Electric Transport Properties And Metallization Of VO₂under High Pressure

VO2is a kind of semiconductor whose physical and chemical properties are very sensitive to temperatures. It undergoes a structural phase transition from monoclinic structure to tetragnal structure at68℃, which simultaneously is accompanied with a temperature-induced metallization. Taking advantage of the big difference in the optical and electrical properties between before and after phase transition, VO2has been widely applied in various technical fields including solar temperature-controlled materials,infrared pulsed laser radiation protective film, photoelectric switches, and lithium electrode. Relevant studies on the physical properties of VO2have also been conducted. But up till now, the study on the physical properties of VO2under the extreme condition of high pressure has seldom reported. So, in this thesis, we have put our main focus on the systematic research of electrical transport properties of VO2under high pressure, including the study of the pressure dependent resistivity, the Hall effect and the energy band structure of VO2under high pressure, and have obtained the results as follow:(1) With the DAC based in-situ electrical resistivity measurement, we have found a discontinuous change in the resistivity at12GPa. It is shown by the temperature dependent resistivity measurement that below12GPa the electrical properties of VO2are of semiconductor as the resistivity decreases with temperature increase, however, above12GPa VO2becomes a metal because the resistivity increases with the temperature increase. Together with the information of activation energy curve, we consider that the discontinuity of VO2resistivity at12GPa results from the pressure induced metallization.(2) With the in-situ Hall effect measurement, we have investigated the pressure dependence of charge carrier concentration, Hall coeficient and mobility of VO2, and abnormal changes could be found at12GPa, which is also caused be the pressure induced metallization of VO2.(3) Meanwhile, we have conducted first-principle calculation on the energy band structure of VO2under high pressure. The theoretical calculation has suggested that the energy band gap width would become narrow as increasing the pressure.