Synthesis, structure and properties of Na4GeS4

Abstract

The Na2S-Na4GeS4 section of the binary system Na2S-GeS2 has been studied and the crystal structure of the Na4GeS4 compound was determined from single-crystal X-Ray diffraction data. The physical properties of Na4GeS4 were characterized by X-ray powder diffraction, TG/DTA, SEM-EDX, Raman spectroscopy and electrochemical impedance spectroscopy. Na4GeS4 crystallizes with a new type of structure in the non-centro-symmetric space group Im, a = 10.9822(9) Å, b = 29.652(2) Å, c = 20.0391(16) Å, β = 105.77(1) (°), V = 6279.96(90) Å3 and Z = 30. The Na4GeS4 asymmetric unit that contains 76 atoms is made up of 10 isolated GeS4 tetrahedra. The 31 sodium atoms are located between these tetrahedra. The distorted NaS6 octahedra, NaS5 trigonal bipyramids, NaS5 square pyramids and NaS4 tetrahedra share corners, edges or faces. The electrical conductivity [2.7 × 10−5 S cm−1 (Ea = 0.41 eV) at 25 °C] of the Na4GeS4 glass phase, prepared by mechanochemical synthesis, is two orders of magnitude higher than that of the glass-ceramic phase, prepared by the combination of mechanochemical and solid-state synthesizes [9.8 × 10−8 (Ea = 0.65 eV) at 25 °C].