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

TL;DR

This study explores the electronic structure and valence states of Yb in Ba(3-x)Yb_xTa5O15 using hard x-ray photoemission spectroscopy, revealing valence evolution and Yb 4f-Ta 5d hybridization that influence its physical properties.

Contribution

It provides new insights into the valence states and hybridization effects of Yb in this tungsten bronze, highlighting the valence evolution with Yb substitution and its impact on electronic interactions.

Findings

Yb primarily in +2 state at low x, shifting to +3 at higher x

Evidence of Yb 4f and Ta 5d hybridization near the Fermi level

Site preference of Yb ions influences valence states

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 $x\leq1$, 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$_{15}$.