Synthesis Of Quasi-one-dimensional VO₂Micro/Nanomaterials And Investigations Of Their High Pressure Structural Phase Transition

Vanadium dioxide (VO2) is a typical strongly correlated material, which containsrich structures and novel physical phenomena, like temperature inducedMetal-Insulator Transition (MIT) with dramatic changes in optical, electrical,magnetic and other physical properties, making it an useful material in the field oflaser blinding guards, infrared remote receiver solar temperature control devices andan important topic in condensed matter physics and material science. Nowdays,finding new structural change, establishing the relation between the structure andproperties and revealing the physical mechanism can be defined as the most importantscientific problems in VO2. As an independent physical parameter besides temperature,pressure can induce structural transition and form new high pressure structure, whichprovides a significant method to explore new structure, new property, newphenomenon and new rule. However, in contrast to temperature induced transition, thehigh pressure research is very limited except for a few low pressure research onVO2(M1). The higher pressure research in VO2(M1) and the research on otherinitiating structure is still blank. For micro and nano size effect on high-pressurestructural phase transition which has recently been very concerned about is stillunknown. Thus, we use in situ high pressure diamond anvil cell experimentaltechniques combined with theoretical calculation to focus on the structural phasetransition on micro/nano materials of VO2(B) and VO2(A), and discuss the initialstructural phase transition of VO2(M1) under higher pressure. We hope to reveal thehigh-pressure structural phase transition rules, find new physical phenomena,understand the strongly correlated systems deeply, and provide a scientific basis forobtaining new functional materials with novel structure and excellent physical properties. the specific research contents and results are as follows:1) By using hydrothermal method and calcine method, we successfullysynthesized VO2(A) quasi-microrods, VO2(B) nanorods and VO2(M1)quasi-microrods. These samples are very satisfactory for further study on the structureand properties under high pressure.2) We apply high pressure structural and electronical transition on monoclinicVO2(M1) quasi-microrods using high pressure XRD measurement, Ramanmeasurement and IR measurement. The highest pressure is33GPa. The first structuralphase transition from M1to Mx1occurs at4.6GPa and completes at10.1GPa; thesecond structural phase transition from Mx1to Mx2occurs at~32.5GPa and Mx2phase was stable up to the highest pressure32.5GPa as well as the whiledecompression process. Also we find a PIM occurred at~14.4GPa, after that theoptical conductivity increases with pressure. Upon decompression a metal-insulatortransition occurs in Mx2phase. Besides, we find the sample is still inquasi-one-dimensional structure. The high pressure behavior on VO2(M1)quasi-microrods is totally different from that on bulk materials, revealing a size effectunder pressure, which has an important guiding significance to explain the VO2structure phase transition mechanism and preparation of new functional material.3) We apply high pressure structural and electronical transition on monoclinicVO2(B) nanorods using high pressure XRD measurement, Raman measurement andIR measurement. The highest pressure is40GPa. We unveil a new metallic phase inVO2(B) nanorods as a result of Pressure-induced metallization(PIM) completed at~15GPa, which is different from all the observed temperature induced or pressure inducedmetallic states in VO2. Furthermore, we also find a pressure-inducedamorphization(PIA) in VO2(B) nanorods accomplished at~30GPa. Upondecompression a metal-insulator transition(MIT) occurs in amorphous VO2(B). Themetallization is due to the enhanced interaction of two layers along b axis, and theamorphization is possibly induced by the disruption of connectivity between theoctahedra in (010) plane. This work proves that the occurrence of both themetallization and amorphization is not just a special case in tetrahedrally coordinated materials SnX4/GeX4(X=halogen I, Br) which is the only family has been found, butcan branch out to other systems like octahedrally coordinated materials VO2.4) We apply high pressure structural and electronical transition on tetragonalVO2(A) quasi-microrods using high pressure XRD measurement, Ramanmeasurement and IR measurement. The highest pressure is46GPa. The observedtetragonal metallic state at~28GPa should be interpreted as a distinct metastable state,while increasing pressure to~32GPa, an amorphization is accomplished. Themetallization is due to V3d orbital electrons delocalization, and the amorphization isattributed to the unique variation of V-O-V bond angle. Upon decompression there isa MIT in amorphous VO2. These results not only give the experimental evidence of anew metallic VO2state directly, but also provide a synthetic guideline for functionalamorphous materials from their crystalline phases.