Li17Sb13S28: A New Lithium Ion Conductor and addition to the Phase Diagram Li2S-Sb2S3.

Abstract

Li17Sb13S28 was synthesized by solid-state reaction of stoichiometric amounts of anhydrous Li2S and Sb2S3. The crystal structure of Li17Sb13S28 was determined from dark-red single crystals at room temperature. The title compound crystallizes in the monoclinic space group C2/m (no. 12) with a=12.765(2) Å, b=11.6195(8) Å, c=9.2564(9) Å, β=119.665(6)°, V=1193.0(2) Å(3), and Z=4 (data at 20 °C, lattice constants from powder diffraction). The crystal structure contains one cation site with a mixed occupation by Li and Sb, and one with an antimony split position. Antimony and sulfur form slightly distorted tetragonal bipyramidal [SbS5E] units (E=free electron pair). Six of these units are arranged around a vacancy in the anion substructure. The lone electron pairs E of the antimony(III) cations are arranged around these vacancies. Thus, a variant of the rock salt structure type with ordered vacancies in the anionic substructure results. Impedance spectroscopic measurements of Li17Sb13S28 show a specific conductivity of 2.9×10(-9) Ω(-1) cm(-1) at 323 K and of 7.9×10(-6) Ω(-1) cm(-1) at 563 K, the corresponding activation energy is EA=0.4 eV below 403 K and EA=0.6 eV above. Raman spectra are dominated by the Sb-S stretching modes of the [SbS5] units at 315 and 341 cm(-1) at room temperature. Differential thermal analysis (DTA) measurements of Li17Sb13S28 indicate peritectic melting at 854 K.