DFT studies on the thermodynamic properties of Na2Ti3O7 under high temperature and high pressure

The equilibrium structure of the compound Na2Ti3O7 has been obtained via the minimization of the total energy within Local Density Approximation (LDA) based on Density Funcitonal Theory (DFT), the calculated equilibrium volume are in agreement with available experimental value. In the mean time, the thermodynamic properties under high temperature and high pressure are investigated applying nonempirical De-bye model combining with the first priniciple theory in the quasi-harmonic approximation. The evaluated equilibrium volume using this model agrees with the value obtained from ab intio and from experiment. The results demonstrate that this method can provide reliable predictions for the temperature and pressure dependence of these quantities such as the equation of state (EOS), the entropy, the isothermal bulk modulus, the heat capacity, and the thermal expansion in detail. In addition, the addition, the calculated thermodynamic properties are all in agreement with available experimental date.