New layered supertetrahedral compounds T2-MSiAs2, T3-MGaSiAs3 and polytypic T4-M4Ga5SiAs9 (M = Sr, Eu)

The new supertetrahedral compounds MSiAs2, MGaSiAs3 and mC/tI-M4Ga5SiAs9 (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 M4Ga5SiAs9 are dimorphic and form new structure types in the space groups C2/c and I4(1)/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 super-tetrahedra. Measurements of the optical band gap, electrical resistivity and Hall Effect support the semiconducting nature of M4Ga5SiAs9. Magnetization measurements confirm Eu2+ in Eu4Ga5SiAs9 and indicate ferromagnetism below T= 2K.