The Carrier Behavior In Phase Transitions Under High Pressure

High pressure method is very important in basic physics research. The structure and interior mutual actions will have special changes under high pressure, such as structure phase transition, electronic phase transition, metallized phase transition, superconductivity etc. Phase transitions under action of pressure present the most interest as they lead to sharp changes both of crystal lattice and also of electron transportation. Carrier is the basic partical in electrical transportation process, and the changes of electrical transportation properties can be obtained through analyzing the changes of carrier behavior. Therefore, to know the the changes of electrical transportation properties in phase transition process, it is necessary to study the carrier behavior under high pressure. In this thesis, using DAC and integrated microcircuit, the structure and electrical properties of LiCr0.35Mn0.65O2 have been studied, and Hall effect of ZnX(X=S,Se,Te)has been investigated.Synchrotron X-ray diffraction showed that the rhombohedral phase of LiCr0.35Mn0.65O2 transforms to a cubic phase with space group Fd/3m at 5.83 GPa, and the LiCr 0.35Mn0.65O2 structure is a tetragonal unit cell with space group I41/amd after 20.35 GPa. Pressure decreases the ionic radius of Cr3+, which leads the Cr3+ ion can not limit the length of Mn-O bond. The spin configuration of the Mn3+ ion varies from low-spin to high-spin. At the same time, the atoms at the octahedral sites are unlike, which lowers the kinetic barriers involved in the redistribution of cations. These lead the structural phase transformation of LiCr0.35Mn0.65O2 occurs at 5.63 GPa. At every phase transition pressure, the discontinuous changes of activation enthalpy and conductivity have been found. The plot of temperature dependence of conductivity indicates that all the phases of LiCr0.35Mn0.65O2 have the semi-conducting character. The conductivity increases with pressure increasing in the whole pressure region, while the change mechanisms of conductivity are different in different phases. In the pressure range 1.10--5.36 GPa, the activation enthalpy increases with pressure increasing, which results in the carrier concentration decreasing, so that the conductivity should be reduced. In fact, the conductivity increases with pressure increasing in this range. This result indicates that the carrier scattering decreases with pressure, and the increase of conductivity induced by carrier scattering variation exceeds the decrease of conductivity induced by carrier concentration decrease, which finally leads to the increase of conductivity. In the pressure ranges 6.32--21.66 GPa and 22.60--26.22 GPa, the activation enthalpy decreases with pressure increasing, which has a positive contribution to the conductivity increase. Moreover, the increase of conductivity is mainly from the decrease of activation enthalpy.Conductivity is an important parameter to characterize electrochemical performance of the cathode materials. Our experimental results indicate that both pressure and temperature have positive contributions to the electrical conductivity increase, which is useful for raising the electrochemical performance of LiCr0.35Mn0.65O2. Although the increase of activation enthalpy is not helpful for increasing the conductivity in pressure range of 1.10--5.36 GPa, the interface feature has been improved, which is confirmed by carrier scattering decrease under compression. As the pressure is unloaded, the activation enthalpy will return to the value under ambient condition, while the interface improvement can be reserved. It can be concluded that to treat LiCr0.35Mn0.65O2 under high pressure, in fact, is an effective method to improve its electrochemical performance.The impedance spectroscopy of the Nyquist representation shows two overlapped arcs in the complex impedance plane which correspond to the electrical transportation of grain and grain boundary respectively under different pressures. Approximately at 5.63GPa and 19.63GPa, both the resistance and the relaxation frequency of grain and grain boundary change discontinuously, corresponding to the phase transitions of LiCr0.35Mn0.65O2. In the lower pressure region, the grain boundary resistance is higher than grain resistance, while it is lower than that after 22.60 GPa, which indicates that the pressure changes the dominant region of charge carrier transportation from the grain boundary to the grain. In the grain region, the transportation property changes are reversible under pressure; while in the grain boundary region, the Pressure induces the variation of the type and amount of defects in the grain boundary region, which is irreversible. These finally leads that the sample resistance in the decompression process is much lower than that in the compression process. Pressure increases the resistance activation energy and the carrier concentration, which finally leads to the decrease of resistance. Pressure decreases the frequency activation energy and the relaxation energy barrier, and increases the relaxation frequency of grain and grain boundary. In the whole region, dERGR/dp is smaller than dERGB/dp, which indicates that pressure has a larger effect on the transportation properties of grain boundary than that of grain.The Hall effect of ZnTe showed that Hall coefficient, carrier concentration and mobility change discontinuously at 9.73 and 12.45 GPa, corresponding to the phase transitions of ZnTe from zinc blende to cinnabar then to Cmcm structure. The carrier concentration and mobility change discontinuously at 14.36 GPa, which is maybe related to the occurrence of unknown phase. At 6.59 GPa, the deep-to-shallow transition of acceptor levels results in the increase of the charge carrier concentration by three orders of magnitude. The decrease of the deep acceptor ionization energy leads to the increase of the charge carrier concentration of zinc blende and cinnabar structure with pressure.The conductivity increases with pressure increasing in the whole pressure region, while the change mechanisms of conductivity are different in different phases. Before 6.59 GPa, the conductivity increase is from the increases of carrier concentration and mobility, and the effect of mobility variation on conductivity is larger than that of carrier concentration. In the pressure region of 6.59-9.73 GPa, the conductivity increase is from the increase of carrier concentration. In the cinnabar and Cmcm phases, the carrier concentration increase leads t the conductivity increase.The theoretic result showed that in the Cmcm phase of ZnTe, the conduction-band and valence-band are overlapped, which would exhibit electron-like conductivity. However, our experimental result indicates that the Cmcm phase has hole-like conductivity. The theoretic calculation bases on the perfect single crystal; while the sample used in our experiment is polycrystalline, which has numbers of acceptor defects, the acceptors are ionized at room temperature, and finally leads to the number of hole is larger than that of electron and the hole transportation is dominant.The Hall effect of ZnSe showed that all the parameters change discontinuously at the transition region from zinc blende to rock salt phase. The conductivity increase of rock salt structure is mainly from the carrier concentration increase. According to the carrier concentration and mobility variation trends, the conductivity increase of zinc blende structure is also mainly from the carrier concentration increase. Hall coefficient changes from negative to positive at 12.32 GPa, which indicates that the dominant carrier changes from electron to hole, then Hall coefficient returns to negative at 22.06 GPa, indicating the dominant carrier returns to electron.The Hall effect of ZnS showed that all the parameters change discontinuously at the transition region from zinc blende to rock salt phase. Hall coefficient changes from negative to positive at 20.71 GPa, which indicates that the dominant carrier changes from electron to hole. By compared the conductivity in compression process with that in decompression process, the phase transition of ZnS exhibits hysteresis character.In conclusion, using high pressure XRD, dc-conductivity and ac-impedance technologies, the structure and electrical properties of LiCr0.35Mn0.65O2 have been investigated, and the carrier behavior under high pressure ZnX (X=S, Se, Te) has been studied.