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

Abstract

We report an x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) study on ${\mathrm{K}}_{2}$V${}_{8\ensuremath{-}x}$Ti${}_{x}$O${}_{16}$ ($x$ = 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 2$p$ XAS across the MIT is similar to that observed in VO${}_{2}$, indicating that the V-V pairing is responsible for the MIT. In the insulating phase, the V 2$p$ XPS spectra are split into the ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{4+}$ features showing the localized character of the V 3$d$ electrons. In addition, the V 2$p$ XAS spectra of the monoclinic insulating phase suggest that the V-V pairing is weakened, and, instead, the ${\mathrm{V}}^{3+}$-V${}^{4+}$ coupling is strengthened by orbital ordering. In contrast to VO${}_{2}$, the existence of the ${\mathrm{V}}^{3+}$ component in ${\mathrm{K}}_{2}$V${}_{8\ensuremath{-}x}$Ti${}_{x}$O${}_{16}$ gives additional orbital instability in the insulating phase, which competes with the V-V pairing.