Valence States,Defects And Metal-insulator Transition In Vanadium Oxides

The variable valence states of vanadium element have led to the formation of a variety of vanadium oxides with different physical and chemical characteristics.Metal-insulator transition?MIT?in vanadium oxides has been demonstrated and of great importance in the fields of fundamental research and real applications in metal oxides.With an ultrafast switching rate in a femtosecond range,MIT ensures unique advantages of vanadium oxides in the advanced electronic devices such as detectors,switchers and storage.However,the physical mechanism of MIT is still in debate although MIT in vanadium oxides has been investigated as early as the 1960s.On the one hand,the critical point and difficulty in correlated vanadium oxides is that MIT is usually accompanied by a structure phase transition?SPT?.On the other hand,the lattice microstructures of vanadium oxides are easily affected by oxygen components and the variable oxidation states of vanadium,and all these will result in different MIT processes.Therefore,how to distinguish MIT from SPT has become one of the key issues in the strongly correlated vanadium oxides.Here,we will systematically study the relationship between MIT and SPT through the influence of the valence states of vanadium,compositions and defects in vanadium oxides.All these are helpful to understand the MIT mechanism.In addition,we believe that they will expand controllable methods for adjusting the compositions and microstructures in vanadium oxides.The main contents are as follows:?1?The oxidation state of vanadium is one of the main factors that determine the lattice microstructure in different vanadium oxides and their corresponding MIT characteristics.However,it is a key issue to accurately regulate the valence state of vanadium during the preparation process.In this paper,we firstly investigate how to prepare vanadium oxides with adjustable oxidation states,and then reveal the relationship between oxidation states and growth conditions of vanadium oxides.In particular,we will study the roles of the reaction temperature,reactant species,and the thermal treatment which affect the valence states and stoichiometric ratio in vanadium oxides.By controlling the growth and post-treatment processes,we can achieve and then understand the transformation between V⁵⁺and V³⁺,which are benefit for controlling microstructures and compositions in vanadium oxides.?2?Oxygen component has a decisive effect on the microstructures and the mixed valence states in vanadium oxides.Moreover,intrinsic defects induced by the changed components will directly affect the MIT characteristics of vanadium oxides.Therefore,we subsequently study the influence of the oxygen components and contents on the microstructures and the composition ratios,and then the roles on MIT and SPT in vanadium oxides.Here,we analyze the change trend of the lattice structure of vanadium oxides by adjusting the growth and post-treatment parameters?e.g.the oxygen partial pressure and the thickness of the thin films?.In addition,the layered vanadium oxide(i.e.V₇O₁₆)is successfully prepared by controlling these parameters during the preparation processes.Finally,we study the optical and electrical properties of the V₇O₁₆,and then the roles of oxygen defects on MIT,which provided an effective way for understanding the growth processes and controlling the microstructures in vanadium oxides.3.In vanadium oxides,it is still in dispute whether the electron-electron correlation?Mott transition?or the electron-phonon interaction induced by the lattice deformation?Peierls transition?drives the MIT.Here,we study the lattice distortion induced by oxygen defects and their effects on the MIT in VO₂.We find that the micro-strain caused by lattice distortion hardly changes the transition temperature of the MIT.Actually,separating SPT from MIT in VO₂ is an alternative and effective method for in-depth understanding of the IMT and SPT behaviors in VO₂,and the strained VO₂ are helpful to reveal the mechanism behind MIT in vanadium oxides.