β-Yb2CdSb2—A Complex Non-Centrosymmetric Zintl Polymorph

The ternary Zintl phase, Yb2CdSb2, was discovered to exist in two different polymorphic forms. In addition to the orthorhombic α-Yb2CdSb2 (space group Cmc21) known for its excellent thermoelectric properties, we present the synthesis and characterization of the crystal and electronic structure of its monoclinic variant, β-Yb2CdSb2. Structural characterization was performed with the single-crystal X-ray diffraction method. β-Yb2CdSb2 crystallizes in a monoclinic crystal system with the non-centrosymmetric space group Cm (Z = 33, a = 81.801(5) Å, b = 4.6186(3) Å, c = 12.6742(7) Å, β = 93.0610(10)°) and constitutes a new structure type. The complex crystal structure of β-Yb2CdSb2 contrasts with the previously studied β-Ca2CdPn2 (Pn = P, As, Sb) polymorphs, although it shares similar structural features. It consists of three different layers, made of corner-sharing [CdSb4] tetrahedra and stacked in the ABC sequence. The layers are interconnected via [CdSb3] trigonal planar units. Multiple Yb and Cd atomic sites exhibit partial occupancy, resulting in extensive structural disorder. Valence electron partitioning within the Zintl–Klemm formalism yields the formulation (Yb2+)1.98(Cd2+)1.01(Sb3−)2(h+)0.02, highlighting the nearly charge-balanced composition. Detailed electronic structure calculations reveal the closed band gap and presumably semimetallic nature of β-Yb2CdSb2 with the band structure features hinting at potential topological properties.