Interplay between electronic correlation and atomic disorder in a low carrier density 4d transition-metal oxide

We have investigated electronic structure evolution by cation substitution in ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_x{\mathrm{Nb}}₅{\mathrm{O}}₁₅$ by means of hard x-ray photoemission spectroscopy. Localization of Nb $4d$ electrons manifests as spectral weight transfer from a coherent component at the Fermi level to an incoherent one at 1--2 eV below it. This behavior is similar to that of electron-doped ${\mathrm{SrTiO}}₃$. On the other hand, Nb $3d$ and $4p$ core level spectra exhibit a screening effect with an energy scale of 1--2 eV by the coherent Nb $4d$ electrons similar to $4d$ electron systems near Mott transitions. The energy scale indicates that electron correlation is involved in the metal to insulator transition in the present system, although the Nb $4d$ band is about 1/10 filled. The present results suggest a mechanism of electron localization due to atomic disorder and electron correlation.