A unique framework in BaGa₂Sb₂: A new Zintl phase with large tunnels

BaGa₂Sb₂ was obtained from a direct element combination reaction in a sealed graphite tube at 950 °C, and its structure was determined by single-crystal X-ray diffraction methods. It crystallizes in the orthorhombic space group Pnma (No. 62) with a = 25.454(5) Å, b = 4.4421(9) Å, c = 10.273(6) Å, and Z = 8. The anionic [Ga₂Sb_6/3]^2- framework is assembled by the ethane-like dimeric [Sb₃Ga-GaSb₃] units sharing Sb atoms, forming parallel tunnels with a 26-membered ring cross section. These tunnels are filled with Ba atoms. The three-dimensional [Ga₂Sb_6/3]^2- framework features a new structure type. The compound satisfies the classical Zintl concept. Band structure calculations indicate that the material is a semiconductor, and this is confirmed by spectroscopic experiments which show E_g ∼ 0.35 eV. The calculations also suggest that the structure is stabilized by strong Ga-Ga covalent bonding interactions. Polycrystalline ingots of BaGa₂Sb₂ show room-temperature electrical conductivity of ∼65 S/cm and a Seebeck coefficient of +65 μV/K.