Quaternary Pnictides with Complex, Noncentrosymmetric Structures. Synthesis and Structural Characterization of the New Zintl Phases Na₁₁Ca₂Al₃Sb₈, Na₄CaGaSb₃, and Na₁₅Ca₃In₅Sb₁₂

Three new Zintl phases, Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na11Ca2Al3Sb8 crystallizes with its own structure type (Pearson index oP48) with the primitive orthorhombic space group Pmn21 (No. 31). The structure is best viewed as [Al3Sb8]15– units of fused AlSb4 tetrahedra, a novel type of Zintl ion, with Na+ and Ca2+ cations that solvate them. Na4CaGaSb3 also crystallizes in its own type with the primitive monoclinic space group Pc (No. 7; Pearson index mP36), and its structure boasts one-dimensional [GaSb3]6– helical chains of corner-shared GaSb4 tetrahedra. The third new compound, Na15Ca3In5Sb12, crystallizes with the recently reported K2BaCdSb2 structure type (space group Pmc21; Pearson index oP12). The Na15Ca3In5Sb12 structure is based on polyanionic layers made of corner-shared InSb4 tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials.本研究通过固相反应（solid-state reactions）合成了三种新型齐特爾相（Zintl phases）：Na₁₁Ca₂Al₃Sb₈、Na₄CaGaSb₃与Na₁₅Ca₃In₅Sb₁₂，并借助单晶X射线衍射（single-crystal X-ray diffraction）解析了其晶体结构。 Na₁₁Ca₂Al₃Sb₈以独有结构型结晶，其皮尔逊指数（Pearson index）为oP48，所属原始正交晶系空间群为Pmn2₁（编号31）。该结构可最优地被描述为由稠合AlSb₄四面体构成的[Al₃Sb₈]¹⁵–单元——一种新型齐特爾离子（Zintl ion），并由Na⁺与Ca²⁺阳离子进行溶剂化稳定。 Na₄CaGaSb₃同样以独有结构型结晶，所属原始单斜晶系空间群为Pc（编号7；皮尔逊指数为mP36），其结构具有由共顶点GaSb₄四面体构成的一维[GaSb₃]⁶–螺旋链。 第三种新型化合物Na₁₅Ca₃In₅Sb₁₂采用近期报道的K₂BaCdSb₂结构型结晶，空间群为Pmc2₁，皮尔逊指数为oP12。Na₁₅Ca₃In₅Sb₁₂的结构以共顶点InSb₄四面体构成的多阴离子层为基础，约六分之一的In位点以统计分布方式存在空位。 上述三种晶体结构均与TiNiSi结构型存在相似性，本文对三者与TiNiSi结构型之间的对应关联展开了讨论。本研究采用紧束缚线性muffin-tin轨道原子球近似（tight-binding linear muffin-tin orbital atomic sphere approximation）方法开展电子能带结构计算，结果表明三种化合物均具有较小的带隙（band gap），暗示此类材料具备本征半导体特性。