Solid Solution Yb_2-CaCdSb₂: Structure, Thermoelectric Properties, and Quality Factor

Solid solutions of Yb_2-ACdSb₂ (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb_2-CaCdSb₂ has end members with different crystal structures. Yb₂CdSb₂ crystallizes in the polar space group Cmc2₁, whereas Ca₂CdSb₂ crystallizes in the centrosymmetric space group Pnma. Other solid solutions, Yb_2-ACdSb₂ (A = Sr, Eu), crystallize in the polar space group for x ≤ 1, and compositions with x ≥ 1 have not been reported. Both structure types are composed of corner-sharing CdSb₄ tetrahedra condensed into sheets that differ by the stacking of the layers. Single crystals of the solid solution Yb_2-CaCdSb₂ (x = 0-1) were studied to elucidate the structural transition between the Yb₂CdSb₂ and Ca₂CdSb₂ structure types. For x ≤ 1, the structures remain in the polar space group Cmc2₁. As the Ca content is increased, a positional disorder arises in the intralayer cation sites (Yb2/Ca2) and the Cd site, resulting in inversion of the CdSb₄ tetrahedral chain. This phenomenon could be indicative of an intergrowth of the opposing space group. The TE properties of polycrystalline samples of Yb_2-CaCdSb₂ (x ≤ 1) were measured from 300 to 525 K. The lattice thermal conductivity is extremely low (0.3-0.4 W/m·K) and the Seebeck coefficients are high (100-180 μV/K) across the temperature range. First-principles calculations show a minimum in the thermal conductivity for the x = 0.3 composition, in good agreement with experimental data. The low thermal conductivity stems from the acoustic branches being confined to low frequencies and a large number of phonon scattering channels provided by the localized optical branches. The TE quality factor of the Yb_1.7A_0.3CdSb₂ (A = Ca, Sr, Eu) series has been calculated and predicts that the A = Ca and Sr solid solutions may not improve with carrier concentration optimization but that the Eu series is worthy of additional modifications. Overall, the x = 0.3 compositions provide the highest zT because they provide the best electronic properties with the lowest thermal conductivity.