Yb Substitution and Ultralow Thermal Conductivity of the Ca_3-YbAlSb₃ (0 ≤ x ≤ 0.81(1)) System

A series of Yb-substituted Zintl phases in the Ca_3-YbAlSb₃ (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 _∞¹[Al(Sb₂Sb_2/2)] 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-Al^1-)(1b-Sb^2-)₂(2b-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 Γ 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)Yb_0.81AlSb₃ 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.