Supertetrahedral Layers Based on GaAs or InAs

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.