Encapsulation of Ba in InSb Framework Introduces Chirality in Clathrate-Like BaIn ₄ Sb ₄

The discovery of Zintl compounds remains a powerful strategy for identifying materials with tunable electronic and thermal transport properties. During a concerted search for new inorganic clathrates with In–Sb frameworks, we discovered BaIn4Sb4. The composition of this phase deviates from that expected for a type-I clathrate with tetrahedral coordination of all In and Sb atoms (Ba8In31Sb15). Instead, in the chiral structure of BaIn4Sb4 (space group P3121, No. 152), a part of the In atoms have a trigonal planar coordination of 1In + 2Sb, forming Sb2–In–In–Sb2 nonplanar fragments isostructural to diborane(4) B2H4 with D2d symmetry. Ba atoms are located inside 16-vertex In8Sb8 polyhedra, which share vertices and edges to form a chiral framework around the 31 screw axes. The title compound is electron-balanced, [Ba2+][In2+]2[In3+]2[Sb3–]4, which was confirmed by characterization of the charge and heat transport properties. BaIn4Sb4 exhibits a low thermal conductivity and high Seebeck coefficient, suggesting its untapped potential for thermoelectric applications. Density functional theory (DFT) calculations indicate that chemical doping may enhance carrier concentration and improve the originally low electrical conductivity, thus enhancing thermoelectric performance.