Double perovskite structure: A vibrational and luminescence investigation providing a perspective on crystal field strength

摘要

The luminescence spectra of Eu ~(3+) doped in a series of double perovskite lattices Ba _2LnMO _6 (Ln = Y, Gd; M = Nb, Ta) have been recorded at room temperature and 10 K. Together with FT-IR and FT-Raman spectra and aided by DFT vibrational energy calculations, assignments have been made for the crystal field levels of the ~5D _J (J = 0,1) and ~7F _J (J = 0-2) multiplets. The luminescence spectra are consistent with monoclinic symmetry of these systems. The crystal field parameters from the fitting of the energy level data set of Ba _2YNbO _6:Eu ~(3+) enable the crystal field strength to be calculated, and the order of magnitude is Cl ~- < O ~(2-) < F ~- for the EuX _6 ~(n-) (n = 6 for halogen, 9 for oxide) moieties. For these systems, an empirical linear relationship between crystal field strength and electronegativity of ligand X has been found. By contrast, the nephelauxetic series from the depression of the Slater parameter F ~2 is Cl ~- ≈ O ~(2-) > F ~- > free ion for these systems.