The Structural And Electrical Transport Properties Study Of BiSb,C₆H₆ And(C₆H₅CH₂NH₃)₂PbI₄ Under High Pressure

In this paper,by combing the high-pressure synchrotron X-ray diffraction,electrical transport properties measurements and first-principles calculations,we have studied systematically the structural and electrical transport properties of compressed BiSb,C₆H₆ and(C₆H₅CH₂NH₃)₂PbI₄,to reaveal the related structural transition,charge carriers transport mechanism,and the correlation between structural modification and abnormal changes of the electrical transport properties,which established general guidelines in designing or optimizing new practical applications of these materials.Detailed experimental and theoretical observations are as following:1.By combing the high-pressure Synchrotron X-ray diffraction,pressure dependence of resistivity,temperature dependence of resistivity measurements and first-principles calculations,we have studied the structural and electrical transport properties of BiSb under compression.Using first-principles structure prediction and synchrotron X-ray diffraction techniques,two structural phase transitions are found at8.5 and 20.3 GPa,respectively.The two-step evolution process in the first phase transition is explained.First,due to the rapid decrease of the interlayer distance,the Bi atoms and Sb atoms between the layers form new bonds.Subsequently,the plane formed by the Sb atom and the surrounding 4 Bi atoms is increasingly parallel to the ab plane,forming tetragonal?-BiSb under pressure.The electrical transport measurement reveal that the pressure-induced carrier concentration increased drastically and the metallization occurred at?4 GPa.There are slight abnormal changes in electrical parameters at?4,8 and 20 GPa.Combined with structural characterizations,in situ resistivity and Hall-effect measurements,and theoretical calculations using density functional theory,results reveal that,the first anomaly is caused by the formation of the Weyl semimetallic state,which is attributed to a Br-Sb bond between layers.The subsequent two are attributed to the trigonal to tetragonal and tetragonal to cubic phase transitions.Our research provides a route to find manipulable topological Weyl semimetallic transport properties in the materials with broken space-inversion symmetry and helps to explore the phase transition of similar non-metallic alloys under high pressure.2.We investigate the pressure effects on the electrical and dielectric properties of C₆H₆ up to 50 GPa at room temperature by combining high pressure in situ impedance spectroscopy with density functional theory(DFT)calculations.It is observed that two discontinuous changes in resistance and relaxation frequency at 4.3 and 9.0 GPa,respectively.The two changes come from the pressure-induced structural phase transitions of Pbca to P4₃2₁2 and P4₃2₁2 to P2₁/c,respectively.The relationship between resistance change and activation energy at high pressure is confirmed,Negative values in the test interval,indicating that pressure can promote the conductivity of C₆H₆.Furthermore,the typical complex dielectric permittivity and dielectric loss of C₆H₆ are also presented,complex dielectric permittivity and dielectric loss increase with increasing pressure.By first-principles calculations,the pressure-dependent change mechanism of the bandgap and bonding state can be understood.The increased overlap of?and?^*at the interstitial site between two adjacent molecules leads to a narrower band gap in benzene,increased conductivity and an increase in polarization with the pressure increases.3.By in situ high pressure impedance spectroscopy measurements,we have systematically studied the electrical transport properties of 2D organic-inorganic hybrid perovskites(C₆H₅CH₂NH₃)₂PbI₄.We found that there are three physical process in(C₆H₅CH₂NH₃)₂PbI₄ through analyzing the Nyquist plots of the impedance spectra:grain conduction,grain boundaries conduction and quantum well effect.Comparing with 2D organic-inorganic hybrid perovskites,the inductive loop of(C₆H₅CH₂NH₃)₂PbI₄ is related to the quantum well effect in 2D organic-inorganic hybrid perovskites.In order to generate the inductive loop in the impedance spectra,the parameter B=(/(3)(?/)must fulfill the condition of B>0.X denotes the unknown variable that can cause the inductive loop.We suggest that the number of non-self-trapped excitons during transport,n,is defined as the variable X.It can be known that the Faradaic electric current IF is proportional to the number of non-self-trapped excitons(/(3=/9)>0).The potential E is proportional to the number of non-self-trapped excitons per unit time(/=((99)/(9)/>0).Hence leading to the appearence of the inductive loop in the impedance spectra.In addition,the grain boundary effect in the electrical transport process originates from the organic spacer layers,and a superlattice-like effect is formed between the organic spacer layers and the inorganic conductive layers.The pressure can regulate the intermolecular forces in the organic spacer layers and reduce the formation of quantum wells,so the inductive loop disappears under high pressure,and the grain boundary resistance drops sharply.