Характеристики Li-=SUP=-+-=/SUP=--ионной проводимости кристаллов Li-=SUB=-3-=/SUB=-R-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=- (R=Fe,Sc) в суперионном состоянии

Abstract

AbstractThe characteristics of Li+-ion conductivity σdc of structural γ modifications of Li_3R_2(PO_4)_3 compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R_2P_3O_12]_∞^3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li_3R_2(PO_4)_3 (Δ H _σ = 0.31 ± 0.03 eV) is lower than in γ-Li_3Fe_2(PO_4)_3 (Δ H _σ = 0.36 ± 0.03 eV). The conductivity of γ-Li_3Fe_2(PO_4)_3 (σ_dc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li_3R_2(PO_4)_3. A decrease in temperature causes a structural transformation of Li_3R_2(PO_4)_3 from the superionic γ modification (space group Pcan ) through the intermediate metastable β modification (space group P 2_1/ n ) into the “dielectric” α modification (space group P 2_1/ n ). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li_3Fe_2(PO_4)_3 σdc gradually decreases in the temperature range T _SIC = 430–540 K, whereas in Li_3R_2(PO_4)_3 σdc undergoes a jump at T _SIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and Δ H _σ values and the thermodynamics and kinetics of the superionic transition for Li_3R_2(PO_4)_3 are discussed.