We investigated the electronic structure of BaEu₂Nb₅O₁₅ and Eu₃Nb₅O₁₅ by means of hard x-ray photoelectron spectroscopy. The Eu $3d$ core level spectra are primarily dominated by Eu²⁺ signals. The well-screened feature in the Nb $3d$ core level spectra of BaEu₂Nb₅O₁₅ is suppressed in Eu₃Nb₅O₁₅, consistent with the insulating behavior of Eu₃Nb₅O₁₅ 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.
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Nakamura et al. (2024) studied this question.
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