Synthesis Of BiOX(X=I,Br And Cl) And Study Of Its Structural And Electrical Transport Properties Under High Pressure

This paper synthesized BiOX（X=I,Br and Cl）powder samples with a two-dimensional layered structure with three-dimensional microsphere morphology,and characterized them by XRD and SEM methods.The lattice structure and electrical transport properties of BiOX（X=I,Br and Cl）series materials under high pressure were studied in depth using a diamond anvil experimental device,using high-pressure XRD testing technology,high-pressure Raman spectroscopy analysis technology,and high-pressure AC impedance spectroscopy testing technology.The specific research results are as follows:1、This article uses hydrothermal synthesis method and carefully controls the growth conditions to prepare BiOI and BiOCl powder samples,while BiOBr powder samples are prepared by co precipitation method.The structure and morphology of the above three samples were characterized by XRD and SEM.The results indicate that all initial samples have a P4/nmm tetragonal structure with extremely high purity.The microscopic morphology is a three-dimensional microsphere self-assembled by layered microsheets,with clear edges and smooth surfaces,and high crystallinity.This will lay a good foundation for subsequent high-pressure experimental testing.In summary,the unique three-dimensional microsphere like morphology and two-dimensional layered structural characteristics of BiOX（X=I,Br and Cl）samples make them a good sample system for studying pressure response.2、The high-pressure Raman spectroscopy analysis of BiOI materials shows that there is no pressure induced structural phase transition of BiOI below 26.1 GPa,indicating that the sample has good stability within this pressure range.The results of AC impedance spectroscopy analysis show that within the experimental pressure range studied in this article,BiOI exhibits a pure electron conduction process,with grain boundary conductivity disappearing after 9.2 GPa.As the pressure increases,the continuous increase in the number of charge carriers per unit volume increases the conductivity by 6 orders of magnitude.Pressure induced lattice fragmentation and grain refinement bring a large number of related energy levels in the gaps,leading to a significant increase in the conductivity of BiOI under pressure.After pressure relief,the conductivity value is still two orders of magnitude higher than the initial value.The spatial charge polarization of the crystal interface layer weakens with pressure,resulting in a decrease in relative dielectric constant.This study found that the conductivity of BiOI significantly increases with increasing pressure,which can significantly increase carrier concentration and mobility,effectively improving its conductivity.3、In situ high-pressure AC impedance spectroscopy,in-situ high-pressure Raman measurement,and in-situ high-pressure X-ray diffraction experiments were conducted on BiOBr powder to explore its structural characteristics and electrical transport process under compression conditions,with a pressure of up to 20.9 GPa.The frequency dependent curves of Z’’and M’’show only one peak,indicating that there is only one electrical conduction mechanism in BiOBr and that grain boundaries do not contribute to the electrical transmission characteristics of BiOBr under high voltage.The results of impedance spectroscopy and resistivity showed turning points around 10.0 and 15.3GPa.High-pressure Raman observed slight discontinuities at the initial pressure（10.0and 15.3 GPa）during the pressure variation of E_g and A_1g-3.Consistent with the Raman results,discontinuous crystal plane spacing was also observed at 9.8 and 15.1 GPa with increasing pressure in the high-pressure synchrotron radiation results.Within this pressure range,the compressibility of the c-axis is 17.1 to 4.0%higher than that of the a-axis.In summary,there are two pressure driven isostructural phase transitions around10.0 and 15.0 GPa:T-T’and T’-T’’（T-tetragonal,T’-tetragonal 1,and T’’-tetragonal 2）.The changes in the structure and electrical transport of BiOBr crystals caused by pressure can provide a reference for explaining the mechanism of structural phase transitions in other similar compounds after compression.4、The electrical operation of BiOCl samples was deeply discussed through high-pressure AC impedance spectroscopy testing,with a pressure of up to 24.1 GPa.The experiment found that grain boundary conduction dominates the electrical transport process.The trends of electrical parameters（resistivity,relaxation frequency,relative dielectric constant）all change at 11.2 GPa and 23.0 GPa.Based on the results of previous high-pressure XRD studies,we have determined that the changes in electrical parameters are due to the change in carrier transport behavior caused by the equistructural phase transition of BiOCl,which is related to the redistribution of Bader charges between Bi,O,and Cl ions.The relative dielectric constant decreases with increasing pressure,possibly due to the localized electrons near the O atom increasing with pressure.Comparative study of three compounds BiOX（X=I,Br and Cl）:All initial samples have a P4/nmm tetragonal structure.Within the applied pressure range,BiOI did not undergo a phase structure transformation,while BiOBr and BiOCl both underwent an isostructural phase transition under high pressure.Both BiOI and BiOCl exhibit mixed conduction between grains and grain boundaries during electrical transport,while grain boundary conduction is largely ineffective in the electron conduction process of BiOBr.Pressure has a good regulatory effect on the conductivity of BiOI,but it does not significantly enhance the conductivity of BiOBr and BiOCl.The overall trend of relative dielectric constant decreases with the increase of pressure,indicating that pressure has an inhibitory effect on its storage capacity.The different results of structural stability and electrical transport properties can be attributed to the significant influence of the ion radius and electronegativity of X^n-on the phase transition behavior under high pressure,as well as the differences in bandgap and defect concentration under pressure.