Oxygen isotope effect in VO2

We investigate the oxygen isotope effect on the VO2 metal-insulator transition. Using an alternative method, we synthesize V16O2 and V18O2 crystals, finding a 1%–3% phonon softening and a 1.3 K increase in the metal-insulator transition temperature for the latter. A simple calculation, further confirmed by density functional theory, shows that this shift can be attributed to changes in the lattice internal energy. Our results show that lattice dynamics plays a key role in setting the electronic transition temperature, indicating that electronic and structural degrees of freedom remain strongly coupled at the transition.