Yb Substitution and Ultralow Thermal Conductivity of the Ca3-x Yb x AlSb3 (0 ≤ x ≤ 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-shapeare 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.

Automated summary

A series of Yb-substituted Zintl phases in the Ca3-xYbxAlSb3 (0 ≤ x ≤ 0.81(1)) system have been successfully synthesized, and their isotypic crystal structures were characterized by X-ray diffraction analysis. The overall structure of these compounds consists of 1-dimensional infinite chains and Ca2+/Yb2+ mixed sites located in between these chains. The charge balance and the independency of the chains were explained by the Zintl-Klemm formalism. DFT calculations showed the heavily doped degenerate semiconducting behavior of the quaternary Ca2YbAlSb3 model and explained the site preference of Yb. Electron localization function calculations revealed the different shapes of lone pairs of Sb atoms and their dependence on local geometry and coordination environment. Thermoelectric measurements showed an enhanced ZT value in the Yb-substituted compound compared to the ternary compound, due to increased electrical conductivity and low thermal conductivity.