Characteristics of the [Li.sup.+]-Ion Conductivity of [Li.sub.3][R.sub.2][ in the Superionic State

The characteristics of [Li.sup.+]-ion conductivity [[sigma].sub.dc] of structural [gamma] modifications of [Li.sub.3][R.sub.2][(P[O.sub.4]).sub.3] compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [[[R.sub.2][P.sub.3][O.sub.12]].sup.3- .sub.[infinity]] affects the [[sigma].sub.dc] value and the [[sigma].sub.dc] activation enthalpy in these compounds. The ion transport activation enthalpy in [gamma]- [Li.sub.3][Sc.sub.2][(P[O.sub.4]).sub.3] ([DELTA][H.sub.[sigma]] = 0.31 [+ or -] 0.03 eV) is lower than in [gamma]-[Li.sub.3][Fe.sub.2][(P[O.sub.4]).sub.3] ([DELTA][H.sub.[sigma]] = 0.36 [+ or -] 0.03 eV). The conductivity of [gamma]-[Li.sub.3][Fe.sub.2][(P[O.sub.4]).sub.3] ([[sigma].sub.dc] = 0.02 S/cm at 573 K) is twice as high as that of [gamma]-[Li.sub.3][Sc.sub.2][(P[O.sub.4]).sub.3]. A decrease in temperature causes a structural transformation of [Li.sub.3][R.sub.2][(P[O.sub.4]).sub.3] from the superionic y modification (space group Pcan) through the intermediate metastable [beta] modification (space group [P2.sub.1]/n) into the "dielectric" [alpha] modification (space group [P2.sub.1]/n). Upon cooling, [[sigma].sub.dc] for both phosphates decreases by a factor of about 100 at the superionic [T.sub.SIC] transition. In [Li.sub.3][Fe.sub.2][(P[O.sub.4]).sub.3][[sigma].sub.dc] gradually decreases in the temperature range [T.sub.SIC] = 430- 540 K, whereas in [Li.sub.3][Sc.sub.2][(P[O.sub.4]).sub.3][[sigma].sub.dc] undergoes a jump at [T.sub.SIC] = 540 [+ or -] 25 K. Possible crystallochemical factors responsible for the difference in the [[sigma].sub.dc] and [DELTA][H.sub.[sigma]] values and the thermodynamics and kinetics of the superionic transition for [Li.sub.3][R.sub.2][(P[O.sub.4]).sub.3] are discussed. DOI: 10.1134/S106378341805030X