Abstract The new supertetrahedral compounds MSiAs2, MGaSiAs3 and mC/tI-M 4Ga5SiAs9 (M = Sr, Eu) have been synthesized by solid-state reactions at high temperatures. The structures were determined by single crystal or powder X-ray diffraction. MSiAs2 and MGaSiAs3 crystallize in the monoclinic TlGaSe2- and RbCuSnS3-type structures, respectively (space group C2/c). These are topologically hierarchical variants of the tetragonal HgI2-type structure with stacked layers of T2 or T3 supertetrahedra. The T4 compounds M 4Ga5SiAs9 are dimorphic and form new structure types in the space groups C2/c and I41/amd, respectively. The latter exhibits coinciding layer stacking as known from tetragonal HgI2. The T4 compounds close the gap between the longer known T2 types and the recently reported compounds with T5 and T6 supertetrahedra. Measurements of the optical band gap, electrical resistivity and Hall Effect support the semiconducting nature of M 4Ga5SiAs9. Magnetization measurements confirm Eu2+ in Eu4Ga5SiAs9 and indicate ferromagnetism below T = 2 K.

中文翻译：

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新型层状超四面体化合物 T2-MSiAs2、T3-MGaSiAs3 和多型 T4-M4Ga5SiAs9 (M = Sr, Eu)

摘要 通过高温固相反应合成了新的超四面体化合物 MSiAs2、MGaSiAs3 和 mC/tI-M 4Ga5SiAs9 (M = Sr, Eu)。通过单晶或粉末X射线衍射确定结构。MSiAs2 和 MGaSiAs3 分别在单斜 TlGaSe2 和 RbCuSnS3 型结构中结晶（空间群 C2/c）。这些是具有 T2 或 T3 超四面体堆叠层的四方 HgI2 型结构的拓扑分层变体。T4 化合物 M 4Ga5SiAs9 是双晶的，分别在空间群 C2/c 和 I41/amd 中形成新的结构类型。后者表现出如从四方 HgI2 已知的重合层堆叠。T4 化合物缩小了已知的较长 T2 类型与最近报道的具有 T5 和 T6 超四面体的化合物之间的差距。光学带隙、电阻率和霍尔效应的测量支持 M 4Ga5SiAs9 的半导体性质。磁化测量证实了 Eu4Ga5SiAs9 中的 Eu2+ 并表明铁磁性低于 T = 2 K。