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Abstract:
Thr electronic structures of Eu2Nb5O9, EuNbO3, and Eu0.7NbO3 have been
investigated by photoemission and total-yield spectroscopy with
synchrotron radiation, and in the case of Eu2Nb5O9 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. Bath 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 4d-->4f technique to determine the Eu valence, The
final-state 4f(b) 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 EuNbO3 and Eu0.7NbO3,
while in Eu2Nb5O9, two 3d doublets have been observed, corresponding to
two chemically distinct niobium atoms in this compound. The 3d(5/2) peak
in EuNbO3 is assigned to the + 4 nominal valence state at a binding
energy of 209.7 eV. The doublet of Eu0.7NbO3 is observed at 0.5 eV
higher binding energy (at 210.2 eV), which then corresponds to a nominal
Nb+4+delta chemical state. In Eu2Nb5O9, the valence of Nb in the NbO6
octahedra is less than + 5 and in the Nb6O12 clusters is close to + 2 as
expected. This is in accordance with the LMTO band-structure
calculations.
