Yb Substitution and Ultralow Thermal Conductivity of the Ca3-x Yb (x) AlSb3 (0 & LE; x & LE; 0.81(1)) System

A series of Yb-substituted Zintl phases in the Ca3-xYbxAlSb3 (0 = x = 0.81(1)) system has been synthesized by initial arc melting and post-heat treatment, and their isotypic crystal structures were characterized by both powder and single crystal X-ray diffraction analysis. All four title compounds adopted the Ca3AlAs3-type structure (space group Pnma, Pearson code oP28, Z = 4). The overall structure can be described as a combination of the 1- dimensional (1D) infinite chain of 8 1 [Al(Sb(2)Sb2/2)] formed by two vertices sharing [ AlSb4] tetrahedral moieties and three Ca2+/Yb2+ mixed sites located in between these 1D chains. The charge balance and the resultant independency of the 1D chains in the title system were explained by the Zintl-Klemm formalism [Ca2+/Yb2+]3[(4b-Al1-)(1bSb(2)-)2((2)b-Sb-1-) 2/2]. A series of DFT calculations proved that (1) the band overlap between the d-orbital states from two types of cations and the p-orbital states from Sb at the high symmetry G point implied a heavily doped degenerate semiconducting behavior of the quaternary Ca2YbAlSb3 model and (2) the site preference of Yb for the M1 site was due to the electronic-factor criterion based on the Q values of each atomic site. The electron localization function calculations also proved that the two different shapes of lone pairs of the Sb atoms the "umbrella-shape" and the "C-shape"are determined by local geometry and the coordination environment on the anionic frameworks. Thermoelectric measurements of the quaternary title compound Ca-2.19( 1)Yb0.81AlSb3 showed an approximately two times larger ZT value than that of ternary Ca3AlSb3 at 623 K due to increased electrical conductivity and ultralow thermal conductivity originated from Yb substitution for Ca.