Study Of The Electrical Transport Properties Of LaAlO₃ And MVO₃（M=Li,Na,Cs） Under High Pressure

Perovskite-type ABO₃ compounds are known for their unique crystal structures that exhibit excellent optical,electrical and magnetic properties.These compounds are not only important research objects in condensed matter physics,but also have significant application prospects in the fields of high-energy physics,quantum information science,and materials science and engineering.Especially in application scenarios such as sensors and solid oxide fuel cells,perovskite-type ABO₃ compounds with high electrical conductivity are often required.Currently,studies to improve the electrical conductivity of perovskite-type ABO₃ compounds are mainly focused on doping,which improves their electrical conductivity by introducing oxygen vacancies.Pressure,as an important thermodynamic parameter,can change the structural and physical properties of the substance and thus regulate the electrical conductivity of a material.In recent years,researchers have often used high pressure to modulate the electrical and structural properties of perovskite-type ABO₃ compounds.For example,when Li Nb O₃ is subjected to a pressure of 25.2 GPa,it undergoes a structural phase transition from the R3c phase to the Pnma phase,leading to an increase in ionic conductivity.Similarly,Na Nb O₃ undergoes a structural phase transition from the Pbcm phase to the Pna2₁ phase at 7.6 GPa,which leads to larger ion channels,thereby increasing the ion migration rate and enhancing the ionic conductivity.Therefore,the unique thermodynamic conditions provided under high pressure can effectively modulate the electrical and structural properties of perovskite-type ABO₃ compounds.The study of perovskite-type ABO₃ compounds under high pressures is of great research significance to enhance the electrical conductivity of the materials and discover novel properties that do not exist under conventional conditions.In this paper,LaAlO₃,LiVO₃,NaVO₃ and CsVO₃ are taken as the research objects,and their electrical transport properties and structural phase transitions under high pressures are systematically investigated through high-pressure in situ alternate-current impedance spectra,high pressure Raman spectra,high pressure synchrotron radiation X-ray diffraction,and first-principles calculations.The findings of the study are outlined as follows:1.The electrical transport properties of LaAlO₃ in the pressure range from 0 GPa to 20 GPa have been investigated using in situ alternate-current impedance spectra measurement and first-principles calculations.LaAlO₃ exhibits an anomalous transition from ionic to polaronic to ionic conduction mechanism from 0 GPa to 20 GPa.Around13.2 GPa,discontinuous changes in the starting frequency of ion migration f _W,relaxation frequency f_b,and ionic resistance R_i have been observed.This can be attributed to the structural phase transition of LaAlO₃ from the rhombohedral phase to the cubic phase.In addition,as pressure increases,f _W and f_b rise while R_i falls,suggesting that pressure enhances the migration of O^2-ions and improves the conductivity in LaAlO₃.Through first-principles calculations,we have elucidated the physical origin of polaronic conduction,which results from the distortion of electron density background around the Al and O atoms.2.A systematic study of the electrical transport properties and structural evolution of LiVO₃ under high pressures has been conducted using high pressure alternate-current impedance spectroscopy,Raman spectroscopy,synchrotron X-ray diffraction measurements,and first-principles calculations.LiVO₃ displays typical ion transport behaviors from ambient pressure to 10.7 GPa.The grain resistance dominates the electrical transport process.The ionic resistance increases with increasing pressure and the long-range diffusion and relaxation frequency decrease.In the pressure range from5 GPa to 10.7 GPa,the ionic resistance increases more rapidly,while the long-range diffusion frequency and relaxation frequency fall more quickly,suggesting that the pressure inhibits the migration of Li⁺ions.Between 10.7 and 19.1 GPa,the inclined straight line representing the long-range diffusion of Li⁺disappears,and the conduction mechanism changes from ionic conduction to electronic conduction.The electronic resistance gradually decreases with increasing pressure and the relaxation frequency increases,indicating that the pressure promotes the electronic conduction.The electrical parameters change discontinuously at 5 GPa and 10.7 GPa.Combing high pressure structure prediction,Raman spectroscopy,and synchrotron X-ray diffraction results,LiVO₃ undergoes a structural phase transition from the monoclinic phase C2/c to the triclinic phase P1 around 5 GPa,during which theVO₄ tetrahedra are transformed intoVO₆ octahedra.The charge density differences calculations show that the structural evolution of theVO₄ tetrahedra intoVO₆ octahedra results in a rearrangement of the electron density around the Li and O atoms,which is responsible for the ionic-electronic transition of LiVO₃ at 10 GPa.3.The electrical transport and structural properties of NaVO₃ under high pressure have been systematically investigated using alternate-current impedance spectroscopy and Raman spectroscopy measurements.NaVO₃ exhibits electronic conduction throughout the experimental pressure range.The resistance and relaxation frequency change discontinuously at 6.7 GPa.In addition,the resistance decreases and the conductivity increases with increasing pressure,indicating that high pressure can increase the conductivity of NaVO₃.The results of high-pressure Raman spectroscopy showed that with the increase of pressure,a new Raman peak appeared in NaVO₃ at 7.9GPa,followed by a gradual broadening and finally disappeared,which signified that NaVO₃ occurs a structural phase transition from crystalline phase to amorphous phase.The discontinuous changes in the electrical parameters of NaVO₃ are related to the structural phase transition from the crystalline phase to the amorphous phase.4.The electrical transport and structural properties of CsVO₃ in the region of 0GPa to 20 GPa have been investigated using alternate-current impedance spectra and Raman spectroscopy measurements.The results show that CsVO₃ exhibits electronic conduction under high pressures.Electrical parameters such as resistance and relaxation frequency change discontinuously at 10.3,11.8 and 15.1 GPa.The resistance rises and the conductivity drops during the high-pressure phase.Based on the Raman investigation under high pressures,with increasing pressure to 11.3 GPa,new Raman peaks appear at 97.5,132.9,148.8,279.8,301.7,666.9,and 985.3 cm^-1,indicating the beginning of the structural phase transition from phase I to phase II.With the pressure increasing to 12 GPa,the structural phase transition from phase I to phase II is completed.As the pressure is above 14.8 GPa,the phenomenon of the generation and disappearance of Raman peaks indicates that the structural phase transition from phase II to phase III occurred in CsVO₃.Therefore,the discontinuous changes in resistance and relaxation frequency are caused by the structural phase transition.