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

Selenotetrelate compounds Na4TtSe4 (Tt = Si, Ge, Sn) were synthesized by solid-state reactions. A new modification of Na4SiSe4 (Na4SiSe4-cP72), which crystallizes in the cubic space group P4̅3n (no. 218) with a = 12.130(1) Å and V = 1784.453(5) Å3, and a new modification of Na4SnSe4 (Na4SnSe4-tI216), which crystallizes in the tetragonal space group I41/acd (no. 142) with a = 14.4053(4) Å, c = 28.5751(8) Å and V = 5929.7(3) Å3, were discovered. All of the title compounds exhibit moderate to good sodium ion conductivities, as revealed by electrochemical impedance spectroscopy. The formation reaction of Na4SiSe4 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 Na4SiSe4 and Na4SnSe4 energetically. Further modifications of Na4SiSe4 and Na4GeSe4 seem plausible, as revealed by density functional theory modeling. The stability of the hypothetic modifications was examined by phonon dispersion calculations.

四硒代碳族元素合钠（Na₄TtSe₄，Tt = Si、Ge、Sn）类化合物通过固相反应合成。本研究发现了两种全新晶型：其一为Na₄SiSe₄的新晶型Na₄SiSe₄-cP72，其结晶于立方晶系空间群P4̅3n（编号218），晶胞参数a = 12.130(1) Å，晶胞体积V = 1784.453(5) ų；其二为Na₄SnSe₄的新晶型Na₄SnSe₄-tI216，其结晶于四方晶系空间群I4₁/acd（编号142），晶胞参数a = 14.4053(4) Å、c = 28.5751(8) Å，晶胞体积V = 5929.7(3) ų。所有标题化合物均表现出中等至优异的钠离子电导率，该结果经电化学阻抗谱（Electrochemical Impedance Spectroscopy）测试证实。通过对球磨后的反应混合物开展高温X射线粉末衍射测试，本研究进一步探究了Na₄SiSe₄的生成反应过程。采用基于密度泛函理论（Density Functional Theory, DFT）的量子化学计算方法，从能量维度对比了Na₄SiSe₄与Na₄SnSe₄的不同晶型。密度泛函理论模拟结果显示，Na₄SiSe₄与Na₄GeSe₄存在其他潜在晶型的可能性。研究者通过声子色散计算（Phonon Dispersion Calculations）对上述假想晶型的稳定性进行了验证。