Interplay between Mott physics and Peierls physics in hollandite-type vanadates with a metal-insulator transition

We report an x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) study on K₂V_8-xTi_xO ₁₆ (x=0.0, 0.3, and 4.0) in which the metal-insulator transition (MIT) and the tetragonal-to-monoclinic structural transition show different responses against the Ti doping. The spectral change of V 2p XAS across the MIT is similar to that observed in VO₂, indicating that the V-V pairing is responsible for the MIT. In the insulating phase, the V 2p XPS spectra are split into the V3⁺ and V4⁺ features showing the localized character of the V 3d electrons. In addition, the V 2p XAS spectra of the monoclinic insulating phase suggest that the V-V pairing is weakened, and, instead, the V3⁺-V4⁺ coupling is strengthened by orbital ordering. In contrast to VO₂, the existence of the V3⁺ component in K₂V_8-xTi_xO₁₆ gives additional orbital instability in the insulating phase, which competes with the V-V pairing.