Synthesis and Characterization of Na 4 Si 2 Se 6 - tP 24 and Na 4 Si 2 Se 6 - oP 48, Two Polymorphs with Different Anionic Structures.

Two different polymorphs of the new selenosilicate Na 4 Si 2 Se 6 were synthesized by solid-state reactions. The high-temperature polymorph Na 4 Si 2 Se 6 - tP 24 crystallizes in the tetragonal space group P 4 2 / mcm (No. 132) with lattice parameters a = 7.2793(2) Å, c = 12.4960(4) Å, and V = 662.14(3) Å 3 . The main structural motifs are isolated Si 2 Se 6 units of two edge-sharing SiSe 4 tetrahedra. The high-pressure/low-temperature polymorph Na 4 Si 2 Se 6 - oP 48 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) Å 3 showing zweier single chains ∞ 1 [Si 2 Se 6 ] 4- . The lattice parameters of Na 4 Si 2 Se 6 - tP 24 were determined by single-crystal X-ray diffraction, whereas those of Na 4 Si 2 Se 6 - oP 48 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 (Δ E = 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 Na 4 Si 2 Se 6 - oP 48.