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 H 2 O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd 1Àx Eu x [B 6 O 9 (OH) 3 ] (x = 0.10-1) and their dehydrated products, a-Gd 1Àx Eu x B 5 O 9 . Efficient energy transfer from Gd 3+ to Eu 3+ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B 6 O 9 (OH) 3 ], the re-crystallization to a-EuB 5 O 9 , and further decomposition to a-EuB 3 O 6 . During those processes, the Eu 3+ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu 3+ . It is observed that the symmetry of the Eu 3+ 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 a-EuB 5 O 9 shows the highest luminescence efficiency when excited by Eu 3+ CT transition. For the first time, complete solid solutions a-Gd 1Àx Eu x B 5 O 9 were synthesized directly by the sol-gel method, and their luminescence properties were also studied.

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-x)Eu(x)[B9O13(OH)4]·H2O (x = 0-1.00) and their dehydrated products α-Sm(1-x)Eu(x)B5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu(3+) activators very well. Sm(3+) and Eu(3+) are basically separated luminescent activators. Comparatively, Sm(3+) shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu(3+) 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 Eu(3+) are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu(3+), even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu(3+) 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 Eu(3+) CT transition. In addition, α-Sm(1-x)Eu(x)B5O9 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.