Chemical bonding and crystal field splitting of the Eu3+ 7F1 level in the pyrochlores Ln2B2O7 (Ln = La3+,Gd3+, Y3+, Lu3+; B = Sn4+, Ti4+)

The crystal field splitting of Eu3+ F-7(1) level in the pyrochlore's Ln(2)Sn(2)O(7) (Ln = La3+, Gd3+, Y3+ and Lu3+) and Ln(2)Ti(2)O(7) (Ln = Gd3+, Y3+ and Lu3+) has been systematically examined. Since the splitting of the Eu3+ F-7(1) manifold in these materials increases with increasing size of the rare earth cation, the electrostatic point charge model is inadequate to account for the observed splitting. It is pointed out that covalency/ligand polarizability contributions to the crystal field splitting are significant. It is shown that the host lattice dependent changes in the polarizability of the oxygen ions occupying the 48f sites of the pyrochlore lattice is responsible for the variation in the splitting of the Eu3+ F-7(1) manifold in these materials. Further, the splitting of the Eu3+ F-7(1) manifold is larger in Ln(2)Sn(2)O(7) than in Ln(2)Ti(2)O(7). This is explained by the differences in the nature of the chemical bonding between Ti4+ (3d(0))/Sn4+(4d(10)) cations and O2-(2p) ligands. The importance of anion polarization towards the Eu3+ ion in determining the magnitude of the Eu3+ F-7(1) manifold splitting is demonstrated in this work. (C) 2008 Elsevier B.V. All rights reserved.