First-principles Calculations Of The Structural Phase Transition Of The High Pressure Vanadium

The same material has different crystal structures, the different crystal structures greatly impact on the the performances of the material, so the study of the structural phase transition and its mechanism is very important for many areas such as materials science, earth science and physical chemistry。This is the hotspot of recent experimental and theoretical study , such as the researchs of structural phase transitions of phase-change materials Ge2Sb2Te5 and GeTe,solid oxygen,magnetoelectric material BiFeO3,low-dimensional ion crystals and structural phase transition mechanism of Pu.The transition metal vanadium has recently been the subject of numerous experi -mental and theoretical studies due to the high superconducting transition temperature Tc . There are differences in the literatures on the study of structural phase transition of vanadium under high pressure. In this paper, structural phase transition of vanadium under high pressure has been calculated by use of the First principles on the basis of the density function theory. The sequence of structures of vanadium is bcc (body centered cubic)- rhombohedral-bcc with increasing pressure, the transition pressure is 70GPa and 380GPa separately. The main contents are the following:1,The ground-state geometry structure of vanadium has been optimized. Vanadium is bcc structure at normal pressure. The calculated equilibrium lattice constand and the isothermal bulk modulus at normal pressure agree well with experiment and theoretical works. The reduced volume is decreased and the degree of decreasing get smaller with the increase of pressure, this is qualitatively good with the experiment.2,What structure phase transition of bcc vanadium under high pressures has been calculated and studied. A mechanical instability in the shear elastic constant ( C 44) is found for bcc vanadium at about 95Gpa, which indicates the existence of the structural transition. By calculation and analysis, we found that there is no bcc-sc (simple cubic) structure transition but a bcc-rhombohedral structure transition at the 70GPa, which is consistent with the experiment data in Phys. Rev. Lett. 98, 085502. Our calculations also firstly give a rhombohedral-bcc structure transition at about 380GPa, which needs to be verified by the experiment.3,The reasons for bcc- rhombohedral structure transition of vanadium under high pressures has been discussed. The primary reason is the nesting properties of the Fermi surface in the 3rd band (intra-band nesting), s, p-d transition and band Jahn-Teller effects are also reasons. Electrons in s, p, and d shells of vanadium at different pressures have been calculated. As a function of compression an s, p-d ttransition shifts the Fermi level beyond the region of electronic instability, thus destabilizing the bcc phase in vanadium. We firstly gave the band structures of vanadium in rhombohedral lattice under high pressures, the results display band Jahn-Teller effects or the initially triple -degenerateΓ25, splitting. One split level goes down below the Fermi level causing the DOS at the Fermi level to decrease when the pressure attaining 120Gpa. This may provide essential information for proper reevaluation of the Tc anomaly at 120Gpa.