The Structural Behavior And Electrical Properties Of CaB₄,MTeO₃(M=Ba, Sr),CuInX₂(X=S, Se) Under High Pressure

The development of high-pressure electrical measurement as a way of in-situbasic electric properties measurement plays an indispensable role in the high-pressureresearch. In this paper, we integrate a microcircuit on the anvil of DAC using thin filmsputtering and photolithograph techniques. The principles of high-pressure phasetransition and the mechanism of conductivity are generally revealed by utilizingVan-der-pauw methods. Under external pressure the structure of the matter will bechanged, which results in the change of electron transport accordingly. According tothe pressure dependence of the electrical transport properties, we can detect thestructural phase transition and the electrical phase transition.It is generally believed that grain boundaries in perovskite-type polycrystallinematerials can play a dramatic role in determination of their macroscopic behaviors.Although extensive efforts were made to understand how the grain boundaries willaffect the transport properties, little is known about the variation of the grainboundaries during the process of a structural phase transition of perovskite-typeoxides. Therefore, with the help of AC impedance spectra technology, a detailedunderstanding of grain boundary effect on the transport properties under compression,especially through a structural phase transition is of major importance.In this paper, the structural phase transition and the electrical transport propertiesof CaB₄, MTeO₃(M=Ba, Sr), CuInX2(X=S, Se) under high pressure have been studiedand the results listed as follows:1. The structural and electrical properties of CaB₄under pressure have been studiedby angle dispersive X-ray diffraction, in situ Hall effect measurement, andfirst-principles calculations. An anomaly of c/a ratio without any symmetricalchange was experimentally observed around12GPa, which was also correctlyreproduced by theory, indicating an isostructural phase transition. To the best of our knowledge, this type transition was just observed in CaB₄of all the MB₄compounds with tetragonal structure. To explore the electronic origin of the c/aanomaly in CaB₄, the variation of the energy band structure and the Fermi surfacewith compression were examined. No noticeable change in the electron topologyof the Fermi surface and no unusual softening in the phonon dispersions can befound as pressure increases up to14GPa. Therefore, the possibility of theexistence of an ETT in CaB₄is excluded. On the other hand, in the cases ofresistivity and Hall coefficient measurements, the isostructural phase transitionhas been detected as the variation trend changed in the transport parameters at11.7GPa. Therefore, the electronic origin for this c/a anomaly is reasonable todue to small changes in the curvature of the electronic band dispersion near theFermi surface. The temperature dependence of CaB₄resistivity was measured atdifferent pressures. The electrical resistivity increased noticeably with increasingtemperatures and therefore CaB₄shows metal behavior. The temperaturecoefficient had a discontinous change around12GPa, provided another evidencethat a pressure induced isostructural phase transition of CaB₄occurred around12GPa.2. The evolutions of alternate current (AC) impedance spectra, direct current (DC)resistivity and Raman spectra in polycrystalline BaTeO3have been investigated athigh pressure. The abrupt changes observed in electrical transport measurementsat12.73GPa indicated a kind of phase transition, which was confirmed to berelated with structural phase transition by high-pressure Raman scatteringexperiment. From the impedance spectra, both grain resistance and grain boundaryresistance decreased with increasing pressure, and two clear discontinuitiesoccurred at12.73GPa and17.47GPa, respectively. After the transition, thecontribution to the total resistance by grain boundary effect dramatically declinedand the grain resistance began to play a dominant role. We found that twoabnormal changes in the pressure dependence of electrical transport parameters ofSrTeO3appear at2.75GPa and14.63GPa, respectively, which means that twounderlying phase transitions occurred. With the measurement of temperaturedependance of resistivity, we obtained the pressure dependence of activationenergy of BaTeO3and SrTeO3. The decrease of the activation energy is the realreason of the decrease of the resistivity under pressure. 3. The structural and electrical transport properties of CuInS2nanocrystals underhigh pressure have been studied by angle dispersive X-ray diffraction, in situHall-effect measurement, temperature dependent resistivity measurement and thefirst-principles calculation. A pressure induced irreversible structural phasetransition from the tetragonal phase to the cubic phase has been confirmed at10.84GPa, which is significantly elevated the transition pressure (1.3GPa) innanocrystals compared to the bulk materials. The electrical transport parameters(the electrical resistivity, Hall coefficient, carrier concentration and mobility) allshow discontinuous changes around11GPa. Specifically, the Hall coefficientchanges its sign from negative to positive around12GPa, indicating that CuInS2undergoes a carrier-type inversion from n-to p-type. The electrical transportproperties of CuInSe2under high pressure also have been studied by in situHall-effect measurement, temperature dependent resistivity measurement and thefirst-principles calculation. The electrical transport parameters (the electricalresistivity, Hall coefficient, carrier concentration and mobility) all showdiscontinuous changes at7.27GPa, which correspond to the structural phasetransition from the chalcopyrite phase to the NaCl phase. The positive Hallcoefficient (RH) was obtained at ambient condition, indicating that CuInSe2is ap-type semiconductor. The variation of the resistivity is some contribution of bothchanges of concentration and mobility.4. The structural properties of Bi₂Sr₂CoO_6+x(0.4＜x＜0.5) under pressure have beenstudied by angle dispersive X-ray diffraction. No structural phase transition can beobserved below25GPa. The resistivity decreases with increasing pressure.However, a significantly discontinuous change of the resistivity appears at12.05GPa, the resistivity decreases over five orders of magnitude and reaches theorder of10-4ohm·cm. The result of temperature dependence of resistivityindicated that below12.13GPa the electrical resistivity of Bi₂Sr₂CoO_6+xdecreaseswith temperature increase and therefore Bi₂Sr₂CoO_6+xshows semiconductorbehavior. Above14GPa the electrical resistivity shows a positive relationshipwith the temperature, which indicates Bi₂Sr₂CoO_6+xshows metallic behavior. Sowe can conclude that a pressure induced metallization of Bi₂Sr₂CoO_6+x(0.4＜x＜0.5) occurred at12.28GPa. In summary, we measured the electrical parameters of CaB₄, MTeO3(M=Ba, Sr),CuInX2(X=S, Se) by using a microcircuit fabricated on a diamond anvil cell. So weobtained the relationship between the resistivity and the pressure, temperature.Furthermore, through the angle dispersive X-ray diffraction experiment and thefirst-principles calculation, we systematically investigated the physical properties andthe structural phase transition of the samples, gained further insight into themechanism of the structural phase transition.