Synthesis and Characterization of NaLaS_2 and NaLaS_(1.5)Se_(0.5)Sodium Fast Ionic Conductors

Novel sodium fast ion conductors of NaLaS_2 and NaLaS_(1.5)Se_(0.5)were prepared by high tem- perature solid state reaction at 750℃.X-ray powder diffraction analysis confirms that NaLaS_(1.5)Se_(0.5) is isostructural to NaLaS_2 and NaLaSe_2(space group FM3-M).Their ionic conductive properties were investigated by AC impedance spectroscopy in the frequency range of 0.1Hz-100kHz at different tem- peratures.The conductivity of NaLaS_(1.5)Se_(0.5)is higher than that of NaLaS_2 at the same temperature. NaLaS_2 shows conductivity of 3.65×10~(-5)S·cm~(-1)at 30℃and 6.23×10~(-5)S.cm~(-1)at 90℃,respectively; while NaLaS_(1.5)Se_(0.5)displays higher conductivity of 8.11×10~(-5)S·cm~(-1)at 30℃and 1.37×10~(-4)S·cm~(-1) at 60℃,respectively.This is probably due to the stronger polarization power of Se~(2-),the larger radius of Se~(2-),and subsequently the enlargement of local lattice.