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

Abstract

Two different polymorphs of the new selenosilicate Na 4 Si 2 Se 6 were synthesized by solid-state reactions. The hightemperature polymorph Na 4 Si 2 Se 6 -tP24 crystallizes in the tetragonal space group P4 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 -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) Å 3 showing zweier single chains ∞ 1 [Si 2 Se 6 ] 4− . The lattice parameters of Na 4 Si 2 Se 6 -tP24 were determined by single-crystal X-ray diffraction, whereas those of Na 4 Si 2 Se 6 -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 Na 4 Si 2 Se 6 -oP48.

Sodium Selenotetrelates with Isolated TtSe4-Tetrahedra (Tt = Si, Ge, Sn): Synthesis, Crystal Structures, Thermal Behavior, DFT Modeling, and Na Ion Conductivities

“…Other selenotetrelates like Na 4 SnSe 4 - tP 18 and Na 4 Si 2 Se 6 - oP 48 show similar ionic conductivities (σ spec = 1.13 × 10 –8 Ω –1 cm –1 at 20 °C and σ spec = 1.4 × 10 –8 Ω –1 cm –1 at 50 °C).…”

“…Two selenide compounds were already checked for the sodium ion mobility. Impedance spectroscopy measurements reveal moderate to good ionic conductivities for Na 4 Si 2 Se 6 - oP 48 (1.4 × 10 –8 S cm –1 at 50 °C) and Na 4 SnSe 4 - tP 18 (1.13 × 10 –8 S cm –1 at 20 °C).…”

“…Adamantan-analogous units are found in Na 4 Si 4 Se 10 and Na 4 Ge 4 Se 10 . Even Tt – Tt bonds are present in hypoditetrelates to form ethane molecule analogous anions. − Edge-sharing SiSe 4 tetrahedra dimers are exclusively observed in Na 4 Si 2 Se 6 - tP 24 . Especially the physical and chemical characterization of sodium selenotetrelates is very scarce in the literature.…”

Trendbericht Festkörperchemie und Materialforschung 2024

“…44) Diese Chalkogensilikate sind die ersten mit zwei verschiedenen Anionen. Die Synthese erfolgt dabei sowohl über HT als auch über HP und resultiert in zwei neuen Strukturtypen: Die HT-Form Na 4 Si 2 Se 6 -(tP24) enthält kantenständige Si 2 Se 64 -Te-traederdimere, die HP-Form Na 4 Si 2 Se 6 -(oP48) zwei Einzelketten der Zusammensetzung ∞ Si 2 Se 6 ] 4-.…”