Yb Substitution and Ultralow Thermal Conductivity of the CaYbAlSb (0 ≤ ≤ 0.81(1)) System.

A series of Yb-substituted Zintl phases in the CaYbAlSb (0 ≤ ≤ 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 CaAlAs-type structure (space group , Pearson code 28, = 4). The overall structure can be described as a combination of the 1-dimensional (1D) infinite chain of [Al(SbSb)] formed by two vertices sharing [AlSb] tetrahedral moieties and three Ca/Yb 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 [Ca/Yb][(4b-Al)(1b-Sb)(2b-Sb)]. A series of DFT calculations proved that (1) the band overlap between the -orbital states from two types of cations and the -orbital states from Sb at the high symmetry Γ point implied a heavily doped degenerate semiconducting behavior of the quaternary CaYbAlSb 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 CaYbAlSb showed an approximately two times larger value than that of ternary CaAlSb at 623 K due to increased electrical conductivity and ultralow thermal conductivity originated from Yb substitution for Ca.