AUTHOR=Liu Yichao , Wang Cangmin , Huang Wenyu , Wang Shaoting , Qiu Huaili , Ge Weifeng , Chen Meixia , Zhang Hui , Gu Yueliang , Zhang Xingmin , Li Xiaolong , Gao Xingyu , Yang Yuanjun 

TITLE=Unraveling Structural Phase Transformation by Simultaneously Determining the Lattice Constants and Mismatch Angle in VO2/Al2O3 Epitaxial Thin Films

JOURNAL=Frontiers in Materials

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2022.866468

DOI=10.3389/fmats.2022.866468

ISSN=2296-8016

ABSTRACT=<p>As a prototype of a strongly correlated electron system, bulk vanadium dioxide (VO<sub>2</sub>) exhibits a large and reversible metal–insulator transition (MIT) near 340 K, concomitantly accompanied by a monoclinic–rutile structural phase transformation (SPT). In this study, we systematically investigated the SPT across the MIT in a (010)-VO<sub>2</sub>/(0001)-Al<sub>2</sub>O<sub>3</sub> epitaxial thin film by simultaneously determining three lattice constants (<italic>a</italic>, <italic>b</italic>, and <italic>c</italic>) and the mismatch angle (<italic>Δβ</italic>) using high-resolution X-ray diffraction. The lattice constants <italic>a</italic>, <italic>b</italic>, and <italic>c</italic> were approximately 5.723, 4.521, and 5.393 Å, respectively, at room temperature, and the mismatch angle was approximately 122.02°. As the temperature increased, the lattice constants and mismatch angle did not change significantly until the temperature reached the MIT point. Then, <italic>a</italic>, <italic>b</italic>, and <italic>c</italic> suddenly increased to approximately 5.689 Å, 4.538 Å, and 5.411 Å, respectively, and retained this value up to nearly 90°C. However, the mismatch angle first slightly increased and then sharply decreased to 122.00°. Additionally, the lattice constants and mismatch angle were almost reproducible with decreasing temperature, except for hysteresis in the MIT region. These results verify that VO<sub>2</sub> undergoes an MIT, simultaneously accompanied by SPT, in thicker films with small strain and weak substrate constraints, analogous to bulk VO<sub>2</sub>. This was further confirmed by <italic>in-situ</italic> varying-temperature Raman characterization. These findings provide insights into the SPT and reveal an angular parameter for judging the SPT in VO<sub>2</sub> systems.</p>