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

Abstract

Selenotetrelate compounds Na 4 TtSe 4 (Tt = Si, Ge, Sn) were synthesized by solid-state reactions. A new modification of Na 4 SiSe 4 (Na 4 SiSe 4 -cP72), which crystallizes in the cubic space group P4̅ 3n (no. 218) with a = 12.130(1) Å and V = 1784.453(5) Å 3 , and a new modification of Na 4 SnSe 4 (Na 4 SnSe 4 -tI216), which crystallizes in the tetragonal space group I4 1 /acd (no. 142) with a = 14.4053(4) Å, c = 28.5751(8) Å and V = 5929.7(3) Å 3 , were discovered. All of the title compounds exhibit moderate to good sodium ion conductivities, as revealed by electrochemical impedance spectroscopy. The formation reaction of Na 4 SiSe 4 was further investigated by high-temperature X-ray powder diffraction of the ball-milled reaction mixture. Density functional-based quantum chemical calculations were performed to compare the different modifications of Na 4 SiSe 4 and Na 4 SnSe 4 energetically. Further modifications of Na 4 SiSe 4 and Na 4 GeSe 4 seem plausible, as revealed by density functional theory modeling. The stability of the hypothetic modifications was examined by phonon dispersion calculations.