High-pressure Behaviours Of Ag₂Te And As₂Te₃

Metal chalcogenides have been focused for the excellent properties. For example,thermoelectric devices made by metal chalcogenides have been widely applied in many fields.; metal chalcogenides have been attracted significant interests as topological insulators. Considerable exotic quantum phenomena have been found in TIs such as quantum anomalous Hall effect and Majorana fermions. Metal chalcogenide has rich pressure-induced phase transition（structural phase transitions and electron phase transitions） information, which can bring new physical properties.Thermoelectric properties have been significantly improved by pressure-induced electronic topological transitions in metal chalcogenide; the topological superconductors can be realized under high pressure. Therefore, it is very significant to carry out the high-pressure research of the change rule in metal chalcogenide.Ag₂Te is a three-dimension topological insulator, and has abundant pressure-induced phase transition information. However, how to improve its topology character has been a problem in scientists; and the high-pressure phase transition sequence of Ag₂ Te is still controversial. Theory prediction suggests that a uniaxial strain induced β-As₂Te₃ transformed from a trivial insulator to a Weyl semimetal, and then to a topological insulator. Therefore, hydrostatic research may find its novel quantum physical states; and its thermoelectric performance undergoes two obvious change, which no one can give a satisfactory explanation.In this thesis, high-pressure phase transitions and physical properties for Ag₂ Te and As₂Te₃ were studied by synchrotron angle-dispersive X-ray diffraction measurements and in-situ high-pressure resistivity measurements, using a diamond-anvil cell, in conjunction with first-principles calculations. The results are listed as follow:1) The structural phase transition of topological insulator Ag₂ Te under high pressure has been systemically studied by in-situ high pressure synchrotron X-ray diffraction. The results suggest that Ag₂ Te undergoes twice structural phase transition around 2.2 GPa and 11.3 GPa. The accurate pressure-induced phase transition sequence is firstly determined as P21/c ® Cmca ® Pnma. It is worth noting that the reported isostructural P21/c phase is not existed, and the reported structure of Cmca phase is corrected by CALYPSO methodology. The second high-pressure structure, a long puzzle to previous reports, is assigned to Pnma phase. In addition, the pressure-induced phase transitions process has been analyzed. Due to the Ag₂ atoms undergo shear slipping along b axis, the distance of Te-Ag₂ decrease from 3.896（4） ? to 2.954（6） ?, which make the [Te Ag8]polyhedron change to the [Te Ag9] polyhedron. Then, under increasing pressure,the [Te Ag9] polyhedron undergo shear slipping when the Cmca phase tansforms to the Pnma phase.2) Here, a pressure-induced electronic topological transition is firstly found in Ag₂ Te at 1.8 GPa by in-situ high pressure synchrotron X-ray diffraction, in-situ high pressure electrical measurements and first-principle calculations. Before the electronic topological transition, there is a positive pressure coefficient of bulk band-gap, which is firstly found in the topological insulators family. Thus, for Ag₂ Te, it is verified that pressure is helpful for increasing the topological nature by inducing the increase of the bulk band-gap. It may provide us an approach of enhancing the topological nature of topological insulators.3) The pressure-induced the metallization phenomenon is uncovered in Ag₂Te:At 4.1 GPa, the results of high-pressure in-situ altering temperature resistivity measurements show that the resistivity of Cmca-phase Ag₂ Te increases with the rise of temperature, which shows the obvious characteristics of metal. Therefore,the pressure-induced the metallization phenomenon of Ag₂ Te is found, which show that the Cmca-phase is not topological.4) The structural phase transitions of As₂Te₃ under high pressure were studied by in-situ high pressure synchrotron X-ray diffraction and first-principles calculations. The results suggest that As₂Te₃ undergoes one reversible structural phase transitions between 17.4 and 36.3 GPa. The ambient monoclinic phase（C2/m, α-As₂Te₃） transformed into a high-pressure γ-Bi2Te3-type monoclinic phase（C2/c, γ-As₂Te₃）. The reported structural transition α-As₂Te₃ ® β-As₂Te₃（R-3m） has not been found at about 6.0 GPa.5) Pressure-induced electron phase transitions in α-As₂Te₃ phase were discovered at about 3.0 and 6.0 GPa for the first time, which can perfectly explain the observed enhancement of thermoelectric property.