Novel Rare Earth Polyborates. 2. Syntheses and Structures

Three novel hydrated rare earth polyborates, Ln[B8O11(OH)5] (Ln = La−Nd) (1), Ln[B9O13(OH)4]·H2O (Ln = Pr − Eu) (2), and Ce[B5O8(OH)]NO3·3H2O (3) have been synthesized by using boric acid as a flux at 240 °C, starting from rare earth oxides or nitrates and an excess of boric acid. All these polyborates crystallize in monoclinic structures (P21/n) and consist of borate sheets as the fundamental unit, that is, [LnB6O11] sheet in 1 and 2 and [CeB5O9] sheet in 3. The borate sheets all contain a nine-membered borate ring, of which the rare earth cations are located around the center. The borate frameworks in 1 and 3 are two-dimensional, which are interlinked via ionic Ln−O bonds forming 3D structures. While in 2 the borate framework is three-dimensional with small channels filled by water molecules. Annealing the hydrated polyborates 1 and 2 at moderate temperature leads to two anhydrous pentaborates, α-LnB5O9 (4) for Ln = Pr−Eu and β-LnB5O9 (5) for Ln = La, Ce. The structure of β-LaB5O9 has been determined by an ab initio method using powder X-ray diffraction data. It crystallizes in a monoclinic structure in the space group P21/c with a = 6.4418(1) Å, b = 11.6888(3) Å, c = 8.1706(2) Å, and β = 105.167(1)°. The structure of β-LnB5O9 contains buckled nine-membered ring borate sheets that are interlinked by BO3 groups forming a three-dimensional framework. The Eu3+-doped β-LaB5O9 materials show dominant 5D0 → 7F2 emission and a low quenching concentration (0.6 at. %).