Mechanochemical Synthesis and Structure of the Alkali Metal Magnesium Chalcogenide Na6MgS4

The new binary sodium magnesium sulfide was preparedby the mechanochemicalsynthesis route from Na2S and MgS as starting materials.Na6MgS4 is extremely sensitive and partiallydecomposes in the presence of oxygen traces. With the use of an excessof MgS in the milling process, the molar ratio of the impurities wassuccessfully decreased from 38% (Na2S + MgO) to 13% MgO.The crystal structure and properties were characterized by X-ray powderdiffraction, thermogravimetry/differential thermal analysis, scanningelectron microscopy-energy-dispersive X-ray spectroscopy, andelectrochemical impedance spectroscopy. The Rietveld refinement confirmedthat Na6MgS4 is isostructural to Na6ZnO4. The compound crystallized in the hexagonal systemin the non-centro-symmetric space group P6(3) mc (No. 186) with a = 9.0265(1), c = 6.9524(1) & ANGS;, V = 490.58(1) & ANGS;(3), and Z = 2. The structure consisted of awurtzite-like 3D framework built up of corner-sharing MgS4 and NaS4 tetrahedra, with 3/4 of the tunnels, parallelto the c axis, filled with octahedrally coordinatedsodium atoms. The ionic conductivity of the composite material (87%Na6MgS4 + 13% MgO) being low (4.4 x 10(-8) S cm(-1) with E (a) = 0.56 eV), indium-doped samples Na6-x x Mg1-x In x S4 (x = 0.05, 0.1) were prepared by the mechanochemical synthesisroute. These samples also contained 13% MgO. Their ionic conductivitiesof 9.3 x 10(-8) S cm(-1) (E (a) = 0.51 eV) and 2.5 x 10(-7) S cm(-1) (E (a) = 0.49eV) at 25 & DEG;C for x = 0.05 and 0.1, respectively,were higher than the ionic conductivity of the undoped sample. The new materials Na6-x Mg1-x In x S4 (x = 0, 0.05, and0.1) thatare isostructural to Na6ZnO4 were prepared bymechanochemical synthesis. Their electrical conductivities were estimatedby electrochemical impedance spectroscopy to check their suitabilityas solid electrolytes for sodium-ion batteries. At room temperature,the electrical conductivity [2.5 x 10(-7) S cm(-1) (E (a) = 0.49 eV)] of theindium-doped sample (x = 0.1) was found to be morethan five times higher than that of the undoped Na6MgS4 sample (x = 0).

Automated summary

A new binary sodium magnesium sulfide (Na6MgS4) was synthesized by mechanochemical synthesis from Na2S and MgS. Na6MgS4 is highly sensitive to oxygen traces and partially decomposes. Excess MgS was used in the milling process to successfully reduce the impurity ratio from 38% (Na2S + MgO) to 13% MgO. The crystal structure and properties of Na6MgS4 were characterized by various techniques, and it was found to be isostructural to Na6ZnO4. Additionally, indium-doped samples (Na6-x Mg1-x In x S4) with improved ionic conductivities were prepared.