Crystal structure of sodium rare earth oxyborates Na2Ln2(BO3)2O (Ln = Sm, Eu, and Gd) and optical analysis of Na2Gd2(BO3)2O : Eu3+

Abstract A new structural family of rare earth oxyborates Na 2 Ln 2 (BO 3 ) 2 O ( Ln =Sm, Eu and Gd) is evidenced. The structure, determined by single crystal X-ray diffraction, is monoclinic, space group P 2 1 / c , Z =4, with a =10.695(6) A, b =6.320(4) A, c =10.328(6) A, β =117.80(4)°, V =617.5(9) A 3 , R 1 =0.039, w R 2 =0.101 for Na 2 Gd 2 (BO 3 ) 2 O. The three-dimensional network is built up from infinite sheets of Ln O 8 polyhedra in the ( b, c ) plane, which are separated by sodium ions. The luminescence of trivalent europium in polycrystalline Na 2 Gd 2 (BO 3 ) 2 O: Eu 3+ is analysed at 77 K. The low symmetry of the rare earth sites, deduced from the X-ray diffraction study, is confirmed. The crystal field strength is high for both europium sites.