Insulator-Metal-Transition of VO₂ with Modified Orbital Occupancy by Octahedral Symmetry

Vanadium dioxide (VO₂) has received much attention due to its insulator-metal-transition (IMT) accompanied by the structural phase transition from an asymmetric to a symmetry octahedral structure near room temperature (~68 ^oC). In the structural aspect, the variation of the asymmetric octahedral structure is known to tune the IMT temperature effectively. However, most studies regarding the effect of the octahedral symmetry carried out on the rutile-like VO₂ having a symmetrical octahedral structure.

In this presentation, we show the correlation between the asymmetry octahedral structure, orbital occupancy, and the IMT temperature of monoclinic VO₂ films with different in-plane compressive strains using x-ray diffraction and x-ray spectroscopy techniques with the theoretical calculation. The octahedral structure with low asymmetry, caused by the in-plane compressive strain, increased the splitting between d_// and d_//^* orbitals and the bandwidth of π^*. The modified orbitals suppressed the hybridization of V 3d - O 2p and subsequently increased interdimer hopping energy, lowering the energy barrier for IMT. As a result, the VO₂ with the low asymmetric octahedral structure has a lower IMT temperature than the VO₂ with a high asymmetric one. These results provide the role of octahedral symmetry in tuning the IMT temperature of VO₂.