Solid Solution Yb2-xCaxCdSb2: Structure, Thermoelectric Properties, and Quality Factor.

Solid solutions of Yb2-xAxCdSb2 (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb2-xCaxCdSb2 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, Yb2-xAxCdSb2 (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 Yb2-xCaxCdSb2 (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 Yb2-xCaxCdSb2 (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 Yb1.7A0.3CdSb2 (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.