Studies of energy transfer process between Gd3+ and Eu3+ ions in oxyfluoride sol-gel materials

Abstract

In this work, the series of silicate xerogels co-doped with Gd3+ and Eu3+ ions have been synthesized via a sol-gel method and further processed into nano-glass-ceramic materials. The mutual molar ratio of trivalent metal ions' acetates in samples’ compositions was gradually modified as follows: La3+:Gd3+:Eu3+ = (0.95-x):x:0.05, x = 0.05, 0.25, 0.45, 0.65, and 0.90; hence the molar ratio of Gd3+:Eu3+ (RGd/Eu) was set from 1 through 5, 9, 13 up to 18. XRD measurements confirmed the amorphous nature of fabricated xerogels, but their heat-treatment successfully triggered the crystallization of hexagonal LaF3 fraction with an average size of about ∼10 nm, which was additionally confirmed by HR-TEM microscopy. Additionally, it was found that the LaF3 phase crystallized in a whole range of proposed La(CH3COO)3:Gd(CH3COO)3 molar ratio, and Gd3+ ions played only the role of dopant and replaced La3+ cation in the fluoride crystal lattice. The optical characterization of prepared sol-gel samples involved the registration of absorption, excitation and emission spectra, along with the luminescence decay analysis from the 6P7/2 (Gd3+) and the 5D0 (Eu3+) excited states. For nano-glass-ceramics defined by RGd/Eu = 13 and 18, the unexpected prolongation of the τ(6P7/2):Gd3+ lifetimes was denoted, compared with samples singly-doped with Gd3+. The changes in photoluminescence behavior of studied samples are strongly dependent on the nature of the fabricated sol-gel samples, the established molar ratios of La3+:Gd3+:Eu3+, and the tendency of cation-exchange in LaF3 nanocrystal lattice by Gd3+ and Eu3+ ions.