First-principle Investigation Of High-pressure Behavior Of Different Selenium Phases

In recent years, the main group VI elements have long been the subject of considerable interest. Its high pressure-induced phases transition have promoted many resistivity, diffraction, density, structure spectroscopy, optical, superconductivity, and theoretical studies. This is due to its various modification and important scientific and technical application as an important semiconductor. Selenium exists in amorphous form in solid state under the ambient condition.Y. Akahama et al. had studied the high pressure structures of crystal Selenium by X-ray diffraction. They reported their experiments of X-ray diffraction and investigated the crystal structure of Se under high pressures up to 150 GPa and observed five structural phase transitions, they are trigonal(Se-I), monoclinic(Se-II), Se-III,Se-IIII, Se-V respectively. But the pressure-induced phases transition sequence, the pressure regions of each phase stable exist and the accurate phase transition pressure point of Se are not clearly now.In this paper, we use the high pressure experimental data of selenium provided by Akahama, M. Kobayashi, Parthasarathy and Holzapfel et al to build crystal model of selenium in various phase in the Materials Studio7.0 package CASTEP module. Under different pressure in order to get the phase equilibrium of the lattice constant, we has carried on the structure optimization, the crystal phase structure optimization standard is: energy convergence condition for 5 x 10-6 e V; Atoms on the stress convergence at 0.01 e V/A; The internal stress of crystal convergence to 0.02 GPa.We use the first principle calculation based on Density Functional Theory, calculated the band structure of selenium in different crystal phase, the density of electronic states in different pressure points. It is concluded that the band-gaps of the crystal decreased and the bands were broadened under pressure, Under 0 GPa We calculated the bandwidth of hexagonal phase is 1.388 e V and it is a kind of semiconductor material, the bandwidth of the rings monoclinic crystal is 2.471 e V belongs to broadband gap semiconductor, and the bandwidth of the chains monoclinic crystal is 0 e V, it is metallic phase. We got the hexagonal crystal’s semi-metal transition pressure for 15 GPa, when the pressure up to 10 GPa chains monoclinic crystal has been finished the semi-metal transition.By comparing the every single atom average gibbs free energy of different crystal phases, along with the change of pressure to determine stability of different phases within 0~20 GPa. After study we found that the hexagonal structure as the most stable crystal phase, in the low pressure range, when the pressure up to 16.2 GPa hexagonal phase transition to ring monoclinic, and the chains monoclinic crystal phase transition pressure is 15.3 GPa, both the results of in good agreement with the experimental value, but the chains monoclinal structure change of pressure is more close to the experimental value, and when the pressure is higher than 7.5 GPa chains monoclinic crystal is more stabile than ring monoclinic, so here we think Se-II phase is chains monoclinal structure.