Study On The Electrical Transport Properties Of Bi₂O₃ Under High Pressure

Bismuth oxide?Bi₂O₃?is a kind of solid electrolytes with perspective applications in ceramic oxygen generators,solid oxide fuel cells and oxygen sensors.Currently,solid electrolytes have attracted much attention due to their extraordinarily high ionic conductivity that can be compared with liquid electrolyte(1×10^-66 S·cm^-1)and low ionic conductivity activation energy??0.40 eV?at a specific temperature range.Because the pressure can change the crystal structure,electronic structure and energy band by means of reducing the atom spacing of the materials,new properties will appear in the materials under high pressure.In this paper,based on the diamond anvil cell?DAC?,the ionic transport properties and the lattice dynamics of Bi₂O₃ under high pressures were investigated with the methods of Raman measurements,high pressure synchrotron radiation X-ray diffraction technique and in situ alternate-current?AC?impedance spectroscopy measurement.The results are listed as follows:Firstly,with the synchrotron radiation X-ray diffraction and GSAS refinement,we found there is no pressure-induced structural phase transition.Through the Raman measurements,we found a tiny shoulder emerges on the left side of Ag-1 mode at 5.3GPa.However,with further increasing pressure,Ag-4 mode and the tiny shoulder of Ag-1 mode disappear at 11.2 GPa.So we think the appearance of tiny shoulder at 5.3GPa and the disappearance of Ag-4 mode and tiny shoulder at 11.2 GPa are attributed to pressure-induced lattice distortion.Secondly,the electrical transport properties of Bi₂O₃ are studied by high pressure in situ AC impedance spectroscopy.From ambient pressure to 8.5 GPa,AC impedance spectroscopy results show that Bi₂O₃ exhibits a typical ionic transporting character and the main conducting ion are O^2-ions.The spectra consist of two parts:a semicircle at high frequencies region and an inclined upward straight line with a slope of?/4 at low frequencies region.From ambient pressure to 4.9 GPa,the total resistance increases with the increasing pressure.From 4.9 GPa to 8.5 GPa,the total resistance decreases with the increasing pressure.When the applied pressure exceeding to 9.0 GPa,the shapes of the spectra change dramatically that the inclined straight line bending toward Z'axis obviously.It suggests that electronic conduction begins to emerge.Finally,by further fitting and analyzing the impedance spectrum data,we have obtained pressure-dependent electron resistance,ionic resistance in bulk and grain boundary respectively,relaxation and conductivity.We find that all the electrical parameters show discontinuous changes at around 5.0 GPa and 9.0 GPa.Generally,changing in lattice vibration modes will alter the electrical transport properties of materials,because the charge carriers is very sensitive to the scattering from crystal lattice.Comparing with the Raman spectroscopy,we believed that the change in electrical parameters at approximately 5.0 GPa is related to the pressure-induced lattice distortion.Because of further distortion of the crystal lattice as increasing pressure which is indicated by Raman spectra,a few of originally localized electrons can participate in the electrical transportation.As a result,electronic conduction is emerged and the electrical transportation in Bi₂O₃ is dominated jointly by both ionic and electronic conductions at above 9.0 GPa.Meanwhile,when electronic conduction begins to emerge,we found that the transfer resistance of electrons is 2-3 orders of magnitude larger than the ionic transfer resistance.The ionic and electronic transportations have a parallel relationship in the equivalent circuit,so it illustrates that the transportation process is dominated by ions.The impedance spectra data indicates that electrical transportation process is ionic conduction at high frequencies region.With the frequency decreasing to zero,the imaginary part of impedance Z will go to zero correspondingly and electrical transport process is electronic conduction.Thus the pressure has endowed Bi₂O₃ with an unexpected function that its electrical transportation property can be tuned freely between the ionic and electronic by applied voltage frequencies.