The optimal charge carrier concentration is engineered by intrinsic defects and extrinsic substitutional atoms where an understanding of the Zintl chemistry is essential . The discovery of the complex Zintl compound, Yb 14 MnSb 11 as a high zT , hightemperature (900 K -1300 K), p-type thermoelectric material has paved the way for 20% or more efficient thermoelectric power generation such that it has been selected by NASA for next-generation radioisotope thermoelectric generators (RTGs) for space missions . With improved processing, and various chemical substitutions of the A or M atom in A 14 M Sb 11 several examples with zT > 1 have been reported . All of the compounds have exceptionally low lattice thermal conductivity (κ L ) because of the large unit cell that limits propagating acoustic waves to a small frequency range and so most of the heat is transported by diffusons .