Site-selective spectroscopy in Eu3+ doped α-cordierite

The location of Eu³⁺ in α-cordierite, synthesized by a sol-gel route, was investigated by using both site-selective spectroscopy and time-resolved spectroscopy. ⁷F₀ → ⁵D_1,2 excitation and ⁵D₀ → ⁷F_0,1,2 emission spectra were performed at 77 K. The Eu³⁺ ions are distributed in two families : the A family in the structure channels and the B family at (4c) position of D_2h¹⁶ in substitution for Mg²⁺ Four B sub-families were characterized and understood as a difference at the second neighbours, i.e., the four possible occupations of the surrounding tetrahedra by Al and Si. The A family was characterized by anomalous spectra: strong ⁵D₀ → ⁷F₀ transition, high position of the ⁵D₀, ⁵D₁ and ⁵D₂ levels and very large ⁷F_1,2 splittings. By comparison with Eu-doped apatites, in which similar features occur, a predominant Eu-O bonding of covalent character with one of the oxygen ions bordering the channel was assumed. When the annealing temperature increases, Eu³⁺ ions migrate from the channel to the (4c) position.