Thermal Stability and Thermoelectric Properties of NaZnSb

A layered Zintl antimonide NaZnSb (PbC1F or Cu2Sb structure type; P4/nmm) was synthesized using the reactive sodium hydride NaH precursor. This method provides comprehensive compositional control and facilitates the fast preparation of high-purity samples in large quantities. NaZnSb is highly reactive to humidity/air and hydrolyzes to NaOH, ZnO, and Sb in aerobic conditions. On the other hand, NaZnSb is thermally stable up to 873 K in vacuum, as no structural changes were observed from high-temperature synchrotron powder X-ray diffraction data in the 300-873 K temperature range. The unit cell expansion upon heating is isotropic; however, interatomic distance elongation is not isotropic, consistent with the layered structure. Low- and high-temperature thermoelectric properties were measured on pellets densified by spark plasma sintering. The resistivity of NaZnSb ranges from 11 m Omega.cm to 31 m Omega.cm within the 2-676 K range, consistent with heavily doped semiconductor behavior, with a narrow band gap of 0.23 eV. NaZnSb has a large positive Seebeck coefficient (244 mu V.K-1 at 476 K), leading to the maximum of zT of 0.23 at 675 K. The measured thermoelectric properties are in good agreement with those predicted by theoretical calculations.