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.