Thesis Colloquium: The fate of phase transitions in VO2 on doping

Ever since its discovery more than six decades ago [1], thermally-induced insulator-metal transitions (IMT) in vanadium dioxide (VO₂) has attracted much attention, not only because of its near-room temperature IMT at ~340 K [1], having potential applications in optoelectronic devices [2], and neuromorphic computing [3], but also because of the intimate connection of the transport transition to this compound’s structural peculiarities, first pointed out by Goodenough [4,5], 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.

The striking difference between these two structures lies in the arrangement of the V ions. In the M1 phase, all the V ions form dimerized zig-zag chains. In contrast, the R phase does not exhibit any dimerization or tilt. The concurrence of the structural and electronic transitions led to a controversy simmering over decades concerning the nature of the IMT with the structural transition (a Peierls transition in d¹ V⁴⁺ chains) and electron-electron correlation (Mott transition) as the possible underlying reasons for the IMT.

In this context, it has also been realized that mild perturbations, like 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) [6,7]. These phases exhibit interesting variations in structural features such as the extents of dimerization and tilt, and consequently, electron-electron correlation strength. In this thesis, we explore the effects of doping Cr³⁺ and Al³⁺ on the phase transitions in VO₂, focusing primarily on temperature and dopant concentration-induced global and local structural modifications, as well as the changes in bulk-sensitive electronic structures. Furthermore, beyond the temperature-induced phase transitions, we investigate the impact of hydrostatic pressure on the M4 phase of Cr-doped VO₂, constructing a comprehensive temperature-pressure phase diagram for the M4 phase.