Chemical bonding and crystal field splitting of the Eu 3+7F 1 level in the pyrochlores Ln 2B 2O 7 (Ln = La 3+, Gd 3+, Y 3+, Lu 3+; B = Sn 4+, Ti 4+)

Abstract

The crystal field splitting of Eu 3+ 7F 1 level in the pyrochlore’s Ln 2Sn 2O 7 (Ln = La 3+, Gd 3+, Y 3+and Lu 3+) and Ln 2Ti 2O 7 (Ln = Gd 3+, Y 3+ and Lu 3+) has been systematically examined. Since the splitting of the Eu 3+ 7F 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 48 f sites of the pyrochlore lattice is responsible for the variation in the splitting of the Eu 3+ 7F 1 manifold in these materials. Further, the splitting of the Eu 3+ 7F 1 manifold is larger in Ln 2Sn 2O 7 than in Ln 2Ti 2O 7. This is explained by the differences in the nature of the chemical bonding between Ti 4+ (3d 0)/Sn 4+(4d 10) cations and O 2−(2p) ligands. The importance of anion polarization towards the Eu 3+ ion in determining the magnitude of the Eu 3+ 7F 1 manifold splitting is demonstrated in this work.