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Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films
Acta Materialia ( IF 9.4 ) Pub Date : 2021-09-26 , DOI: 10.1016/j.actamat.2021.117336
Laura Rodríguez 1 , Felip Sandiumenge 2 , Carles Frontera 2 , José Manuel Caicedo 1 , Jessica Padilla 1 , Gustau Catalán 1, 3 , José Santiso 1
Affiliation  

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.



中文翻译:

VO2外延薄膜中金属-绝缘体转变处的强应变梯度和相共存

热力学三相点的接近和瞬态亚稳态相的形成可能导致应变 VO 2薄膜中金属-绝缘体转变的复杂相和微观结构轨迹。使用原位同步加速器 X 射线衍射的详细分析揭示了外延膜结构中这种复杂性的微妙指纹。在相变过程中,低温单斜 M1 相被共存的高温 R 相域沿 {111} R平面约束,这些域保持外延夹在衬底上。这种几何约束引起抵消局部应力,导致 M1 域的倾斜和单轴平面内压缩的组合,以及伴随的异常c R- 轴伸长率。这种机制逐渐将 M1 相转变为过渡三斜相 (T),并最终转变为单斜 M2 相,产生强烈的应变和倾斜梯度,在 R 相完全转变后保持冻结状态。VO 2薄膜的转变路径、稳定和亚稳态VO 2多晶型物之间的复杂竞争及其对低温单斜晶态结构的影响,为理解纳米级薄膜的电子和机械性能提供了重要的见解,以及以控制它们在功能设备中的使用。

更新日期:2021-10-02
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