The fate of phase transitions in VO2 on doping

Abstract

Ever since its discovery more than six decades ago, thermally-induced insulator-metal transition (IMT) in vanadium dioxide (VO2) has attracted much attention, not only because of its near-room temperature IMT at ~340 K, having potential applications in optoelectronic devices, and neuromorphic computing, but also because of the intimate connection of the transport transition to this compound’s structural peculiarities, with the low-temperature insulating monoclinic (M1) phase having the P2₁/c space group while the high-temperature metallic rutile (R) phase being in the tetragonal P4₂/mnm space group. This thesis focuses on how mild perturbations, such as doping trivalent ions (e.g., Cr3+, Al3+, Fe3+, etc.) at the V site, can alter the structure of VO2, leading to several other phases like T (P-1), M2 (C2/m), and M4 (P2/m). These phases exhibit interesting variations in structural features such as the extents of dimerization and tilt, and consequently, electron-electron correlation strength. Specifically, using single-crystal and powder X-ray diffraction along with differential scanning calorimetry measurements, this work establishes the primitive unit cell of the triclinic T phase and the first-order nature of the T-M2 phase transition through constructions of unique microscopic order parameters. Furthermore, it presents the local-structural peculiarities of the higher-doped M4 phase using extended X-ray absorption fine structure measurements. These measurements reveal the presence of V-V dimerization on the local scale, even when the global crystal structure indicates otherwise. The change in electronic structure across the various structural transitions in Al and Cr-doped VO2 systems has also been studied using bulk-sensitive hard X-ray photoelectron spectroscopy. These results show minimal changes across the various low-temperature insulating phases (M1, T, and M2), highlighting the importance of electron-electron correlation in these systems. Additionally, they indicate that the M4-R transition is an insulator-bad metal transition and reveal an unusual increase in electron-electron correlation with doping in the high-temperature correlated metallic R phase. Finally, this thesis investigates the impact of hydrostatic pressure on the M4-R phase transition of Cr-doped VO2, which leads to several new phases, such as monoclinic M4’ and orthorhombic O, thereby enabling the construction of a comprehensive temperature-pressure phase diagram for the M4 phase.