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本文以覆盖Nb薄膜的半赫斯勒合金Nb0.8CoSb为研究对象，成功利用原位加热透射电镜技术在高温下诱导Nb扩散，致使Nb0.8CoSb转变为有序度更高的Nb0.8+δCoSb，即倒空间漫散带代表的短程有序结构转变为超结构衍射点代表的长程有序结构。进一步的分析表明，这种超结构的调制波矢为q = 1/3（a* + b* - c*），其形成主要源自于Sb和Nb组分的变化。与离位合成的Nb0.84CoSb的微观结构进行对比，发现二者中超结构不同，这种超结构的调制波矢为 q = 1/3（2a* - 2c*），主要源自于Nb组分的变化。此项研究揭示了组分导致超结构的多样性以及半赫斯勒合金结构相变的复杂性，丰富了对半赫斯勒合金材料的理解，对相变材料的设计以及功能调控具有重要指导意义。This study focused on the investigation of Nb0.8CoSb half-Heusler alloy covered with Nb films. By employing in-situ heating transmission electron microscopy (TEM) technique, diffusion of Nb was observed at high temperature, resulting in the ordering transformation from Nb0.8CoSb to Nb0.8+δCoSb. 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 Nb0.8CoSb samples transitioned into superlattice diffraction spots representing long-range order. The modulation wave vector of this superstructure was determined to be q=1/3(a* + b*-c*). This structural evolution primarily arised from the diffusion of Nb atoms from the Nb film into the Nb0.8CoSb sample at high temperature, leading to compositional changes in Sb and Nb. Further comparative analysis revealed significant differences between in-situ synthesized Nb0.8+δCoSb samples and ex-situ synthesized Nb0.84CoSb samples despite both exhibiting superstructures. In the ex-situ synthesized Nb0.84CoSb, the modulation wave vector of the superstructure was <q=1/3(2a*-2c*), mainly attributed to Nb compositional variations. Moreover, the superstructure in Nb0.84CoSb samples could remain stable from room temperature tor high temperature, whereas in Nb0.8+δCoSb samples, it only existed at elevated temperatures and gradually weakened as the temperature decreased, suggesting it might be a metastable structure between Nb0.8CoSb and Nb0.84CoSb. 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
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Keywords:
- solid-state phase transition /
- in-situ heating /
- short-range ordering /
- half-Heusler alloy
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