Eu 5 Ga 2 As 6 and Ba 5 In 2 As 6– x Sb x : new As-containing members of the Sr 5 Al 2 Sb 6 structure type

The ternary Zintl arsenide Eu 5 Ga 2 As 6 and two members of the Ba 5 In 2 As 6– x Sb x [ x  = 0.49 (2) and 2.92 (2)] solid solution series were obtained from Pb-flux reactions and characterized by single-crystal X-ray diffraction methods. They adopt the uncommon Sr 5 Al 2 Sb 6 structure type and represent the first As-bearing compounds adopting this structure type. The polyanionic substructure consists of 1D chains of edge- and corner-sharing [ TrPn 4 ] ( Tr  = Ga, In; Pn  = As, Sb) tetrahedra extending along the [100] direction, with pendant [ Pn 2 ] dumbbells. Non-random As/Sb site preferences within the polyanionic substructure of Ba 5 In 2 As 6– x Sb x are also discussed. Formal electron counting supports valence-precise Zintl notation for all reported compositions. First-principles electronic structure calculations for Eu 5 Ga 2 As 6 reveal a moderate indirect band gap of ∼0.8 eV. Comparative calculations for the chemically related Eu 3 GaAs 3 phase, also isolated during the present study, predict a narrow direct band gap of ∼0.2 eV and provide additional context for the bonding and electronic structure of Eu 5 Ga 2 As 6 .