Metal valence states in Eu0.7NbO3, EuNbO3, and Eu2Nb5O9 by TB-LMTO-ASA band-structure calculations and resonant photoemission spectroscopy

摘要

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. [References: 21]