LAUSR

Yb10MgSb9 is a new Zintl compound (with a composition closer to Yb10.5MgSb9) and a promising thermoelectric material first reported in this work. Undoped Yb10MgSb9 has an ultralow thermal conductivity due to crystallographic complexity and exhibits a relatively high peak p‐type Seebeck coefficient and high electrical resistivity. This is consistent with Zintl counting and density functional theory (DFT) calculations that the composition Yb10.5MgSb9 should be a semiconductor. Na is found experimentally to be an effective p‐type dopant potentially due to the replacement of Na+ for Yb2+, allowing for a significant decrease in electrical resistivity. With doping, a dramatic improvement of electrical conductivity is observed and the glass‐like thermal conductivity remains low, allowing for a significant enhancement of the thermoelectric figure of merit, zT. Doping increases the zT from 0.23 in undoped Yb10MgSb9 to 1.06 in 7 at% Na‐doped Yb10MgSb9 at 873K. This high thermoelectric performance found through Na‐doping places this material amongst the leading p‐type Zintl thermoelectrics, making it a promising candidate for future studies and high‐temperature thermoelectric applications.