Yb 4 <i>f</i> -Ta 5 <i>d</i> Hybridization and Valence Evolution in Tetragonal Tungsten Bronze Ba ₃₋ <i> _x </i> Yb <i> _x </i> Ta ₅ O ₁₅

Here we investigate the electronic structure of the tetragonal tungsten bronze Ba$_{3-x}$Yb$_x$Ta$_{5}$O$_{15}$ by making use of hard x-ray photoemission spectroscopy. The core level spectroscopy shows that the substitution with Yb ions in the series first occurs on the compact S1 site. For $x\leq1$, Yb is found to be dominantly Yb$^{2+}$ with a small mixing of Yb$^{3+}$, while for $x>1$, a significant increase of Yb$^{3+}$ is found, suggesting not only that site S2 favours Yb$^{3+}$, but also that their presence affects also the valency of the ions in site S1. The valence band spectra shows a relatively deep Yb$^{2+}$ doublet, but at the same time indications of a Ta~$5d$-Yb~$4f$ interaction are found, suggesting the presence of Yb~$4f$ carriers at the Fermi level through this hybridization. Our results thus point towards an exotic form of $d$-$f$ electronic interplay that together with the structural degrees of freedom can result in the unusual trends observed in the physical properties of Ba$_{3-x}$Yb$_x$Ta$_{5}$O$_{15}$.