Mechanochemical Synthesis and Structure of the Alkali Metal Magnesium Chalcogenide Na₆MgS₄

The new binary sodium magnesium sulfide was prepared by the mechanochemical synthesis route from Na₂S and MgS as starting materials. Na₆MgS₄ is extremely sensitive and partially decomposes in the presence of oxygen traces. With the use of an excess of MgS in the milling process, the molar ratio of the impurities was successfully decreased from 38% (Na₂S + MgO) to 13% MgO. The crystal structure and properties were characterized by X-ray powder diffraction, thermogravimetry/differential thermal analysis, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and electrochemical impedance spectroscopy. The Rietveld refinement confirmed that Na₆MgS₄ is isostructural to Na₆ZnO₄. The compound crystallized in the hexagonal system in the non-centro-symmetric space group P6₃mc (No. 186) with a = 9.0265(1), c = 6.9524(1) Å, V = 490.58(1) ų, and Z = 2. The structure consisted of a wurtzite-like 3D framework built up of corner-sharing MgS₄ and NaS₄ tetrahedra, with 3/4 of the tunnels, parallel to the c axis, filled with octahedrally coordinated sodium atoms. The ionic conductivity of the composite material (87% Na₆MgS₄ + 13% MgO) being low (4.4 × 10^-8 S cm^-1 with E_a = 0.56 eV), indium-doped samples Na_6-□Mg_1-InS₄ (x = 0.05, 0.1) were prepared by the mechanochemical synthesis route. These samples also contained 13% MgO. Their ionic conductivities of 9.3 × 10^-8 S cm^-1 (E_a = 0.51 eV) and 2.5 × 10^-7 S cm^-1 (E_a = 0.49 eV) at 25 °C for x = 0.05 and 0.1, respectively, were higher than the ionic conductivity of the undoped sample.