Metal valence states inEu0.7NbO3,EuNbO3,andEu2Nb5O9by TB-LMTO-ASA band-structure calculations and resonant photoemission spectroscopy

Abstract

The electronic structures of ${\mathrm{Eu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{9},$ ${\mathrm{EuNbO}}_{3},$ and ${\mathrm{Eu}}_{0.7}{\mathrm{NbO}}_{3}$ have been investigated by photoemission and total-yield spectroscopy with synchrotron radiation, and in the case of ${\mathrm{Eu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{9}$ by tight-binding linear muffin-tin orbital (LMTO) band-structure calculations. A central question for reduced europium niobates is that of the valence of Eu and Nb. Both europium and niobium atoms can appear in different valence states so that various electronic configurations in the title compounds are possible. For this reason, the valence band was studied by the resonant Eu $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ technique to determine the Eu valence. The final-state ${4f}^{6}$ multiplet of divalent Eu is dominant in all spectra. Since there are no $4f$ density of states at the Fermi level, valence fluctuations are not expected. The niobium valence states were investigated by core-level spectroscopy. We found only one ${3d}_{5/2}{3d}_{3/2}$ doublet for the Nb $3d$ core level in ${\mathrm{EuNbO}}_{3}$ and ${\mathrm{Eu}}_{0.7}{\mathrm{NbO}}_{3},$ while in ${\mathrm{Eu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{9},$ two $3d$ doublets have been observed, corresponding to two chemically distinct niobium atoms in this compound. The ${3d}_{5/2}$ peak in ${\mathrm{EuNbO}}_{3}$ is assigned to the $+4$ nominal valence state at a binding energy of 209.7 eV. The doublet of ${\mathrm{Eu}}_{0.7}{\mathrm{NbO}}_{3}$ is observed at 0.5 eV higher binding energy (at 210.2 eV), which then corresponds to a nominal ${\mathrm{Nb}}^{+4+\mathrm{\ensuremath{\delta}}}$ chemical state. In ${\mathrm{Eu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{9},$ the valence of Nb in the ${\mathrm{NbO}}_{6}$ octahedra is less than $+5$ and in the ${\mathrm{Nb}}_{6}{\mathrm{O}}_{12}$ clusters is close to $+2$ as expected. This is in accordance with the LMTO band-structure calculations.