Enhanced sodium ion mobility in sodium tellurosilicates and crystal structures of Na 4 SiTe 4 and Na 10 Si 2 Te 9 with isolated [SiTe 4 ] 4- tetrahedra and isolated Te 2- anions.

The sodium tellurosilicates Na 4 SiTe 4 , Na 10 Si 2 Te 9 , Na 6 Si 2 Te 6 and Na 8 Si 4 Te 10 were synthesized by ball milling and subsequent high temperature solid state reactions and analyzed by electrochemical impedance spectroscopy. All compounds show moderate to remarkable sodium ion conductivities. The crystal structures of the novel materials Na 4 SiTe 4 and Na 10 Si 2 Te 9 were determined by X-ray diffraction. Both compounds represent new structure types with isolated SiTe 4 tetrahedra. The crystal structure of Na 10 Si 2 Te 9 exhibits a single telluride anion besides two SiTe 4 tetrahedra. Na 4 SiTe 4 crystallizes in the cubic space group Pa 3̄ (no. 205) with lattice parameters a = 13.0312(1) Å and V = 2212.84(2) Å 3 . Na 10 Si 2 Te 9 crystallizes in the orthorhombic space group Pna 2 1 (no. 33) with lattice parameters a = 12.8235(7) Å, b = 14.8398(8) Å, c = 12.9530(7) Å and V = 2464.9(2) Å 3 . The presence of two different anionic units makes this compound stand out from other alkali chalcogenotetrelates. The electronic structure of all compounds was investigated by density functional theory, revealing their semiconducting behaviour.