Study On The Electrical Transport Properties Of AMoO₄?A=Ca,Ba,Sr? And MoX₂?X=O,S? Under High Pressure

In this thesis,through combining a variety of high pressure in situ electrical transport properties measurements,high pressure synchrotron X-ray diffraction and first-principles calculations,we systematically investigated the dielectric behavior of scheelite molybdates AMo O4(A=Ca,Ba,Sr)and the electrical transport properties of transition-metal oxides Mo O2 and Mo S2 under high pressure in a diamond anvil cell.The detailed research results are as follows:1.In situ impedance measurements and first-principles calculations were employed to investigate the dielectric behavior of Ca Mo O4,Ba Mo O4 and Sr Mo O4 under high pressure.The relations between carrier transport behavior and crystal structure were analyzed,and high pressure dielectric properties of the three compounds were compared.(1)Ca Mo O4: The anomalous inflection points at 8.0 and 13.9 GPa in the electrical parameters(resistance,relaxation frequency and relative dielectric constant)of polycrystalline Ca Mo O4 were related to pressure-induced structural phase transitions.The grain boundary effect was observed to dominate the electrical transport process.The density of grain boundaries changed as a function of pressure,with rearrangement of the grain boundary microstructure,resulting in the resistance of grain boundary changed with pressure.The relaxation activation energy increased with increasing pressure in the tetragonal phase but decreased in the monoclinic phase.In the tetragonal phase,the increase of the relative dielectric constant with pressure can be attributed to the weakened electronic localization around O atoms.In addition,the dispersion in ?0 versus f weakened with increasing pressure.The dielectric loss decreased significantly in the low frequency region after a pressure cycle.(2)Ba Mo O4: Two discontinuous changes appeared in the electrical parameters of polycrystalline Ba Mo O4 at 4.8 and 8.8 GPa,respectively,which can be attributed to the pressure-induced structural phase transitions.The electrical transport mechanism of Ba Mo O4 was dominated by the grains.The increased resistance with increasing pressure in the tetragonal and monoclinic phases were caused by the increasing band gap and defect levels.In the whole pressure range,the activation energy increased with increasing pressure.The decrease of the relative dielectric constant with pressure in the tetragonal and monoclinic phase was attributed to the enhanced electronic localization around O atoms.The dielectric loss at low frequency decreased significantly after pressure released.(3)Sr Mo O4: The discontinuous variations in the electrical parameters of polycrystalline Sr Mo O4 at 7.0 and 13.3 GPa corresponded to the beginning and the end of the pressure-induced structural phase transition,respectively.The electrical transport mechanism was grain-dominated.The increase of resistance with increasing pressure in the tetragonal phase was caused by the reduction of carrier mobility.The decrease of resistance in the monoclinic phase was due to the increased carrier concentration.The activation energy increased with increasing pressure in the tetragonal phase but decreased in the monoclinic phase.The dielectric loss at low frequency was reduced significantly after a pressure cycle.A comparison of the three compounds: The dielectric performance of Ca Mo O4,Ba Mo O4 and Sr Mo O4 can be regulated by pressure.Differences existed in the carrier transport behavior among the three compounds.From a microscopic point of view,it is due to the difference of chemical bonds and structure stability caused by the discrepancy of Ca,Ba,Sr atomic radius and electronegativity.From a macroscopic point of view,the discrepancy may be caused by the difference of band gap and defect concentration under pressure.Systematic study on the dielectric behavior of AMo O4(A=Ca,Ba,Sr)improved the understanding of the properties of ABO4 materials under high pressure,and provided useful guidance and help in potential applications of AMo O4 and related ABO4 materials.2.The influence of pressure and temperature on the electrical transport properties of polycrystalline Mo O2 was investigated by in situ direct-current resistivity measurements at room temperature and variable temperature.In addition,the stability of Mo O2 was studied by high pressure synchrotron X-ray diffraction experiments.The inflection points of the room temperature resistivity(at 13.7 GPa,16.7 GPa,21.5 GPa and 27.2 GPa)reflected the transformations of the electron transport mechanism in the lattice.The variable temperature resistivity results indicated that Mo O2 presented good metallic conduction behavior in the whole pressure range.From 100 K to room temperature,no temperature-induced phase transition occurred in polycrystalline Mo O2,and the temperature coefficient kept a low value from ambient to 34.4 GPa.The abnormal changes of the temperature coefficient at 10.8 GPa,12.3 GPa,20.7 GPa and 26.7 GPa indicated the transition of lattice vibration trend.The X-ray diffraction experiment indicated that two structural phase transitions occur at 14.3~17.3 GPa and 22.6~25.5 GPa,respectively.Therefore,the discontinuous variations of the electrical parameters can be attributed to the pressure-induced structural phase transitions.The study on the high pressure electrical transport properties and structural stability of Mo O2 could strengthen the understanding of high pressure properties of the transition-metal oxides,and provide useful guidance for practical applications.3.The carrier transport behavior of single crystalline 2Hc-Mo S2 under high pressure was studied with in situ direct current resistivity and Hall effect measurements.With the pressure range of 0 ~ 9.1 GPa,the value of Hall coefficient was negative,which indicated that the main charge carriers were the electrons.The decrease of resistivity with increasing pressure can be attributed to the increased carrier concentration.At 9.1 GPa,the carrier concentration increased by one order of magnitude in comparison with ambient pressure.The research on the Hall effect of Mo S2 can deepen the understanding of high-pressure electrical transport properties of two-dimensional layered transition-metal sulfide,and open up a new area of research for other layered materials.