Enhanced sodium ion mobility in sodium tellurosilicates and crystal structures of Na4SiTe4 and Na10Si2Te9 with isolated SiTe44- tetrahedra and isolated Te2- anions

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 Pa3̄ (no. 205) with lattice parameters a = 13.0312(1) Å and V = 2212.84(2) Å3. 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) Å3. 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.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 Pa3̄ (no. 205) with lattice parameters a = 13.0312(1) Å and V = 2212.84(2) Å3. 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) Å3. 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.