Congruent melt terbium-rich borate Na2Tb2B2O7: Synthesis, crystal structure, optical and magnetic properties

Single crystals of Na2Tb2B2O7 were successfully grown from a Na2O - B2O3 - NaF flux, and its lanthanide homotypic compounds were synthesized using the solid-state reaction method and of pure phase according to the refinements of the X-ray diffraction patterns. Na2Tb2B2O7 belongs to monoclinic system, P21/c space group, with cell parameters of a = 10.643(2) Å, b = 6.2724(13) Å, c = 10.247(2) Å, β = 117.72(3)° and Z = 4. In the structure, the terbium atoms coordinate to eight oxygen atoms to form a TbO8 polyhedra, which are connected through edge-sharing O atoms to form a layered [Tb2(BO3)2O]2- sheets in the bc plane. Then the sheets are further linked along a direction to build three dimensional framework with the separation of Na+ cations. Na2Tb2B2O7 exhibits high transparent in the range of 500-1500 nm. Meanwhile, it shows a green-emitting band due to the characteristic electronic 5D4 to 7F5 transitions of Tb3+ ions. Magnetic measurements show that Na2Tb2B2O7 exhibits typical paramagnetic behaviors from 2 K to 300 K.