Study On The Structural And Electrical Properties Of ABO₄ Tungstates And Molybdate Under High Pressure

ABO₄ compounds are common accessory minerals on the earth and important functional materials.The studies of ABO₄ compounds under high pressure are helpful for understanding the physical and chemical properties of these minerals and improving their physical and mechanical properties.Tungstates and molybdates are typical ABO₄ compounds.Especially,the scheelite is also a high-pressure phase of other ABO₄ compounds?e.g.zircon,monazite and Cr VO₄-type compounds?.So,the study on scheelite at high pressures will provide significant reference for exploring the high-pressure behaviors of other ABO₄ compounds.In this thesis,by combining the high-pressure synchrotron X-ray diffraction,alternating current impedance spectra measurements and first-principles calculations,we have studied systematically the high-pressure structural and electrical transport properties of ABO₄ tungstates and molybdate,including scheelite-structured Ca WO₄,Ca Mo O₄,Sr WO₄,wolframite-structured Mg WO₄,and distorted wolframite-structured Cu WO₄.The results of study are as follows:1.By combining first-principles structure prediction and X-ray diffraction measurements,we found that Ca WO₄ transformed from tetragonal scheelite structure?I41/a?to monoclinic fergusonite structure?I2/a?at 10.0 GPa,and then to layered monoclinic structure?P21/m?at 38.0 GPa.These two phase transitions are secondand first-order phase transitions,respectively.In I41/a phase,Ca WO₄ compresses anisotropically with c > a.In I2/a phase,the unit-cell parameter of Ca WO₄ shows a strong non-linear behavior.Besides,we found that the P21/m phase of Ca WO₄ has a smaller band gap than the other two phases.By alternating current impedance spectra measurements,we found that the pressure-induced structural phase transitions significantly change the electronic structure of Ca WO₄ that gives rise to abnormal variations in electrical transport properties.Besides,we found a monoclinic structure?P21/c?with the lowest energy and we attribute this structure to a high-temperature high-pressure phase.This work observes the I2/a ? P21/m phase transition for the first time in the X-ray diffraction experiment and enriches the information of ABO₄ compounds under high pressure.It is an important reference for the high-pressure research of other ABO₄ compounds.2.Ca Mo O₄ displays similar behaviors with Ca WO₄ under high pressure.Ca Mo O₄ undergoes a second-order structural phase transition from tetragonal scheelite structure?I41/a?to monoclinic fergusonite structure?I2/a?around 10.0 GPa.The second structural phase transition occurs at a pressure of 42.1 GPa,changing the space group from I2/a to P21/m.Under high pressure,in I41/a phase,compression is anisotropic,being the c-axis more compressible than the a-axis.The unit-cell parameter of I2/a phase shows a non-linear change.In addition,we calculated the electronic band structures under different pressures.The results show that the I41/a,I2/a,and P21/m phases are direct bandgap semiconductor,direct bandgap semiconductor,and indirect bandgap semiconductor,respectively.The bandgap of Ca Mo O₄ decreases with increasing pressure.These results will be significant for further understanding the physical properties of ABO₄ compounds at high pressures.3.The phase-transition sequence of Sr WO₄ at high pressures is quite different from those of Ca WO₄ and Ca Mo O₄.In Sr WO₄ a phase transition from tetragonal scheelite structure?I41/a?to monoclinic fergusonite structure?I2/a?was found at 9.1 GPa.The second structural phase transition takes place at 12.3 GPa.By combining theory with experiment,the high-pressure new phase is defined to have an orthorhombic structure?Cmca?,rather than decomposition as reported in previous study.Meanwhile,the structural phase transitions give rise to abnormal variations in bandgap and electrical transport parameters.In all structures,the resistance of Sr WO₄ decreases with increasing pressure.All the activation energies decrease with increasing pressure and the charge-discharge process becomes easier.This work for the first time prove the prediction from the experiment that the scheelite structure can transform to the Cmca structure at high pressures.4.In Mg WO₄,we found two phase transitions at 17.2 GPa and 37.0 GPa,respectively.The first phase transition is from monoclinic wolframite structure?P2/c?to triclinic Cu WO₄ structure?P-1?and the second one is to an unknown high-pressure phase.At pressures below 16.1 GPa,in monoclinic wolframite structure,Mg WO₄ compresses anisotropically with b > a > c.The electrical transport parameters of Mg WO₄ vary anormalously along with the structural transitions.At pressures below 16.1 GPa,the band gap becomes larger and the resistance increases slowly with increasing pressure.However,at the first phase transition pressure,the resistance decreases five orders of magnitude that may give rise to narrower bandgap.In P-1 phase,the activation energy decreases with increasing pressure and the charge-discharge process becomes easier.In new high-pressure phase,the activation energy increases with increasing pressure and the charge-discharge process becomes more difficult.5.The effect of crystallization water on the structural and electrical properties of Cu WO₄ under high pressure has been investigated by X-ray diffraction and alternating current impedance spectra measurements.The crystallization water was found to be a key role in modulating the structural stability of Cu WO₄ at high pressures.The anhydrous Cu WO₄ undergoes two pressure-induced structural transitions at 8.8 and 18.5 GPa,respectively,while Cu WO₄?2H2O keeps its original structure up to 40.5 GPa.Besides,the crystallization water makes the electrical transport behavior of anhydrous Cu WO₄ and Cu WO₄?2H2O quite different.The charge carrier transportation is always isotropic in Cu WO₄?2H2O,but anisotropic in the triclinic and the third phase of anhydrous Cu WO₄.The grain resistance of Cu WO₄?2H2O is always larger than that of anhydrous Cu WO₄ in the entire pressure range.By analyzing the relaxation response,we found that the large number of hydrogen bonds can soften the grain characteristic frequency of Cu WO₄?2H2O over Cu WO₄ by one order of magnitude.