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

초록

Five novel Zintl phase solid solutions in the Ba1-x Sr x Zn2-y Cd y Sb2 (0 & LE; x & LE; 0.13(1); 0 & LE; y & LE; 0.32(2)) system were successfully synthesized by the moltenPb metal-flux method, and the powder X-ray diffraction and single-crystalX-ray diffraction analyses proved that all five title compounds adoptedthe BaCu2S2-type phase having the orthorhombic Pnma space group (Z = 4, Pearson code oP20) with five crystallographically independent atomicsites. The previously studied BaCu2S2-type antimonidesdemonstrated a limited tolerance for doping in contrast to the CaAl2Si2-type antimonides. To understand the relativelynarrower phase width and limited dopability of the title BaCu2S2-type phase than the CaAl2Si2-type phase in the overall Ba1-x Sr x Zn2-y Cd y Sb2 system, the radiusratio of cations and anionic elements r (+)/r (-) for two structure types werethoroughly investigated. For the first time, the r (+)/r (-) ratio was identifiedas a critical factor for the phase selectivity: (1) r (+)/r (-) > 1 favoredtheBaCu(2)S(2)-type phase, and (2) r (+)/r (-) < 1 favoredthe CaAl2Si2-type phase. We also revealed thestructural transformation mechanism from the more widely observedCaAl(2)Si(2)-type phase to the title BaCu2S2-type phase as the relatively larger cationic elementswere introduced to the system. A series of DFT calculations usingthe three hypothetical models indicated that a resonance peak near E (F) in the density of states curves was descendedfrom the relatively flat band structure at several special symmetrypoints rationalizing the enhanced Seebeck coefficients of Ba0.94(1)Sr0.06Zn1.86(3)Cd0.14Sb2 and Ba0.96(1)Sr0.04Zn1.68(2)Cd0.32Sb2. Electron localization function analysisrationalized the correlation between the polarity change of anionicZn/Cd-Sb bonds and the charge carrier mobility on the anionicframeworks. Temperature-dependent thermoelectric properties were studiedfor the four title compounds, and the results proved that the Sr andCd doping in the title Ba1-x Sr x Zn2-y Cd y Sb2 system successfully enhancedthe ZT values through the increased Seebeck coefficientsand the reduced total thermal conductivities.

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