The Electrical Transport Properties Of Superionic Conductor AgX（X=Cl,Br） And YF₃（Y=La,Lu） Under High Pressure

Superionic conductors are solid electrolytes that have extraordinarily high ionic conductivity（≥0.01S/cm）.They having huge advantages in safety and reliability over liquid electrolytes,and present tremendous potential in technical fields of energy source and solid ionic device.In this paper,the electrical transport behaviors of superionic conductors AgCl,AgBr,La F₃ and Lu F₃ were systematically studied under high pressure by combining the electrochemistry alternating current impedance spectra measurements and first-principles calculations.The results of study are as follows:1.Ionic transport behaviors of superionic conductor AgCl have been revealed with electrochemistry alternating current impedance spectra measurement under high pressures and high temperature.From impedance spectra measurement under high pressure,AgCl always presented ionic conducting under experimental pressures.But electronic conduction can coexist with ionic conduction within the pressure range from 6.7 to 9.3 GPa,the emergence of electronic conduction is induced by pressure-induced rocksalt-structure to KOH-type structure and then to T1I-type structure transformation.The conductivity of AgCl decreases by 3 orders of magnitude under compression,indicating that Ag⁺ions migrations are suppressed by high pressure.A parameter,f_W,was defined as the starting frequency at which Ag⁺ions begin to show obvious long-distance diffusion in AgCl.The higher the f_W value,the faster the Ag⁺ion diffusion.The f _W showed similar trend with the ionic conductivity under high pressures,indicating that the speed of Ag⁺ions diffusion slows down as increasing pressure.Unlike AgI,Ag⁺ions diffusion in AgCl is controlled by indirect-interstitial mechanism.Due to the stronger ionic bonds and larger lattice deformation,Ag⁺ions diffusion in rigid Cl^-lattice is more difficult than in I^-lattice under high pressures.From impedance spectra measurement under high pressure and high temperature,the ionic conductivity of AgCl increases by 2-4 orders of magnitude under high temperature.It is indicated that Ag⁺ions migrations are accelerated by temperature even under compression.2.The electrical transport behaviors of superionic conductor AgBr was systematically studied under high pressure and high temperature with alternating current impedance spectroscopy measurements and first-principles calculations.From impedance spectra measurement under high pressure,a pressure-induced abnormal ionic-polaronic-ionic transition sequence was found.The ionic to polaronic transition at 5.0 GPa occurs with the absence of structure phase transition.The polaronic to ionic transition at 8.6 GPa occurs with the structure phase transition from rocksalt-structure to KOH-type structure.The conductivity of AgBr decreases by 2orders of magnitude under compression indicating that the speed of Ag⁺ions diffusion slows down as increasing pressure.From impedance spectra measurement under high pressure and high temperature,at 0.4 GPa 485 K,the conductivity of AgBr reaches to superionic states（0.01S/cm）.The temperature of superionic states under high pressure is much less than 555 K at ambient condition.At 7.3 GPa 443 K,temperature-induced polaronic-ionic transition indicates that temperature can renew the migration of Ag⁺ions even though the transporting mechanism of AgBr has been changed by high pressure.The increasing E_a under high pressure means that Ag⁺ions need overcome higher energy barriers,which is the reason that the conductivity of AgBr decreases with increasing pressure.Based on first-principles calculations,a localized-electron-soup model was proposed to explain the physical origin of the ionic-polaronic transition.In the model,more localized electrons around Br atoms are pressed into interstitial space and simultaneously polarons are formed between Ag⁺ions and localized electrons background at 5.0 GPa.Therefore,the diffusion of Ag⁺ions are effectively screened by the movement of localized electrons background from its equilibrium position as if beans are completely trapped in a cup of thick soup.3.The electrical transport behaviors of superionic conductor LaF₃ was systematically studied under high pressure with alternating current impedance spectroscopy measurements and first-principles calculations.From impedance spectra measurement under high pressure,LaF₃ presented ionic conducting under ambient conditions.At 15.5 GPa,a pressure-induced ionic to electronic coexist with ionic conduction transition was found with the pressure-induced structure phase transition from P3c1 to Cu₃Ti-type.The ionic conductivity of LaF₃ decreases by one order of magnitude upon compression indicating that the speed of F^-ions diffusion slows down as increasing pressure.Furthermore,from ambient to 5.0 GPa,the accelerated speed of F^-ions,the decreased ionic conductivity,indicates that the carrier concentration decreased under compression.From first-principles calculations,P3c1 structured LaF₃ always presented ionic conducting,no charge distribution during the interstitial lattice.The distance of La⁺-La⁺,F^--F^-and La⁺-F^-decrease with increasing pressures,indicating that F^-ions diffusion in rigid La⁺lattice is difficult with the shrinking distances.4.The electrical transport behaviors of superionic conductor LuF₃ was systematically studied under high pressure with alternating current impedance spectroscopy measurements and first-principles calculations.From impedance spectra measurement under high pressure,LuF₃ always presented ionic conducting under experimental pressures with the absence of conductive mechanism transition.From ambient to 13.0 GPa,the ionic conductivity of LuF₃ is decreased with increasing pressures.From 13.0 GPa to 28.5 GPa,the ionic conductivity of LuF₃ increasing.The abrupt transition with ionic conductivity at 13.0 GPa may be attributed to pressure-induced structural phase transition.The ionic conductivity of LuF₃ increased upon compression,indicating that our assumption is correct which the conductivity increased and no change in conductive mechanism under high pressure in type I superionic conductor.From first-principles calculations,Lu F₃ always presented ionic conducting,no charge distribution during the interstitial lattice,from 0 GPa to 13.0GPa.The atomic distance is decreased under high pressure.But the larger distance in Lu⁺-Lu⁺can satisfy F^-ions diffusion in rigid Lu⁺lattice.