Crystal Structure of Sodium Rare Earth Oxyborates Na2Ln2(BO3)2O (Ln=Sm, Eu, and Gd) and Optical Analysis of Na2Gd2(BO3)2O:Eu3+

A new structural family of rare earth oxyborates Na2Ln2(BO3)2O (Ln=Sm, Eu and Gd) is evidenced. The structure, determined by single crystal X-ray diffraction, is monoclinic, space groupP21/c,Z=4, witha=10.695(6) ?,b=6.320(4) ?,c=10.328(6) ?,BETA=117.80(4)°,V=617.5(9) ?3,R1=0.039, wR2=0.101 for Na2Gd2(BO3)2O. The three-dimensional network is built up from infinite sheets ofLnO8polyhedra in the (b, c) plane, which are separated by sodium ions. The luminescence of trivalent europium in polycrystalline Na2Gd2(BO3)2O: Eu3+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.