Calculation of the Raman frequencies and thermodynamic quantities at high pressures for the cubic gauche nitrogen (cg-N)

Abstract

This study gives the calculation of the frequencies from the V-P Vinet EOS which has been modified as the frequency-pressure (υ-P) relation for the spectroscopic parameters in the cubic gauche nitrogen(cg-N). Calculation of the Raman frequencies is performed by means of the mode Grüneisen parameter as a function of pressure for the cg-N phase. From the Raman frequencies calculated, the pressure dependence of the thermodynamic quantities such as the free energy, entropy, heat capacity and the enthalpy are predicted by using the quasi-harmonic approximation for the cg-N structure. Our results show that the υ-P relation derived from the V-P relation (Vinet-EOS) can be used to predict the frequencies and that the quasi-harmonic approximation is satisfactory to predict the thermodynamic quantities of interest, as exemplified for the cg-N structure in this study.