Reversal of Strain State in a Mott Insulator Thin Film by Controlling Substrate Morphology

The phase diagram of V₂O₃ contains two insulating phases and one metallic phase with distinct lattice structures whose stability is highly strain sensitive. In epitaxial thin films, strain is typically controlled through lattice mismatch with the substrate. Here, we show that substrate morphology itself can become a key control parameter by enabling thermal expansion mismatch to dominate the strain state. We investigate V₂O₃ films grown on sapphire, where lattice mismatch induces compressive strain while thermal expansion mismatch produces tensile strain. By modifying the sapphire surface morphology through annealing, which generates either atomically flat or stepped surfaces, the compressive strain can be partially relaxed. This relaxation allows the tensile thermal strain to overcome the lattice-mismatch contribution, resulting in either strongly compressive or strongly tensile strain states in otherwise nominally identical films. High-resolution STEM reveals crystallographic defects forming near substrate steps, identifying the microscopic origin of the enhanced relaxation. These morphology-controlled strain states strongly affect the electronic properties: compressive strain suppresses the metal-insulator transition, whereas tensile strain stabilizes insulating phases at all temperatures, producing resistivity changes spanning many orders of magnitude. Our results establish substrate morphology as a powerful route for strain engineering in correlated oxide thin films.