The Melting Properties For Mo Under High Pressure

Many transition metals have demonstrated the complex physical properties and behaviors due to their electronic structure features (such as the partially filled d,f shells electrons, various valences etc.), exemplified particularly by molybdenum(Mo). It’s high-pressure melting temperature has been controversial in theory and experiment, for example the melting temperature for Mo of diamond anvil cell(D AC) measured, in which the dTm/dP is almost zero, was much lower than that of shock wave experiment results, This dissertation is devoted to the study of the melting properties of Mo under high pressure by the ab initio molecular dynamics.In order to determine the stable structure of solid phase at zero temperature, the enthalpies for different crystal structures(bcc, fcc,hcp) of OK were calculated. The results show that bcc Mo has the lowest enthalpy within600GPa, indicating that Mo maintain a bcc phase below600GPa, which is consistent with the experimental data at room temperature. The equilibrium volume and bulk modulus were obtained by fitting Murnagahan isothermal equation of state (EOS),the results are in good agreement with experiments.We investigated the melting behaviors of Mo by molecular dynamics method, in which the main three factors were taken into account. First, the different melting mechanisms should be used different methods, e.g. melting comes of inside the solid or solid surface. Secondly, the effect of solid-solid phase transition on melting should be considered since the shock wave experiments of measuring the sound speed suggested that solid-solid phase transition may occur before melting at high pressure. Although there has not been a final conclusion, the investigation of solid structure effect on melting was meaningful. Thirdly, the change in liquid short range order should also be studied. Large number of experiments had found that the liquid has special structure, which is not necessarily similar to solid structure, including nearest-neighbor coordination number.Based on the system of128atoms with the initial bcc structure configuration, and different volume and temperatures states were calculated by the single-phase method in the NVT ensemble, then fitted the melting temperatures to the melting curve. The results showed that the melting temperature was higher than that of DAC experiment, close to that of the impact experiment. The "Z " method was used to simulate the melting, the results showed the melting points was about10%-20%lower than the melting points of overheating, and the latter agree with the simulated melting temperatures of the single phase method. Taking into account solid-solid phase transition before melting, the initial configuration was changed to fee structure, the melting temperature decreased significantly compared with the results of the bec structure simulations.The short range order of liquid may change after melting, experimental results showed that the liquid structure of Mo may become similar to like-fcc structure, rather than close to normal bcc structure. Thus bcc structure solid and like-fcc structure liquid were formed into the two-phase coexistence system, then the simulation of this system in NVE ensemble was performed, the results showed that the melting temperatures were also significantly reduced.Because the system scale of the first principle molecular dynamics simulation is small, solid-liquid coexistence simulation did not get the final equilibrium state of the solid-liquid coexistence. The classical molecular dynamics method was used to study the phase transformation and liquid-structure effects on the melting behavior, the force-matching method got the parameters of EAM potential of Mo. Single-phase method, Z method and two-phase coexistence method were used to simulate the melting. The results also showed that the transitions of solid structure and liquid structure impact the melting.Although different factors we considered affected the melting temperature, melting point was lower than the simulation results of single-phase method, which still could not explain the early DAC experiments. The recent DAC melting experiments of Ta found that chemical reaction between the sample and diamond as well as the pressure transmitting medium may occur under high temperature and high pressure, and this affected the temperature measurement. Thus the corrected melting temperature was significantly higher the original results, and closed to the impact experimental results.In order to obtain a large amount of equation of state data in engineering application, the mean field potential method based on first principle and Lindemann melting law for melting curve of Mo was also studied, and the calculated melting curve was consistent with that from first-principles molecular dynamics simulation of single-phase method based on bec structure. The other thermodynamic properties of Mo were also studied, including the normality properties and Hugoniot curves, which agree well with the experimental data. At the same time using Gray three-phase equation of state model to build the experience equation of state for Mo describing solid-liquid phase change.In summary, we thought the simulation results that consideration of phase transition and liquid structure were closer to the new experimental results, thus the further experiment are needed to provide some direct confirmation, and meanwhile the corresponding theoretical studies are also demanded.