The distribution of luminescent Eu3+ ions in crystals with the cubic bixbyite-type structure is subject of debate. In this work, the actual occupancy of the two cation sites available for europium in yttria and lutetia nanocrystals with sizes of 10-20 nm has been evaluated by means of Eu-151 Mossbauer spectroscopy. The spectral contribution of the ion at different crystalline sites has been resolved, allowing for the quadrupole splitting. The spectra of the nanocrystalline Y1.8Eu0.2O3 and Lu1.8Eu0.2O3 compounds have been analyzed in detail; the C-3i and C-2 sites are occupied by the europium ion in a statistical way. The hyperfine parameters have been discussed in terms of symmetry and bonding. An increase of the covalence of the Eu-O bond has been found in the series of compounds Gd2O3, Eu2O3, Y2O3 and Lu2O3
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