The Study Of Heating And Temperature Measurement Methods In Diamond Anvil Cell And The Electrical Transport Properties Of MnXO₃（X=Ti,Sn） Under High Pressure

In this article,the new method of heating and temperature measurement on the diamond anvil cell（DAC）is proposed,and detailed introduced the design ideal and fabrication progress.Also,the methods are systematically verified by experiments under ambient and high pressure.On the basis,the electrical transport properties of the multiferroic materials MnXO₃（X=Ti,Sn）are systematically studied by impedance spectrum and the first properties calculation under ambient and high temperature and high pressure.The results are as follows:1.As an important physical quantity,the measurement accuracy of temperature will directly decide the rationality and reliability of experiment results,when the DAC was applied to high temperature and pressure experiment.Based on the laws of thermocouples,the new method of directly temperature measurement in DAC is proposed in this article,which takes the S-type thermocouple as prototype.The new method is integrated the platinum-rhodium 10 thermocouple（S-type）into sample chamber of DAC to realize directly temperature measurement of sample.The stability test results of integrated thermocouple indicate that the characteristic of it is consistent with S-type thermocouple and the response speed is no obvious latency or out-synchronization compared with reference thermocouple.The heating experiment in pressure chamber of DAC（at 5.3 GPa）also proves that the built-in integrated thermocouple can measure temperature stably and correctly.The working performance test results show that integrated thermocouple still maintains excellent working performance under pressure（at 15.8 GPa）.The insulation layer of integrated thermocouple maintains complete during heating and high-pressure progress,indicated that it has stable and reliable working performance under high pressure.2.Realizing stable and uniform sample heating in sample chamber of DAC is a significant topic of high-pressure experiment method.Therefore,researchers have made much efforts to improve and develop new heating technology.In this article,annular electric furnace is directly integrated into sample chamber of DAC,which is realized directly heating for sample.The heating performance test results prove that it has excellent working performance,and can hold extended-period（30 min）effective heating（1476 K）in oxidation condition.Though auxiliary temperature measurement of finite element simulation method,heating performance experiments under high pressure obtain temperature distribution of pressure chamber.At 8.0 GPa,the integrated electric furnace still realizes stable heating for 1443.6 K.The finite element simulation calculation results show that temperature difference less than 10 K and 20 K of region is diameter for 120μm and 170μm respectively,when the diameter of sample is 210μm and no heat-insulating materials is used in experiment（centered on the center of the anvil）.Besides,the results indicates that built-in annular electric furnace can form relatively uniform temperature filed in sample chamber.Besides,heating electric furnace integrated into sample chamber can decrease the oxidation risk of diamond,and be compatible with all in-situ spectroscopic measurement methods under high temperature and pressure for DAC.The manufacture of heating electric furnace is simple and repeatable,and has stable heating performance under high pressure.After added the insulation layer,the integrated electric furnace can heat sample over 1800 K without any auxiliary heating source under high pressure of 12.3 GPa,which is significant progress in high-pressure experimental technology of DAC.3.By the AC impedance spectrum measurement of DAC and the first principles calculation method,the electrical transport behavior and structural evolution of MnTi O₃and MnSnO₃ under high temperature and pressure have been systematically explored in this article,and compared the dielectric properties:（1）.The impedance spectrum measurement and theoretical calculation results show that the resistant,relaxation frequency and relative dielectric constant of MnTi O₃occurred discontinues changes during the pressure range of 4.0-8.1 GPa.And the conduction mechanism has changed from ionic conduction to electronic conduction,resulted from that MnTi O₃ was occurred structural phase transition from lithium niobate to perovskite structure.The differential charge density function calculation results indicate that the polarization ability of MnTi O₃ with perovskite structure was weakened in electric filed,and the relative dielectric constant was gradually decreased,because the electronic locality of O atoms increased with increasing pressure,which is consistent with electrical experiment results.（2）.By analyzed the AC impedance spectrum measurement results can know that the conduction mechanism of MnSnO₃ changed from ionic conduction to electronic conduction at 7.8 GPa,that because pressure induced the structural phase transition of MnSnO₃ from lithium niobate structure to perovskite structure.In MnSnO₃,the contribution of grain resistant to total resistant is more than grain boundary resistant,thus,grain resistant is occupied dominant position in perovskite structure.The relative dielectric constant increases with increasing pressure,indicating that it has more excellent energy storage capacity of lithium niobate structure.According to the electronic local function calculation results,it can be known that the covalent bond was enhanced after structural phase transition,so the relative dielectric constant of MnSnO₃with perovskite structure decreased with increasing pressure.The distance of Mnatoms with O atoms is suddenly decreased and coulomb force is increased,leading to the closure of ion channel.Thus,the conduction mechanism is changed to electronic conduction after MnSnO₃ transformant into perovskite structure,which is consistent with electric experimental results.At 8.4 GPa and 450 K,the conduction mechanism of MnSnO₃ is transformed from electronic conduction to ion-electron mixed conduction,which indicates that temperature can induce the participate in conductive behavior of Mn²⁺ions.The relation of resistivity changed with temperature under different pressure show that the combined action of temperature and pressure can regulate the resistivity of MnSnO₃.The comparative study:pressure induce the conduction mechanism to change,indicated that pressure can effectively regulate and control the electric transport properties of MnTi O₃ and MnSnO₃.The resistance of MnTi O₃ is decreased with increasing pressure,but the resistance variation trend of MnSnO₃ is completely different from MnTi O₃,resulted from that their band gap has different change under pressure,and induced the different variation of carrier concentration and mobility.Besides,the structural stability discrepancy made the dielectric properties of MnTi O₃and MnSnO₃ shows obvious difference under pressure.The study of electric transport properties on MnTi O₃ and MnSnO₃ can provide experimental and theoretical data support for the application,development and evaluation of ABO₃-type multiferroic material.