Structures And Properties Of Topological Compounds Under High Pressure

By the means of the in situ high-pressure synchrotron radiation X-ray diffractions and electrical resistance measurements under high pressure, we studied the high pressure structural phase transition and properties of V-VI family topological insulator compounds Bi₂Te₃, Sb₂Te₃ and Bi₂Se₃. We also conducted the structural phase transition research of BiTe under high pressure, which has the same elements and very similar structure with Bi₂Te₃. The main results of this thesis are included as follows:(1) The pressure phase diagram of Bi₂Te₃ was obtained by in situ high-pressure synchrotron radiation X-ray diffractions at room temperature and low temperature around 10K. We found a new high pressure phase which is named as Phase IV. The XRD pattern of Phase IV is simple and can be indexed into body-centered cubic unit cell in which Bi and Te atoms randomly shared the same atomic sites and formed a substitutinal alloy. The lattice parameters and atomic sites of Phase I and Phase IV were achieved by refining the patterns with Rietveld method using GSAS package. The unit cell volumes of Phase I and Phase IV under pressure were fitted with Birch-Murnaghan equation of states and the bulk modulus of Phase I and Phase IV are 35GPa and 51GPa respectively. The pressure of the phase transition from Phase I to Phase II at low temperature is higher than that at room temperature. The phase transition did not occur even if the pressure rised up to 10GPa at 8K. The measurements of temperature dependent of resistance under high pressure were conducted. Both p-type and n-type samples existed an electronic phase transition in the ambient structure phase under high pressure, where the p-type samples changed from insulator phase to semi-metal phase while the n-type samples changed from insulator phase to metal phase. Superconducting phases were found either before the electronic phase transition or after transition in both carrier-type samples. We studied the topological properties via first-principles calculation method. Both superconducting phase before and after electronic phase transition are topological non-trivial for p-type samples, however, only the phase before the transition remained topological non-trivial for n-type samples. We also discussed the possibility that the topological non-trivial states could be topological superconductors. All the high pressure phases were found to be superconductors at low temperature. (2) High pressure synchrotron radiation X-ray diffractions of Sb₂Te₃ at room temperature and low temperature were performed. Several structure phase transitions were observed and a pressure-temperature phase diagram was obtained. The measurements of resistance as a function of temperature at various pressures were carried out. An electronic phase transition was found at the ambient structure phase at the pressure of around 6 GPa, where the insulator phase at ambient pressure turned into a semi-metal phase. Both two electronic phases with ambient phase structure are superconductors. We discussed the topological properties of the two ambient structure superconducting phase. All the structure phases were found to be superconductors at higher pressure. The changing of superconducting critical temperature and resistance under pressure were perfectly in accordance with the phase diagram.(3) High pressure in situ synchrotron X-ray diffractions of Bi₂Se₃ was conducted and a structure phase transition occurred at 12GPa was found. No other high pressure phase was found at higher pressure up to 30GPa. An insulator-metal transition was found at the pressure range between 5GPa and 6.6GPa via high pressure resistance measurements. The transition temperature under high pressure is higher than that under low pressure.(4) The structure phase transition of BiTe was studied, which has the same elements and very similar structure with the topological insulator material Bi₂Te₃. Three high pressure phases was observed and the phase transition pressure was determined. The Phase IV is a bcc structure phase which is similar to the Bi₂Te₃, indicating the bcc substitutional alloy phase in Bi₂Te₃ is possible.