Lee Junsu, Shin Seungeun, Jo Hongil, Shin Weon Ho, Moon Dohyun, Ok Kang Min, You Tae-Soo
Department of Chemistry and BK21Plus Research Team, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea.
Department of Chemistry, Sogang University, Seoul 04107, Republic of Korea.
Inorg Chem. 2020 Sep 21;59(18):13572-13582. doi: 10.1021/acs.inorgchem.0c01944. Epub 2020 Sep 2.
A series of quaternary and quinary Zintl phase thermoelectric (TE) compounds, CaYbAlInSb (3.07(1) ≤ ≤ 4.88(2); 0.16(2) ≤ ≤ 2.00), containing Al/In mixed sites as well as Ca/Yb mixed sites has been successfully synthesized by a direct arc-melting method, and the X-ray diffraction analyses indicated that the products initially adopted an orthorhombic BaAlBi-type structure (space group , = 2). However, after a postannealing process at 973 K for 1 month, the particular Yb rich compounds underwent a transformation of the original structure type to a CaGaSb-type phase regardless of the In substitution for Al. The noticeable site preference of cationic Ca and Yb in the three available cationic sites could be understood on the basis of a size match between the central cation and the volume of the anionic polyhedra. The observed phase transition was nicely explained by DFT calculations, proving that the CaGaSb-type phase was energetically more favorable than the BaAlSb-type phase for the particular Yb-rich compound. Moreover, this energy difference between the two title phases was originally the result of both the site energy in the Ca site and the bond energies in the [(Al/In)Sb] anionic building blocks. A series of thermoelectric property data indicated that a two-step process involving a partial/full In substitution for Al and a phase transition from the BaAlSb-type to the CaGaSb-type phase successfully enhanced the electrical conductivities and the Seebeck coefficients of the title compounds. This kind of combined effect eventually resulted in a improvement for the quinary compound CaYbAlInSb by approximately 4 times in comparison to its quaternary predecessor CaYbAlSb.
通过直接电弧熔炼法成功合成了一系列含Al/In混合位点以及Ca/Yb混合位点的四元及五元Zintl相热电（TE）化合物CaYbAlInSb（3.07(1) ≤ ≤ 4.88(2)；0.16(2) ≤ ≤ 2.00），X射线衍射分析表明产物最初采用正交晶系的BaAlBi型结构（空间群 ， = 2）。然而，在973 K下进行1个月的后退火处理后，无论In对Al的取代情况如何，特定的富Yb化合物都经历了从原始结构类型到CaGaSb型相的转变。基于中心阳离子与阴离子多面体体积之间的尺寸匹配，可以理解阳离子Ca和Yb在三个可用阳离子位点上明显的位点偏好。密度泛函理论（DFT）计算很好地解释了观察到的相变，证明对于特定的富Yb化合物，CaGaSb型相比BaAlSb型相在能量上更有利。此外，这两个标题相之间的能量差最初是Ca位点的位点能量和[(Al/In)Sb]阴离子结构单元中的键能共同作用的结果。一系列热电性能数据表明，涉及部分/完全用In取代Al以及从BaAlSb型到CaGaSb型相的相变这一两步过程成功提高了标题化合物的电导率和塞贝克系数。这种综合效应最终使五元化合物CaYbAlInSb与其四元前体CaYbAlSb相比， 提高了约4倍。
