Nb<sub>0.8</sub>CoSb ordering transformation caused by <i>in situ</i> heating-induced Nb diffusion
Authors: Li Qi-Zhu , Fan Hao-Han , Gao Zi-Heng , Nan Peng-Fei , Zhu Tie-Jun , Ge Bing-Hui ,   Information Materials and Intelligent Sensing Laboratory of Anhui Province, Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, China,   State Key Laboratory of Silicon and Advanced Semiconductor Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
Published: April 2024

<sec>This study focuses on the investigation of Nb<sub>0.8</sub>CoSb half-Heusler alloy covered with Nb films. By employing <i>in-situ</i> heating transmission electron microscopy (TEM) technique, diffusion of Nb is observed at high temperature, showing the ordering transformation from Nb<sub>0.8</sub>CoSb to Nb<sub>0.8+δ</sub>CoSb. Through observations of high-angle annular dark-field (HAADF) images and selected-area electron diffraction (SAED) patterns, it is found that under elevated temperatures, the diffuse streaks representing short-range disorder in Nb<sub>0.8</sub>CoSb sample transform into superlattice diffraction spots representing long-range order. The modulation wave vector of this superstructure is determined to be <inline-formula><tex-math id="M3">\begin{document}$ q={1}/{3}({a}^{*}+{b}^{*}-{c}^{*}) $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M3.png"/></alternatives></inline-formula>. This structural evolution primarily arises from the diffusion of Nb atoms from the Nb film into the Nb<sub>0.8</sub>CoSb sample at high temperature, leading to compositional changes in Sb and Nb.</sec><sec>Further comparative analysis reveals significant differences between <i>in-situ</i> synthesized Nb<sub>0.8+δ</sub>CoSb samples and <i>ex-situ</i> synthesized Nb<sub>0.84</sub>CoSb samples despite both exhibiting superstructures. In the <i>ex-situ</i> synthesized Nb<sub>0.84</sub>CoSb, the modulation wave vector of the superstructure is <inline-formula><tex-math id="M4">\begin{document}$ q={1}/{3}({2a}^{*}-2{c}^{*}) $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M4.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M4.png"/></alternatives></inline-formula>, which is mainly attributed to Nb compositional variations. Moreover, the superstructure in Nb<sub>0.84</sub>CoSb sample can remain stable from room temperature to high temperature, whereas in Nb<sub>0.8+δ</sub>CoSb samples, it only exists at elevated temperatures and gradually weakens as the temperature decreases, suggesting that it may be a metastable structure between Nb<sub>0.8</sub>CoSb and Nb<sub>0.84</sub>CoSb.</sec><sec>This study reveals the diversity of superstructures induced by compositional variations and the complexity of structural phase transitions in half-Heusler alloys, enriching the understanding of these materials and providing important guidance for the design and functional control of phase-change materials.</sec>