Electrical Transport Properties And Structures Of Tungsten Trioxide And Barium Tungstate Under High Pressure

In this thesis, the powder tungsten trioxide （WO₃）、nano-WO₃and bariumtungstate （BaWO₄） are selected as the research objects, high pressure effects on theelectrical transport properties and structures have been systematically researched bytaking advantage of various in situ measurement technologies such as integratedmicrocircuit on a diamond anvil cell, and it is discussed on their electrical resistivity、grain resistance、grain boundary resistance and relaxation frequency under highpressure.The high-pressure electrical transport behavior of microcrystalline WO₃hasbeen investigated by direct current electrical resistivity measurement and alternatecurrent impedance spectrum techniques in a diamond anvil cell up to36GPa. Thediscontinuous changes of electrical resistivity are observed at1.8,21.2and30.4GPawhich reflect pressure induced structural phase transitions. The irreversible resistivityreveals that the structure phase transition is not reversible. In addition, the abnormalchanges of grain resistance and transport activation energy are found at about3and10GPa, which are related to the isostructural phase transition reported by previousRaman study. The change of resistivity under different temperature indicates thatWO₃is a semiconductor from ambient pressure to25.3GPa. The new peaks appear at24and31GPa confirm the structural phase transition by Synchrotron radiation X-raydiffraction experiment, and demonstrate the correctness of electrical measurementresults and correlation analysis.By high-pressure in situ electrical resistivity measurement, nano tungstentrioxide shows discontinuous changes of electrical resistivity at4.3and10.5GPa,which reflect the pressure induced electronic structural phase transitions, and grainsize effect leads to the hysteresis of phase transitions in the meantime; the slopechanges of electrical resistivity at24.8and31.6GPa reflect the pressure structuralphase transitions; the electrical resistivity do not return to the initial state from36GPato ambient pressure which reveals the structural phase transition is also notreversible；nano WO₃always keep the conductive property of semiconductor fromambient pressure to36GPa according to the resistivity under different temperatures,this is similar with the bulk WO₃. The nano WO₃shows the grain boundary effectunder high pressure by variable-frequency alternate current impedance spectrum; Thediscontinuous changes of grain boundary also provide the evidence for electronicstructural phase transition at4.6and10.3GPa; the change of pressure dependentrelaxation frequency indicates the relaxation process needs much shorter time during the electronic structural phase transitions at10.3GPa; the trace of grain boundaryrelaxation frequency also shows the grain boundary effect have not disappeared, thisresult is in accordance with Nyquist impedance spectrum.In present study, the electrical transport behaviors of BaWO₄under highpressure have been studied. For getting accurate data, a kind of insulating gasket wasdeveloped and employed to avoid introducing additional error in the experiment.Taking advantage of the integrated microcircuits on a DAC, in-situ impedancespectrum measurements were conducted and the grain and grain boundary effectshave been studied. Pressure-induced relaxation frequency and activation energy werealso discussed. For BaWO₄, two semicircles merge together and the wire inductancehas no effect on the impedance and the correction is not necessary for the sample withlower conductance. However, not all impedance curves can show the well-definedsemicircle, some impedance spectrums also show an incomplete semicircle in the highfrequency region and a compressed semicircle in the low frequency region. Thewell-defined semicircles at high and low frequencies are shown in our experiment bythe improvement of measurement technique. The grain and grain boundary resistancescan be fitted by semicircle and the intercepts represent the resistances. The change ofgrain resistance is different from variation of grain boundary resistance. In fact, mostanomalies of electrical parameters are caused by pressure-induced phase transition.The inflections of grain and grain boundary resistances indicate the changes ofelectrical transport properties, which can reflect the structural phase transition underhigh pressure.The increase of defect varieties and the change of lattice structure may also leadto the abrupt decrease of relaxation frequency during the phase transitions forrelaxation process of BaWO₄. The number of Frenkel and Schottky defects increasesbecause the atoms can be thermally activated caused by the fluctuations of entropyduring the phase transition. The fbhas an increasing trend from6.9to8.9GPa, whichis in accordance with previous relaxation peaks shift and caused by the differentrelaxation time belonged to different kinds of bulk inside. The scheelite, fergusoniteand BaWO₄-II phases coexist in this pressure range, in which the fergusonite andBaWO₄-II phases have lower relaxation time than the scheelite phase. During thephase transition, the compressure decreases the energy barrier height so that theelectrical charge moves more easily in fergusonite and BaWO₄-II phases resulting inthe decreasing relaxation time. The average relaxation time leads to an increase ofrelaxation frequency and the increasing relaxation frequency also indicates the frequency response property is inductive with increasing pressure.For grain boundary of BaWO₄powders, the pressure dependence of theactivation energy is–9.09meV/GPa from2.6to6.9GPa, indicating that theactivation energy of the grain boundary decreases with increasing pressure. Theactivation energy of the grain boundary decreases from6.9to8.9GPa and then therate of decrease is much larger comparing with the pressure zone of2.6-6.9GPa. Thisindicates that in the pressure range of2.6-8.9GPa, the pressure has a negativecontribution to the activation energy and the transport of charge carriers through theboundary becomes easier. Above8.9GPa, on the contrary, the activation energyincreases with pressure. This indicates that in the pressure range of8.9-13.7GPa, thepressure has a positive contribution to the activation energy and makes the transportof charge carriers difficult. After structural phase transition beyond13.7GPa, theactivation energy also decreases with increasing pressure and makes the transport ofcharge carriers easy.For BaWO₄powders, the structural phase transitions lead to the discontinuouschanges of grain and grain boundary resistance at about7and14GPa. The impedancebulk arcs indicate structural transition is firstly caused by grain and then involves ingrain boundary at about7GPa. The abrupt variations of relaxation frequency for grainand grain boundary are also caused by the structural phase transitions. The decreasingactivation energy of grain boundary shows that the pressure has a negativecontribution to the activation energy and the transport of charge carriers through theboundary becomes easier from6.9to8.9GPa. In addition, the ascending relaxationfrequency of garin and grain boundary indicates that the polarization process needsmuch shorter time during the phase transitions between6.9and8.9GPa.In summary, the electrical properties and structures of bulk WO₃、nano WO₃andBaWO₄are detailedly investigated and analysed under high pressure by direct currentelectrical resistivity and alternate current impedance spectroscopy combining withsynchrotron radiation X-ray diffraction experiment in the paper.