Ternary Zinc Antimonides Unlocked Using Hydride Synthesis

Three new sodium zinc antimonides Na₁₁Zn₂Sb₅, Na₄Zn₉Sb₉, and NaZn₃Sb₃ were synthesized utilizing sodium hydride NaH as a reactive sodium source. In comparison to the synthesis using sodium metal, salt-like NaH can be ball-milled, leading to the easy and uniform mixing of precursors in the desired stoichiometric ratios. Such comprehensive compositional control enables a fast screening of the Na-Zn-Sb system and identification of new compounds, followed by their preparation in bulk with high purity. Na₁₁Zn₂Sb₅ crystallizes in the triclinic P1 space group (No. 2, Z = 2, a = 8.8739(6) Å, b = 10.6407(7) Å, c = 11.4282(8) Å, α = 103.453(2)°, β = 96.997(2)°, γ = 107.517(2)°) and features polyanionic [Zn₂Sb₅]^11- clusters with unusual 3-coordinated Zn atoms. Both Na₄Zn₉Sb₉ (Z = 4, a = 28.4794(4) Å, b = 4.47189(5) Å, c = 17.2704(2) Å, β = 98.3363(6)°) and NaZn₃Sb₃ (Z = 8, a = 32.1790(1) Å, b = 4.51549(1) Å, c = 9.64569(2) Å, β = 98.4618(1)°) crystallize in the monoclinic C2/m space group (No. 12) and have complex new structure types. For both compounds, their frameworks are built from ZnSb₄ distorted tetrahedra, which are linked via edge-, vertex-sharing, or both, while Na cations fill in the framework channels. Due to the complex structures, Na₄Zn₉Sb₉ and NaZn₃Sb₃ compounds exhibit low thermal conductivities (0.97-1.26 W·m^-1 K^-1) at room temperature, positive Seebeck coefficients (19-32 μV/K) suggestive of holes as charge carriers, and semimetallic electrical resistivities (∼1.0-2.3 × 10^-4 Ω·m). Na₄Zn₉Sb₉ and NaZn₃Sb₃ decompose into the equiatomic NaZnSb above ∼800 K, as determined by in situ synchrotron powder X-ray diffraction. The discovery of multiple ternary compounds highlights the importance of judicious choice of the synthetic method.