Novel Rare Earth Polyborates. 2. Syntheses and Structures

Three novel hydrated rare earth polyborates, Ln[B 8 O 11 (OH) 5 ] (Ln = La−Nd) ( 1 ), Ln[B 9 O 13 (OH) 4 ]·H 2 O (Ln = Pr − Eu) ( 2 ), and Ce[B 5 O 8 (OH)]NO 3 ·3H 2 O ( 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 ( P 2 1 / n ) and consist of borate sheets as the fundamental unit, that is, [LnB 6 O 11 ] sheet in 1 and 2 and [CeB 5 O 9 ] 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-LnB 5 O 9 ( 4 ) for Ln = Pr−Eu and β-LnB 5 O 9 ( 5 ) for Ln = La, Ce. The structure of β-LaB 5 O 9 has been determined by an ab initio method using powder X-ray diffraction data. It crystallizes in a monoclinic structure in the space group P 2 1 / 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-LnB 5 O 9 contains buckled nine-membered ring borate sheets that are interlinked by BO 3 groups forming a three-dimensional framework. The Eu 3+ -doped β-LaB 5 O 9 materials show dominant 5 D 0 → 7 F 2 emission and a low quenching concentration (0.6 at. %).