well as the formation of various crystalline phases. In the most cases the melting temperature of crystals is described by the empirical Simon equation ln P a c ln T b where a b and c are constant and P and T respectively are the melting pressure and the melting temperature 1 . However this equation cannot be used for crystals at extremely high pressure. On the theoretical side in order to determine the melting temperature we must use the equilibrium condition of the liquid and solid phases. However a clear expression of the melting temperature has not yet been obtained in this way. Notice that the limiting temperature of absolute stability for the crystalline state at a determined pressure is not far from the melting temperature. Therefore some researchers had identified the melting curve with the curve of absolute stability for the crystalline state.