Tuning the Radius Ratio to Enhance Thermoelectric Properties in the Zintl Compounds AM₂Sb₂ (A = Ba, Sr; M = Zn, Cd)

Five novel Zintl phase solid solutions in the Ba_1–xSr_xZn_2–yCd_ySb₂ (0 ≤ x ≤ 0.13(1); 0 ≤ y ≤ 0.32(2)) system were successfully synthesized by the molten Pb metal-flux method, and the powder X-ray diffraction and single-crystal X-ray diffraction analyses proved that all five title compounds adopted the BaCu₂S₂-type phase having the orthorhombic Pnma space group (Z = 4, Pearson code oP20) with five crystallographically independent atomic sites. The previously studied BaCu₂S₂-type antimonides demonstrated a limited tolerance for doping in contrast to the CaAl₂Si₂-type antimonides. To understand the relatively narrower phase width and limited dopability of the title BaCu₂S₂-type phase than the CaAl₂Si₂-type phase in the overall Ba_1–xSr_xZn_2–yCd_ySb₂ system, the radius ratio of cations and anionic elements r₊/r_– for two structure types were thoroughly investigated. For the first time, the r₊/r_– ratio was identified as a critical factor for the phase selectivity: (1) r₊/r_– > 1 favored the BaCu₂S₂-type phase, and (2) r₊/r_– < 1 favored the CaAl₂Si₂-type phase. We also revealed the structural transformation mechanism from the more widely observed CaAl₂Si₂-type phase to the title BaCu₂S₂-type phase as the relatively larger cationic elements were introduced to the system. A series of DFT calculations using the three hypothetical models indicated that a resonance peak near E_F in the density of states curves was descended from the relatively flat band structure at several special symmetry points rationalizing the enhanced Seebeck coefficients of Ba_0.94(1)Sr_0.06Zn_1.86(3)Cd_0.14Sb₂ and Ba_0.96(1)Sr_0.04Zn_1.68(2)Cd_0.32Sb₂. Electron localization function analysis rationalized the correlation between the polarity change of anionic Zn/Cd–Sb bonds and the charge carrier mobility on the anionic frameworks. Temperature-dependent thermoelectric properties were studied for the four title compounds, and the results proved that the Sr and Cd doping in the title Ba_1–xSr_xZn_2–yCd_ySb₂ system successfully enhanced the ZT values through the increased Seebeck coefficients and the reduced total thermal conductivities.