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

A new structural family of rare earth oxyborates Na(2)Ln(2)(BO3)(2)O (Ln = Sm, Eu and Gd) is evidenced, The structure, determined by single crystal X- ray diffraction, is monoclinic, space group P2(1)/c, Z=4, with a 10.695(6) Angstrom, b = 6.320(4) Angstrom, c = 10.328(6) Angstrom, beta = 117.80(4)de grees, V = 617.5(9)Angstrom(3), R-1 = 0.039, wR(2) = 0.101 for Na2Gd2(BO3)( 2)O. The three-dimensional network is built up from infinite sheets of LnO( 8) polyhedra in the (b,c) plane, which are separated by sodium ions. The lu minescence of trivalent europium in polycrystalline Na2Gd2(BO3)(2)O: Eu3+ i s analysed at 77K, The low symmetry of the rare earth sites, deduced from t he X-ray diffraction study, is confirmed, The crystal field strength is hig h for both europium sites. (C) 1999 Academic Press.