Coordination environment evolution of Eu3+ during the dehydration and re-crystallization processes of Sm1−xEux[B9O13(OH)4]·H2O by photoluminescent characteristics

Abstract

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H₂O or –OH may lead to emission quenching. We investigated the PL properties of Sm_1−xEu_x[B₉O₁₃(OH)₄]·H₂O (x = 0–1.00) and their dehydrated products α-Sm_1−xEu_xB₅O₉. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu³⁺ activators very well. Sm³⁺ and Eu³⁺ are basically separated luminescent activators. Comparatively, Sm³⁺ shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu³⁺ luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B₉O₁₃(OH)₄]·H₂O and a re-crystallization process to α-EuB₅O₉, during which luminescent spectra of Eu³⁺ are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu³⁺, even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu³⁺ in the amorphous state is the lowest. The high efficiency of the f–f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB₅O₉ shows the highest luminescent efficiency excited by Eu³⁺ CT transition. In addition, α-Sm_1−xEu_xB₅O₉ was synthesized by a sol–gel method directly for the first time, and α-EuB₅O₉ shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.