Lim Sung-Ji, Nam Gnu, Shin Seungeun, Ahn Kyunghan, Lee Yunho, You Tae-Soo
Department of Chemistry and BK21Plus Research Team , Chungbuk National University , Cheongju , Chungbuk 28644 , Republic of Korea.
Department of Applied Physics , Kyung Hee University , Yong-in 17104 , Republic of Korea.
Inorg Chem. 2019 May 6;58(9):5827-5836. doi: 10.1021/acs.inorgchem.9b00181. Epub 2019 Apr 15.
Three Zintl phase compounds belonging to the CaYbAlSbGe ( x = 0.2, 0.5, 0.7; nominal compositions) system with various Ge-doping contents were successfully synthesized by arc-melting and were initially crystallized in the BaAlBi-type phase (space group Pbam, Pearson codes oP26). However, after post-heat treatment at an elevated temperature, the originally obtained crystal structure was transformed into the homeotypic CaGaSb-type structure according to powder and single-crystal X-ray diffraction analyses. Two types of crystal structures share some isotypic structural moieties, such as the one-dimensional anionic chains formed by [AlSb] and the void-filling Ca/Yb mixed cations, but the slightly different spatial arrangements in each unit cell make these two structural types distinguishable. This series of title compounds is originally investigated to examine whether anionic p-type doping using Ge can successfully enhance thermoelectric (TE) properties of the Yb-rich CaYbAlSbGe series even after the phase transition from the BaAlBi-type to the CaGaSb-type phase. More interestingly, we also reveal that the given structural transformation is triggered by the particularly different site-preference of Ca and Yb among three available cationic sites in each structure type, which is significantly affected by thermodynamic conditions of this system. Band structure and density of states analyses calculated by density functional theory using the tight-binding linear muffin-tin orbital method also prove that the Ge-doping actually increases band degeneracies and the number of resonant peaks near the Fermi level resulting in the improvement of Seebeck coefficients. Electron localization function analyses for the (0 1 0) sliced-plane and the 3D isosurface nicely illustrates the distortion of the paired-electron densities due to the introduction of Ge. The systematic TE property measurements imply that the attempted anionic p-type doping is indeed effective to improve the TE characteristics of the title CaYbAlSbGe system.
通过电弧熔炼成功合成了三种属于CaYbAlSbGe（x = 0.2、0.5、0.7；标称成分）体系且具有不同Ge掺杂含量的津特耳相化合物，它们最初结晶为BaAlBi型相（空间群Pbam，皮尔逊代码oP26）。然而，在高温后热处理后，根据粉末和单晶X射线衍射分析，最初获得的晶体结构转变为同型的CaGaSb型结构。两种晶体结构共享一些同型结构部分，例如由[AlSb]形成的一维阴离子链和填充空隙的Ca/Yb混合阳离子，但每个晶胞中略有不同的空间排列使这两种结构类型得以区分。最初研究这一系列标题化合物是为了检验即使在从BaAlBi型相转变为CaGaSb型相之后，使用Ge进行阴离子p型掺杂是否能成功提高富Yb的CaYbAlSbGe系列的热电（TE）性能。更有趣的是，我们还揭示了给定的结构转变是由每种结构类型中三个可用阳离子位点之间Ca和Yb特别不同的位点偏好引发的，这受到该体系热力学条件的显著影响。使用紧束缚线性 muffin-tin轨道方法通过密度泛函理论计算的能带结构和态密度分析也证明，Ge掺杂实际上增加了能带简并度以及费米能级附近的共振峰数量，从而导致塞贝克系数的提高。对（0 1 0）切片平面和3D等值面的电子定位函数分析很好地说明了由于引入Ge导致的成对电子密度的畸变。系统的TE性能测量表明，尝试的阴离子p型掺杂确实有效地改善了标题CaYbAlSbGe体系的TE特性。
