Yb 4f-Ta 5d Hybridization and Valence Evolution in Tetragonal Tungsten Bronze Ba_3-xYb_xTa₅O₁₅

Here we investigate the electronic structure of the tetragonal tungsten bronze Ba3-xYbxTa5O15 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 <= 1, Yb is found to be dominantly Yb2+ with a small mixing of Yb3+, while for x > 1, a significant increase of Yb3+ is found, suggesting not only that site S2 favours Yb3+, but also that their presence affects also the valency of the ions in site S1. The valence band spectra show a relatively deep Yb2+ 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 Ba3-xYbxTa5O15.