Non-congruence of thermally driven structural and electronic transitions in VO₂

The multifunctional properties of vanadium dioxide (VO₂) arise from coupled first-order phase transitions: an insulator-to-metal transition (IMT) and a structural phase transition (SPT) from monoclinic to tetragonal. The characteristic signatures of the IMT and SPT are the hysteresis loops that track the phase transition from nucleation to stabilization of a new phase and back. A long-standing question about the mechanism of the VO₂ phase transition is whether and how the almost-simultaneous electronic and structural transitions are related. Here, we report independent measurements of the IMT and SPT hystereses in epitaxial VO₂ films on c-sapphire with distinct morphologies. The measurements show that the IMT and the SPT are not congruent, in that the structural phase transition requires more energy to reach completion than the electronic, insulator-to-metal transition. This result is independent of nanoscale film morphology and grain orientation on the substrate, so that the non-congruence is an intrinsic property of the VO₂ phase transition. Our conclusion is supported by effective-medium calculations of the dielectric function incorporating the measured volume fractions of the monoclinic and tetragonal states. The results are consistent with the existence of an intermediate metallic state in which the electron-electron correlations characteristic of the monoclinic state begin to disappear before the transition to the tetragonal structural state.