Energy Transfer in Eu _x Gd_1-x (OH)_3,am

This study examined the luminescence properties and quenching mechanism in amorphous Eu-(OH)₃ and Eu _x Gd_1-x (OH)₃ under alkaline conditions (pH 12.5) representative of cement porewaters. Different Gd-(III) contents were used to alter the average Eu-(III)-Eu-(III) distances in the amorphous solids. Gd-(III) was considered "inert" due to the similar ionic radius and lack of spectral overlap between Eu-(III) and Gd-(III). The Eu-(III) luminescence was investigated using time-resolved laser-induced luminescence spectroscopy (TRLFS), and the luminescence decay curves were analyzed according to the Inokuti-Hirayama model. It was found that the luminescence quenching in the investigated materials is primarily caused by energy migration between Eu-(III) ions best described by dipole-dipole interactions with an estimated critical Eu-(III)-Eu-(III) distance R ₀ of 5.8 ± 0.3 Å. Temperature-dependent experiments revealed that the quenching efficiency significantly decreased below 250 K, indicating that Eu-(III) ions in thermally populated ⁷F₁ and ⁷F₂ levels take part in the energy migration. Employing the RET pair Tb-(III)/Eu-(III) in Tb _x Eu_0.0005x Gd_1-x (OH)₃ offered additional evidence that interlanthanide energy transfer occurs in the amorphous lanthanide hydroxides. In addition, concentration-independent quenching processes, such as energy transfer to vibrational overtones of OH^- or H₂O, were observed. These findings contribute to understanding the photophysics of amorphous lanthanide hydroxides and may also offer insights into other Eu-(III)-rich phases that may form under conditions relevant for nuclear waste repositories.