Unusual valence state and metal-insulator transition in BaV10O15 probed by hard x-ray photoemission spectroscopy

We have studied the electronic structure of ${\mathrm{BaV}}₁₀{\mathrm{O}}₁₅$ across the metal-insulator transition with V trimerization by means of hard-x-ray photoemission spectroscopy (HAXPES) and mean-field calculations. The V $2p$ HAXPES indicates ${\mathrm{V}}^2.5+\ensuremath{-}{\mathrm{V}}³⁺$ charge fluctuation in the metallic phase, and ${\text{V}}²⁺\ensuremath{-}{\text{V}}³⁺$ charge order in the insulating phase. The ${\mathrm{V}}^2.5+\ensuremath{-}{\mathrm{V}}³⁺$ charge fluctuation is consistent with the mean-field solution where a V $3d {a}_1g$ electron is shared by two V sites with face-sharing ${\mathrm{VO}}₆$ octahedra. The valence-band HAXPES of the metallic phase exhibits pseudogap opening at the Fermi level associated with the charge fluctuation, and a band gap $\ensuremath{\sim}200$ meV is established in the insulating phase due to the switching of charge correlation.