VO2金属-绝缘体相变机理的研究进展

VO₂是一种热致相变金属氧化物. 在341 K附近, VO₂发生由低温绝缘体相到高温金属相的可逆转变, 同时伴随着光学、电学和磁学等性质的可逆突变, 这种独特的性质使得VO₂在光电开关材料、智能玻璃、存储介质材料等领域有着广阔的应用前景. 因此, VO₂金属-绝缘体可逆相变一直是人们的研究热点, 但其相变机理至今未有定论. 首先, 简要概述了VO₂相变时晶体结构和能带结构的变化情况: 从晶体结构来讲, 相变前后VO₂从低温时的单斜相VO₂(M)转变为高温稳定的金红石相VO₂(R), 在一定条件下此过程也可能伴随着亚稳态单斜相VO₂(B)与四方相VO₂(A)的产生; 从能带结构来看, VO₂处于低温单斜相时, 其d_//能带和π^*能带之间存在一个禁带, 带宽约为0.7 eV, 费米能级恰好落在禁带之间, 表现出绝缘性, 而在高温金红石相时, 其费米能级落在π^*能带与d_//能带之间的重叠部分, 因此表现出金属导电性. 其次, 着重总结了VO₂相变物理机理的研究现状. 主要包括: 电子关联驱动相变、结构驱动相变以及电子关联和结构共同驱动相变的3种理论体系以及支撑这些理论体系的实验结果. 文献报道争论的焦点在于, VO₂是否是Mott绝缘体以及结构相变与MIT相变是否精确同时发生. 最后, 展望了VO₂材料研究的发展方向.

Research progress of metal-insulator phase transition mechanism in VO2

VO₂ is a metal oxide that has a thermally-induced phase-transition. In the vicinity of 341 K, VO₂ undergoes a reversible transition from the high-temperature metal phase to the low-temperature insulator phase. Associated with the metal-insulator transition (MIT), there are drastic changes in its optical, electrical and magnetic characteristics. These make VO₂ an attractive material for various applications, such as optical and/or electrical switches, smart glass, storage media, etc. Thus, the reversible metal-insulator phase transition in VO₂ has long been a research hotspot. However, the metal-insulator transition mechanism in VO₂ has been a subject of debate for several decades, and yet there is no unified explanation. This paper first describes changes of the crystal structure and the energy band structure during VO₂ phase transition. With regard to the crystal structure, VO₂ transforms from the low-temperature monoclinic phase VO₂(M) into the high-temperature stable rutile phase VO₂(R), and in some special cases, this phase transition process may also involve a metastable monoclinic VO₂(B) phase and a tetragonal VO₂(A) phase. In respect of the energy band structure, VO₂ undergoes a transition from the low-temperature insulator phase into a high-temperature metal phase. In the band structure of low-temperature monoclinic phase, there is a band gap of about 0.7 eV between d_// and π^* bands, and the Fermi level falls exactly into the band gap, which makes VO₂ electronically insulating. In the band structure of high-temperature rutile phase, the Fermi level falls into the overlapping portion of the π^* and d_// bands, which makes VO₂ electronically metallic. Next, this paper summarizes the current research status of the physical mechanism underlying the VO₂ MIT. Three kinds of theoretical perspectives, supported by corresponding experimental results, have been proposed so far, which includes electron-correlation-driven MIT, Peierls-like structure-driven MIT, and MIT driven by the interplay of both electron-correlation and Peierls-like structural phase transition. It is noted that recent reports mostly focus on the controversy–whether VO₂ is a Mott insulator, and whether the structural phase transition and the MIT accurately occur simultaneously in VO₂. Finally, the paper points out the near-future development direction of the VO₂ research.