Ultralow Lattice Thermal Conductivity in Ba23M10Ge10Sb25-delta (M = Ga, In): Quaternary Compounds Containing Ba-Centered Dodecahedra

Two new quaternary compounds, Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79, have been synthesized by using the high temperature solid-state reactions. These new phases exhibit huge unit cells (V = 17 514.6 and 18 455.8 angstrom(3) for Ga- and In-containing compounds, respectively),which comprise 544 atoms and feature onion-like M20Ge20Sb30 clusters connected through the Sb Sb bonds. Among these clusters, substantial isolated units such as Ba, Sb, or Sb-3 trimers are filled. With such a complex structure as well as the large unit cell and substantial heavy constituent atoms, ultralow lattice thermal conductivity (kappa(1)) was observed for Ba23Ga10Ge10Sb25, ranging from 0.2 to 0.38 W.m(-1).K-1 from 323 to 823 K. As a result, a maximun ZT of 0.27 was achieved at 823 K.