Topological Z2 Invariants in the Metallic Bismuthide Series A1-xT2Bi2 (A = Na, K, Rb, Cs; T = Zr, Hf; x = 0-0.27)

The bismuthides A1-xT2Bi2 (A = Na, K, Rb, Cs; T = Zr, Hf; x = 0 - 0.27(2)) were synthesized as air-sensitive black powders with small metallic crystals. X-ray powder diffraction determined their tetragonal structure with space group I4/mmm as a ternary ordered i5-superstructure of bcc packing. This is an isopointal variant of the Er2Mg2Ru structure type, with interatomic distances and density functional theory (DFT) calculations suggesting layers of edge-sharing TBi5 pyramids alternating with layers of alkali metal in a cubic coordination. Electrical resistivity measurements of Rb1-xHf2Bi2 show metallic behavior matching the crystal and powder appearance. Magnetic susceptibility measurements show a weak, nearly temperature-independent paramagnetic signal suggesting Pauli paramagnetism. Analysis of the fully relativistic band structure employing the Fu-Kane approach reveals nontrivial topological properties characterized by Z2 = (1;111) and Z4= 3 invariants for all eight compounds. This robust topological nature remains unaffected by the substitution of alkali metal and transition metal components, demonstrating that nontrivial topology is an intrinsic property of this structure type, driven by the strong spin-orbit coupling of the bismuth sublattice.