Syntheses, Structure, and Luminescent Properties of Novel Hydrated Rare Earth Borates Ln2B6O10(OH)4•H2O (Ln = Pr, Nd, Sm, Eu, Gd, Dy, Ho, and Y)

Ln2B6O10(OH)4•H2O (Ln = Pr, Nd, Sm−Gd, Dy, Ho, and Y), a new series of hydrated rare earth borates, have been synthesized under hydrothermal conditions. A single crystal of Nd analogue was used for the structure determination by X-ray diffraction. It crystallizes in the monoclinic space group C2/c with lattice constants a = 21.756(4), b = 4.3671(9), c = 12.192(2) Å, and β = 108.29(3)°. The other compounds are isostructural to Nd2B6O10(OH)4•H2O. The fundamental building block (FBB) of the polyborate anion in this structure is a three-membered ring [B3O6(OH)2]5−. The FBBs are connected by sharing oxygen atoms forming an infinite [B3O5(OH)2]3− chain, and the chains are linked by hydrogen bonds, establishing a two-dimensional (2-D) [B6O10(OH)4•H2O]6− layer. The 2-D borate layers are thus interconnected by Ln3+ ions to form the complex three-dimensional structure. Ln2B6O10(OH)4•H2O dehydrates stepwise, giving rise to two new intermediate compounds Ln2B6O10(OH)4 and Ln2B6O11(OH)2. The investigation on the luminescent properties of Gd2−2xEu2xB6O10(OH)4•H2O (x = 0.01−1.00) shows a high efficiency of Eu3+ f−f transitions and the existence of the energy transfer process from Gd3+ to Eu3+. Eu2B6O10(OH)4•H2O and its two dehydrated products, Eu2B6O10(OH)4 and Eu2B6O11(OH)2, present the strongest emission peak at 620 nm (5D0 → 7F2 transition), which may be potential red phosphors.