Alloying-Induced Structural Transition in the Promising Thermoelectric Compound CaAgSb
Alloying-Induced Structural Transition in the Promising Thermoelectric Compound CaAgSb
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DOI：
10.1021/acs.chemmater.3c02621
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发表时间：
2024-02
影响因子：
8.6
通讯作者：
A. Shawon;Weeam Guetari;Kamil M Ciesielski;Rachel Orenstein;Jiaxing Qu;Sevan Chanakian;Md. Towhidur Rahman;Elif Ertekin;Eric Toberer;Alexandra Zevalkink
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作者：
A. Shawon;Weeam Guetari;Kamil M Ciesielski;Rachel Orenstein;Jiaxing Qu;Sevan Chanakian;Md. Towhidur Rahman;Elif Ertekin;Eric Toberer;Alexandra Zevalkink
AMX Zintl 化合物以几种密切相关的层状结构结晶，最近由于其令人兴奋的热电特性而引起了人们的关注。在这项研究中，我们证明斜方晶系 CaAgSb 可以与六方晶系 CaAgBi 合金化，以实现在 x ∼ 0.8 时发生结构转变的固溶体。这种转变可以看作是从三维 (3D) 到二维 (2D) 共价键的转变，其中层间 M-X 键距离扩大，而面内 M-X 距离收缩。弹性模量的测量表明，随着 Bi 含量的增加，CaAgSb1-xBix 变得更软，但在 3D 到 2D 相变时观察到的阶梯状 10% 硬化除外。热电输运测量揭示了本征 CaAgSb 的霍尔迁移率和 620 K 时 0.47 的峰值 zT，这高于之前报告中未修饰的 CaAgSb 的值。然而，发现与 Bi 合金化会使空穴浓度超过最佳值，从而有效降低 zT。有趣的是，对热导率和电导率的分析表明，富Bi合金是低洛伦兹数（L）材料，尽管具有类似金属的传输特性，但L的估计值远低于非简并极限L = 1.5 × 10–8 W Ω K–2。低洛伦兹数可解耦晶格和电子热导率，为增强热电性能提供更大的灵活性。
AMX Zintl compounds, crystallizing in several closely related layered structures, have recently garnered attention due to their exciting thermoelectric properties. In this study, we show that orthorhombic CaAgSb can be alloyed with hexagonal CaAgBi to achieve a solid solution with a structural transformation at x ∼ 0.8. This transition can be seen as a switch from three-dimensional (3D) to two-dimensional (2D) covalent bonding in which the interlayer M–X bond distances expand while the in-plane M–X distances contract. Measurements of the elastic moduli reveal that CaAgSb1–xBix becomes softer with increasing Bi content, with the exception of a steplike 10% stiffening observed at the 3D-to-2D phase transition. Thermoelectric transport measurements reveal promising Hall mobility and a peak zT of 0.47 at 620 K for intrinsic CaAgSb, which is higher than those in previous reports for unmodified CaAgSb. However, alloying with Bi was found to increase the hole concentration beyond the optimal value, effectively lowering the zT. Interestingly, analysis of the thermal conductivity and electrical conductivity suggests that the Bi-rich alloys are low Lorenz-number (L) materials, with estimated values of L well below the nondegenerate limit of L = 1.5 × 10–8 W Ω K–2, in spite of the metallic-like transport properties. A low Lorenz number decouples lattice and electronic thermal conductivities, providing greater flexibility for enhancing thermoelectric properties.