The First-principle Calculation Of Physical Properties Under High Pressure

The multi-electron system can be converted to single-electron system by using density function theory (DFT). The lattice parameter, elastic constants and the bulk modulus of semiconductor and metal materials are calculated which are in agreement with that of experiment. It has become an important achievement of electron theory. The elastic constant is an important parameter to decide the intensity of materials, so obtaining the precise elastic constants has important significance in the engineer and studying of ground theory. NaI has the property of high intensive and uniform illumination, and therefore can be used as the high-temperature measuring standard materials of shocking wave. So the accurate theory calculation of properties of NaI under high pressure is necessary to understand the properties of the extreme conditions of high temperatures and pressures and theoretic explanation of the experiment phenomena. NaI has the stable face-centre-cubic (FCC) structure under lower pressures and has the phase transition under the domain (30-200 GPa) of high pressure. It transforms from the B1 structure to the stable orthogonal B33 structure which is different from NaCl (B1-B2). This paper studied the elastic and thermodynamic properties of NaI with different structures under different pressures which accords to the experimental results.The elastic modulus and sound velocity of NaI has been calculated by using the generalized gradient approximate (GGA) based on the first-principle calculation. The obtained elastic constants and lattice parameter accord with those of experimental results, demonstrating the validity of simulations. We also obtained the elastic constants at various temperature and pressure with the absence of experimental data, and compared them with simulation results of other methods. Due to the correlation of the elastic constants and the volume, the implication of temperature on that are exhibited by the influence of volume change on elasticity, so the finite- temperature elasticity can be obtained.By fitting the Vinet equation of second order associated with the quasi-harmonic Debye model, we obtained the thermodynamic properties of B1 and B33 structure of NaI and the heat capacity C_p of B1 structure which is according to that of experiment. Therefore we also calculated the iso-pressure heat capacity Cp and iso-volume heat capacity Cv of B1 and B33 structure of NaI under high temperature, the Gruneisen parameter was calcualted by the relation of the heat capacity and Debye temperature.