ABSTRACT
The solid-state compounds M15Tr22As32 and M3Ga6As8 (M = Sr, Eu; Tr = Ga, In) were synthesized by heating the elements, and their crystal structures were determined by single-crystal and powder X-ray diffraction (space group C2/c). The structures are hierarchical variants of the HgI2 type and consist of layers of polymeric T5 (M15Tr22As32) or T6 supertetrahedra (M3Ga6As8), separated by strontium or europium cations. These compounds constitute hitherto unknown GaAs- or InAs-based supertetrahedral structures and represent the first binary vacancy-free T5 and T6 supertetrahedra. Vacancies or mixed-metal strategies for charge compensation, as known from related chalcogenides, are not required for supertetrahedra based on charge-neutral GaAs or InAs. Optical band gap, resistivity, and Hall-effect measurements together with DFT calculations reveal that the supertetrahedral compounds are direct band gap semiconductors similar to binary GaAs or InAs. Magnetic susceptibility measurements confirm Eu2+ in Eu15Ga22As32, Eu15In22As32, and Eu3Ga6As8 and indicate antiferromagnetic ordering below 10 K.
