Nb_0.8CoSb ordering transformation caused by <i>in situ</i> heating-induced Nb diffusion

Abstract

This study focused on the investigation of Nb&lt;sub&gt;0.8&lt;/sub&gt;CoSb half-Heusler alloy covered with Nb films. By employing &lt;i&gt;in-situ&lt;/i&gt; heating transmission electron microscopy (TEM) technique, diffusion of Nb was observed at high temperature, resulting in the ordering transformation from Nb&lt;sub&gt;0.8&lt;/sub&gt;CoSb to Nb&lt;sub&gt;0.8+&lt;i&gt;δ&lt;/i&gt;&lt;/sub&gt;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&lt;sub&gt;0.8&lt;/sub&gt;CoSb samples transitioned into superlattice diffraction spots representing long-range order. The modulation wave vector of this superstructure was determined to be &lt;i&gt;q&lt;/i&gt;=1/3(&lt;i&gt;a&lt;/i&gt;&lt;sup&gt;*&lt;/sup&gt; + &lt;i&gt;b&lt;/i&gt;&lt;sup&gt;*&lt;/sup&gt;-&lt;i&gt;c&lt;/i&gt;&lt;sup&gt;*&lt;/sup&gt;). This structural evolution primarily arised from the diffusion of Nb atoms from the Nb film into the Nb&lt;sub&gt;0.8&lt;/sub&gt;CoSb sample at high temperature, leading to compositional changes in Sb and Nb. Further comparative analysis revealed significant differences between &lt;i&gt;in-situ&lt;/i&gt; synthesized Nb&lt;sub&gt;0.8+&lt;i&gt;δ&lt;/i&gt;&lt;/sub&gt;CoSb samples and &lt;i&gt;ex-situ&lt;/i&gt; synthesized Nb&lt;sub&gt;0.84&lt;/sub&gt;CoSb samples despite both exhibiting superstructures. In the &lt;i&gt;ex-situ&lt;/i&gt; synthesized Nb&lt;sub&gt;0.84&lt;/sub&gt;CoSb, the modulation wave vector of the superstructure was &lt;&lt;i&gt;q&lt;/i&gt;=1/3(2&lt;i&gt;a&lt;/i&gt;&lt;sup&gt;*&lt;/sup&gt;-2&lt;i&gt;c&lt;/i&gt;&lt;sup&gt;*&lt;/sup&gt;), mainly attributed to Nb compositional variations. Moreover, the superstructure in Nb&lt;sub&gt;0.84&lt;/sub&gt;CoSb samples could remain stable from room temperature tor high temperature, whereas in Nb&lt;sub&gt;0.8+&lt;i&gt;δ&lt;/i&gt;&lt;/sub&gt;CoSb samples, it only existed at elevated temperatures and gradually weakened as the temperature decreased, suggesting it might be a metastable structure between Nb&lt;sub&gt;0.8&lt;/sub&gt;CoSb and Nb&lt;sub&gt;0.84&lt;/sub&gt;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