Enhanced sodium ion mobility in sodium tellurosilicates and crystal structures of Na4SiTe4 and Na10Si2Te9 with isolated [SiTe4]4− tetrahedra and isolated Te2− anions

Abstract

The sodium tellurosilicates Na₄SiTe₄, Na₁₀Si₂Te₉, Na₆Si₂Te₆ and Na₈Si₄Te₁₀ 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 Na₄SiTe₄ and Na₁₀Si₂Te₉ were determined by X-ray diffraction. Both compounds represent new structure types with isolated SiTe₄ tetrahedra. The crystal structure of Na₁₀Si₂Te₉ exhibits a single telluride anion besides two SiTe₄ tetrahedra. Na₄SiTe₄ crystallizes in the cubic space group Pa (no. 205) with lattice parameters a = 13.0312(1) Å and V = 2212.84(2) ų. Na₁₀Si₂Te₉ crystallizes in the orthorhombic space group Pna2₁ (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) ų. 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.