Positive and Negative Pressure Regimes in Anisotropically Strained V₂O₃ Films

The metal-insulator phase transitions in V2O3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V2O3 films are explored with nearly the same contraction along the c-axis, but different degrees of ab-plane expansion. With small ab-plane expansion, the films behave similar to bulk V2O3 under hydrostatic pressure. However, with large ab-plane expansion, the films are driven into the "negative pressure" regime, similar to that of Cr-doped V2O3, exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180-500 K. This shows that c-axis contraction alone, or an increase in c/a ratio is insufficient for inducing "negative pressure" effects. Actually, c-axis contraction alone destabilizes the two insulating phases of V2O3, whereas a-axis expansion tends to stabilize them. The effects of strain are modeled using density functional theory providing good agreement with experimental results. The findings show that chemical pressure alone cannot account for the phase diagram of (V1-xCrx)(2)O-3. This work enables to manipulate a Mott transition above room temperature, thereby expanding the opportunities for applications of V2O3 in novel electronics.