Interplay between strongly localized Eu 4f and weakly localized Nb 4d electrons in Eu3Nb5

We investigated the electronic structure of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ and ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ by means of hard x-ray photoelectron spectroscopy. The Eu $3d$ core level spectra are primarily dominated by ${\mathrm{Eu}}^{2+}$ signals. The well-screened feature in the Nb $3d$ core level spectra of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ is suppressed in ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$, consistent with the insulating behavior of ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious $A2$ site. In the valence band spectra, the Nb $4d$ states at the Fermi level are close to the Eu $4f$ states located around 2 eV below it. This suggests that the degree of localization of the Nb $4d$ electrons can be enhanced through the Eu $4f$--Nb $4d$ hybridization with the atomically and magnetically disordered Eu ions at the $A2$ site.