Syntheses and luminescence of complete solid solutions Gd1−xEux[B6O9(OH)3] and α-Gd1−xEuxB5O9

Abstract

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H₂O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd_1−xEu_x[B₆O₉(OH)₃] (x = 0.10–1) and their dehydrated products, α-Gd_1−xEu_xB₅O₉. Efficient energy transfer from Gd³⁺ to Eu³⁺ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B₆O₉(OH)₃], the re-crystallization to α-EuB₅O₉, and further decomposition to α-EuB₃O₆. During those processes, the Eu³⁺ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu³⁺. It is observed that the symmetry of the Eu³⁺ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB₅O₉ shows the highest luminescence efficiency when excited by Eu³⁺ CT transition. For the first time, complete solid solutions α-Gd_1−xEu_xB₅O₉ were synthesized directly by the sol–gel method, and their luminescence properties were also studied.