Electrical Transport Properties Of Mg₂Si And CdS Under High Pressure

In our paper, a novel high-pressure electrical measurement method is constructedbased on diamond anvil cell （DAC）, which effectively eliminates the bypass effectinduced by the sample chamber and the measurement errors caused byhigh-pressure in situ thickness measurement, and then breaks the technical obstacle ofhigh-pressure accurate electrical conductivity measurement. We have researchedsystematically on high-pressure electrical properties of crystal samples （Mg₂Si andCdS） using the integrated electrodes on DAC, displaying their electronic transportproperties and interface conductance variations with pressure，which makes sure theirmetallization and enriches the physical meaning of new High-pressure phases.In High-pressure electrical measurement，the thickness measurement of sampleneed be extremely accurate and the insulation between gasket and sample should bebest accomplished. However，it is still a prominent technical problem that therealization of the insulation between sample and chamber inner wall because of thenarrowness of sample chamber. Uninsulation of inner wall produces leakage currentwhich results in a large error on experimental results. The deformations of diamondand gasket also affect the accurate thickness measurement. If these questions aren’tresolved very well， the judgment on the properties of materials could be broughtwith negative effects.As the questions mentioned above, we have designed a three-electrodeconfiguration. There are three parts for this electrode: one is the around electrode inthe center of the diamond anvil; two is the arc electrode around the round electrode;gasket is used to be the third electrode. Using the gasket as an electrode, we solve theproblem about the defective insulation of the sample chamber and avoid the bypassingeffects. The accuracy of the experiment is improved. An in situ measurementparameter is reduced due to the introduction of a reference electrode. Meanwhile,during the experiment the measurement of sample thickness need to be detected due to the introduction of the reference electrode. So we can get the accurate result ofexperiment as we use the new electrode. We obtain accurate thickness through theanalysis of the electric field distribution and cleverly avoid the in situ measurementsof the sample thickness. According to this conceptual design, the accuracy ofexperimental measurements is significantly improved and the complexity of electricalparameters under high pressure is simplified. It is a technological innovation.The electrical resistivity of CdS is studied using the new electrode under highpressure. The electric field analysis showed that there was the exponential change forthe thickness under high pressure. In the initial stage of pressure, the thickness ofsample is changing rapidly. Below and above15GPa, the rate of change of thicknessis about3.3μm/GPa and0.8μm/GPa, respectively. This result is consistent with thethickness variation of in situ experiments, which prove the new method isreasonable.We also find that the electrical resistivity of CdS drops over three orders ofmagnitude at about2.8GPa, which is due to the phase transition of CdS from wurtziteto rocksalt. In a word, the electrical resistivity can be detected accurately using ournew electrode due to eliminating the effect of defective insulation of sample cavitywall and avoiding the measurement of in situ sample thickness. This technologyprovides a new way in which we can have a better study of materials electricaltransport properties and behavior under high pressure.The pressure-dependent electrical resistivity results show that the electricalresistivity of Mg₂Si decreases with the increasing pressure. The inflexion aboutelectrical resistivity at7GPa inicated thatthe Mg₂Si undergoes the phase transitionfrom Anti-fluorite to Anti-cotunnite; From7GPa to12.2GPa, Anti-fluorite phasecoexists with the Anti-cotunnite; At12.2GPa, the phase transition from Anti-fluoriteto Anti-cotunnite completes, which results in the change of electrical resistivity. At22.2GPa, another inflexion of electrical resistivity demonstrates that Mg₂Siundergoes the phase transition from Anti-cotunnite to Ni2In-type. Above22.2GPa,the electrical resistivity almost remains unchanged with the pressure increasing.The temperature-dependent resistivity of Mg₂Si under high pressure is alsostudied. It can be observed that below22.2GPa the electrical resistivity of Mg₂Si shows the negative relationship with the temperature and Mg₂Si remains thesemiconductor behavior. However, above22.2GPa, the electrical resistivity exhibitsthe positive behavior with the temperature, which illustrates that Mg₂Si has becomemetallic. The change of activation energy shows that the discontinuous change ofactivation energy is consistent with the structural phase transition at7GPa,12.2GPaand22.2GPa, which reveals that the change of carrier activation barrier results frompressure-induced structural phase transitions.The phase transitions of Mg₂Si under high pressure were researched by using thefirst-principles density functional theory. Meanwhile, the properties of metallic phase（Ni2In-type of Mg₂Si） were researched through the band structures, density of states（DOS） and electron density.Impedance spectroscopy of CdS with different size is also studied under highpressure. There are two mechanisms about CdS powder response of AC signal: one isthe grain transportation in the high frequency region, another is the grain boundarytransportation the low frequency region. With the decreasing of the grain size, grainand grain boundary electrical resistance can be separated easily. For the CdSnanocrystals, the impedance spectroscopy includes more information. At the pressurebelow3.8GPa, the impedance spectroscopy is composed of a high impedance value（almost straight line） at low frequency range and an oblate-like semicircle at highfrequency range. The semicircle and the straight line represent the charge carriertransportation in the bulk and the space charge depletion at grain boundary,respectively. With the pressure increasing, the straight line at low frequency range ispressed to the real axis in the complex plane, indicating the relaxation timedistribution of sample under low pressure has non-Debye characteristic. At thepressure above3.8GPa, the spectra at high frequency range move to real axis andbecome the well-defined semicircles. The activation energy of grain boundarybecomes smaller with the increasing pressure, which makes the transport of chargecarriers through boundary easier and the grain boundary resistance smaller. Thechange of impedance spectrum from line-type to closed-type reveals the occurring ofstructure phase transition. The bulk resistance plays a dominant role at the pressure above11.9GPa.The change of the relaxation frequency with pressure showed that the grainboundary relaxation peaks at low frequency range broaden under pressure whichdisplayed the continuous change of the grain boundary. With pressure increasing, therelaxation time become long and the height of relaxation peak minish. Due to thevalue of boundary resistance is proportional to the value of the peaks, the carrierbecome easier to cross the boundary and formed the current. At the pressure above18GPa, the peaks at low frequency disappear.In summary, a new experimental method has been founded to measure the highpressure electrical properties of sample. Meanwhile, we determine the metal phasetransition and electrical transport properties with pressure of Mg₂Si; we also study thetransformation of grain boundaries and grain conductance of CdS under high pressure.These lay a foundation for research of electrical transport properties and interfaceconductance.