Characteristics of the Li+-Ion Conductivity of Li3R2(PO4)3 Crystals (R = Fe, Sc) in the Superionic State

Abstract

The characteristics of Li+-ion conductivity σdc of structural γ modifications of Li3R2(PO4)3 compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R2P3O12]∞3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li3R2(PO4)3 (ΔHσ = 0.31 ± 0.03 eV) is lower than in γ-Li3Fe2(PO4)3 (ΔHσ = 0.36 ± 0.03 eV). The conductivity of γ-Li3Fe2(PO4)3 (σdc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li3R2(PO4)3. A decrease in temperature causes a structural transformation of Li3R2(PO4)3 from the superionic γ modification (space group Pcan) through the intermediate metastable β modification (space group P21/n) into the “dielectric” α modification (space group P21/n). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li3Fe2(PO4)3 σdc gradually decreases in the temperature range TSIC = 430–540 K, whereas in Li3R2(PO4)3 σdc undergoes a jump at TSIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and ΔHσ values and the thermodynamics and kinetics of the superionic transition for Li3R2(PO4)3 are discussed.