From NaZn₄Sb₃ to HT-Na_1–xZn_4–ySb₃: Panoramic Hydride Synthesis, Structural Diversity, and Thermoelectric Properties

Two new sodium zinc antimonides NaZn₄Sb₃ and HT-Na_1–xZn_4–ySb₃ were synthesized by using reactive sodium hydride, NaH, as a precursor. The hydride route provides uniform mixing and comprehensive control over the composition, facilitating fast reactions and high-purity samples, whereas traditional synthesis using sodium metal results in inhomogeneous samples with a significant fraction of the more stable NaZnSb compound. NaZn₄Sb₃ crystallizes in the hexagonal P6₃/mmc space group (No. 194, Z = 2, a = 4.43579(4) Å, c = 23.41553(9) Å) and is stable upon heating in vacuum up to 736 K. The layered crystal structure of NaZn₄Sb₃ is related to the structure of the well-studied thermoelectric antimonides AeZn₂Sb₂ (Ae = Ca, Sr, Eu). Upon heating in vacuum, NaZn₄Sb₃ transforms to HT-Na_1–xZn_4–ySb₃ (x = 0.047(3), y = 0.135(1)) due to partial Na/Zn evaporation/elimination, as was determined from high-temperature in situ synchrotron powder X-ray diffraction. HT-Na_1–xZn_4–ySb₃ has a complex monoclinic structure with considerable degrees of structural disorder (P2₁/c (No. 14), Z = 32, a = 19.5366(7) Å, b = 14.7410(5) Å, c = 20.7808(7) Å, β = 90.317(2)°) and is stable exclusively in a narrow temperature range of 736–885 K. Further heating of HT-Na_1–xZn_4–ySb₃ leads to a reversible transformation to NaZnSb above 883 K. Both compounds exhibit similarly low thermal conductivity at room temperature (0.9 W m^–1 K^–1) and positive Seebeck coefficients (38–52 μV/K) indicative of holes as the main charge carriers. However, resistivities of the two phases differ by 2 orders of magnitude.