Materials design, synthesis, and transport properties of disordered rare-earth Zintl bismuthides with the anti-Th₃P₄ structure type

The synthesis, structural elucidation, and transport properties of the extended series Ca_4-REBi₃ (RE = Y, La-Nd, Sm, Gd-Tm, and Lu; x ≈ 1) and Ca_4-REBi_3-Sb (RE = La, Ho, Er, and Lu; x ≈ 1, δ ≈ 1.5) are presented. Structural elucidation is based on single-crystal X-ray diffraction data and confirms the chemical drive of Ca₄Bi₃ with the cubic anti-Th₃P₄ structure type (space group I4̄3d, no. 220, Z = 4) into a Zintl phase by the introduction of trivalent rare-earth atoms. The structure features complex bonding, heavy elements, and electron count akin to that of valence-precise semiconductors, making it an ideal target for thermoelectrics development. Introducing crystallographic site disorders at the cation site for the Ca_4-REBi₃ phase and both the cation and anion sites for the Ca_4-REBi_3-Sb phase brings about additional desirable characteristics for thermoelectric materials in the context of tuning knobs for lowering thermal conductivity. Electronic structure calculations of idealized Ca₃YBi₃ and Ca₃LaBi₃ compounds indicate the opening of indirect bandgaps at the Fermi level with magnitudes E_g = 0.38 eV and 0.57 eV, respectively. The electrical resistivity ρ(T) of some of the investigated phases measured on single crystals evolve in a metallic manner with magnitudes of order 1.4 mΩ cm near 500 K, thus supporting the notion of a degenerate semiconducting state, with the temperature dependence of the Seebeck coefficient α(T) suggesting the p-type behavior. The low electrical resistivity and the realization of a degenerate semiconducting state in the title phases present a window of opportunity for optimizing their carrier concentrations for enhanced thermoelectric performance.