Yb 4f-Ta 5d Hybridization and Valence Evolution in Tetragonal Tungsten Bronze Ba(3-x)YbxTa5O15

Abstract

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 $x\le 1$, 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${}_{15}$.

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