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)]NO3.3H(2)O (3) have been synthesized by using boric acid as a flux at 240 degreesC, starting from rare earth oxides or nitrates and an excess of boric acid. All these polyborates crystallize in monoclinic structures (P2(1)/n) and consist of borate sheets as the fundamental unit, that is, [LnB(6)O(11)] 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 I and 2 at moderate temperature leads to two anhydrous pentaborates, alpha-LnB(5)O(9) (4) for Ln = Pr-Eu and beta-LnB(5)O(9) (5) for Ln = La, Ce. The structure of beta-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 P2(1)/c with a = 6.4418(l) Angstrom, b = 11.6888(3) Angstrom, c = 8.1706(2) Angstrom, and beta = 105.167(1)degrees. The structure of beta-LnB(5)O(9) contains buckled nine-membered ring borate sheets that are interlinked by BO3 groups forming a three-dimensional framework. The Eu3+-doped beta-LaB5O9 materials show dominant D-5(0 -->) F-7(2) emission and a low quenching concentration (0.6 at. %).