Ordering transformation in Nb<sub>0.8</sub>CoSb induced by <i>in situ</i> heating-induced Nb diffusion

Abstract

This study focused 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 was observed at high temperature, resulting in the ordering transformation from Nb<sub>0.8</sub>CoSb to Nb<sub>0.8+<i>δ</i></sub>CoSb. Through observations of high-angle annular dark-field (HAADF) images and selected-area electron diffraction (SAED) patterns, it was found that under elevated temperatures, the diffuse streaks representing short-range disorder in Nb<sub>0.8</sub>CoSb samples transitioned into superlattice diffraction spots representing long-range order. The modulation wave vector of this superstructure was determined to be <i>q</i>=1/3(<i>a</i><sup>*</sup> + <i>b</i><sup>*</sup>-<i>c</i><sup>*</sup>). This structural evolution primarily arised 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. Further comparative analysis revealed significant differences between <i>in-situ</i> synthesized Nb<sub>0.8+<i>δ</i></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 was <<i>q</i>=1/3(2<i>a</i><sup>*</sup>-2<i>c</i><sup>*</sup>), mainly attributed to Nb compositional variations. Moreover, the superstructure in Nb<sub>0.84</sub>CoSb samples could remain stable from room temperature tor high temperature, whereas in Nb<sub>0.8+<i>δ</i></sub>CoSb samples, it only existed at elevated temperatures and gradually weakened as the temperature decreased, suggesting it might be a metastable structure between Nb<sub>0.8</sub>CoSb and Nb<sub>0.84</sub>CoSb. In summary, 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