Pressure-induced amorphization (PIA) has drawn great attention due to the fundamental importance in the fields of physics, chemistry, materials, and earth sciences. Previous works are mainly focused on the microscopic structure and the driving forces of amorphization, but few are focused on the physical property change during the phase transition. A team from State Key Laboratory of Superhard Materials of Jilin University has gained insight into the pressure-induced metallization and amorphization in VO₂(A) nanorods. Their research has been published on May 24th, 2016 in Physical Review B.

The team observed pressure-induced metallization (PIM) and amorphization (PIA) in VO₂ (A). at about 28 and 32 GPa, respectively, and obtained a metallic state VO₂(A). They shown that the remarkable anisotropic compression along crystallographic directions and the unique variation of V-O-V bond angle results in the occurrence of PIA. In addition, the PIM was attributed to the delocalization of the V3d orbital electrons. Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials and provides important physical foundation in experimental understanding of MIT in VO₂.

(a)    Variation of relative lattice parameters a/a₀ and c/c₀ for VO₂(A) nanorods, (b) Pressure dependence of the bond angle of V-O-V in tetragonal phase upon compression.

The high pressure structural transition behaviors of VO₂(A) nanorods were studied by using the X-ray diffraction (XRD) of Beijing Synchrotron Radiation Facility (BSRF). VO₂(A) nanorods show an anisotropic compressibility as a axis is more compressible than c axis. Above 20 GPa, the bond angle of V-O1-V (the angle of two O atom shared octahedra along c axis) decreases sharply, theV-O2-V (the angle of two O atom shared octahedral in [110] plane) increases slightly. This multiple rotation of the octahedra makes a huge variation in lattice, leading to the amorphization in VO₂(A).

The research provides further insight into the PIA and PIM of VO₂, which is beneficial for exploring the phase diagram of VO₂. In this study, synchrotron sources have helped the team to perform the in situ study of the structural evolution of VO₂(A) nanorods under high pressure. "The PIA and PIM in VO₂(A) depend on the lattice structure and atom arrangement. Brilliant synchrotron beams would surely help to examine the crystals structure of samples under pressure, which is significant for further understanding the structural phase transitions and the change of properties under high pressure. " explains Bingbing Liu, the team leader and the professor of State Key Laboratory of Superhard Materials, Jilin University.

Article: Benyuan Cheng, Quanjun Li, Huafang Zhang, Ran Liu, Bo Liu, Zhen Yao, Tian Cui, Jing Liu, Zhenxian Liu, Bertil Sundqvist, and Bingbing Liu*, Pressure induced metallization and amorphization in VO₂(A) nanorods, Physical Review B 2016, 93,184109.