METAL VALENCE STATES IN EU0.7NBO3, EUNBO3, AND EU2NB5O9 BY TB-LMTO-ASA BAND-STRUCTURE CALCULATIONS AND RESONANT PHOTOEMISSION SPECTROSCOPY

Thr electronic structures of Eu2Nb5O9, EuNbO3, and Eu0.7NbO3 have been investigated by photoemission and total-yield spectroscopy with synch rotron radiation, and in the case of Eu2Nb5O9 by tight-binding linear muffin-tin orbital (LMTO) band-structure calculations. A central quest ion for reduced europium niobates is that of the valence of Eu and Nb. Bath europium and niobium atoms can appear in different valence state s so that various electronic configurations in the title compounds are possible. For this reason, the valence band was studied by the resona nt Eu 4d-->4f technique to determine the Eu valence, The final-state 4 f(b) multiplet of divalent Eu is dominant in all spectra. Since there are no 4f density of states at the Fermi level, valence fluctuations a re 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 N b 3d core level in EuNbO3 and Eu0.7NbO3, while in Eu2Nb5O9, two 3d dou blets have been observed, corresponding to two chemically distinct nio bium 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 do ublet 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 ac cordance with the LMTO band-structure calculations.