Molecular orbital formation and metastable short-range ordered structure in VO2

The metal-insulator transition (MIT) in vanadium dioxide $({\mathrm{VO}}_{2})$ due to V-V dimerization has been extensively discussed for decades. While it is widely acknowledged that electron correlations, Peierls instabilities, and molecular orbital formations are crucial for understanding the MIT of ${\mathrm{VO}}_{2}$, the primary origin of the MIT remains controversial. In this study, we delve into the crystal structure and orbital state of ${\mathrm{VO}}_{2}$ through synchrotron x-ray diffraction experiments. The molecular orbital formation corresponding to the V-V dimerization is directly observed in the low-temperature insulating monoclinic phase, called the M1 phase, as indicated by the valence electron density distribution. Moreover, diffuse scattering observed in the high-temperature metal phase of rutile structure suggests the presence of short-range correlation of V displacements, which is not directly attributed to the structural fluctuation of the M1 phase. The short-range order in the rutile phase will be the key to understanding the MIT in this system.