Crystal structure of hexasodium heptaselenodisilicate, Na6Si2Se7

Synthesis and Characterization of Na4Si2Se6-tP24 and Na4Si2Se6-oP48, Two Polymorphs with Different Anionic Structures

Two different polymorphs of the new selenosilicate Na₄Si₂Se₆ were synthesized by solid-state reactions. The high-temperature polymorph Na₄Si₂Se₆-tP24 crystallizes in the tetragonal space group P4₂/mcm (No. 132) with lattice parameters a = 7.2793(2) Å, c = 12.4960(4) Å, and V = 662.14(3) ų. The main structural motifs are isolated Si₂Se₆ units of two edge-sharing SiSe₄ tetrahedra. The high-pressure/low-temperature polymorph Na₄Si₂Se₆-oP48 crystallizes in the orthorhombic space group Pbca (No. 61) with lattice parameters a = 12.9276(1) Å, b = 15.9324(1) Å, c = 6.0349(1) Å, and V = 1243.00(2) ų showing zweier single chains _∞¹[Si₂Se₆]^4-. The lattice parameters of Na₄Si₂Se₆-tP24 were determined by single-crystal X-ray diffraction, whereas those of Na₄Si₂Se₆-oP48 were investigated by powder X-ray diffraction. Both modifications crystallize in new structure types. An energetic comparison of the two polymorphs and further hypothetical structure types was carried out by density functional theory modeling. Calculations reveal that the polymorphs are very close in energy (ΔE = 3.4 kJ mol^-1). Impedance spectroscopic measurements show ionic conductivity (σ_spec = 1.4 × 10^-8 S cm^-1 at 50 °C and 6.8 × 10^-6 S cm^-1 at 200 °C) with an activation energy of E_A = 0.54(2) eV for Na₄Si₂Se₆-oP48.