Two new rare-earth borates Sr2Tb3B27−δO46 and Ba2Eu3B27−δO46 (δ = 2/3): syntheses, crystal structures, and luminescence properties

Abstract

Two new alkali-earth and rare-earth metal borates, Sr₂Tb₃B_27−δO₄₆ and Ba₂Eu₃B_27−δO₄₆ (δ = 2/3), have been prepared by high-temperature solution reactions at 950 °C. The single-crystal XRD analysis showed that the borates crystallize in the rhombohedral space group R3 with the cell parameters a = 7.7468(1) Å, c = 46.3657(5) Å, V = 2409.75(3) ų, Z = 3 and a = 7.7964(1) Å, c = 46.5086(6) Å, V = 2448.23(7) ų, Z = 3, respectively, which further confirms that “LnMB₉O₁₆” has the right formula of M₂Ln₃B_27−δO₄₆ (δ = 2/3). Their crystal structures consist of 3D networks constructed by Tb(Eu)O₆ trigonal prisms, BO₃ planar triangles, and BO₄ tetrahedra, with large cavities accommodating Sr²⁺ (Ba²⁺) cations. Furthermore, the phase composition, morphology, IR, Raman, and XPS spectra as well as photoluminescence properties of these two compounds were studied. Upon excitation by ultraviolet radiation, Sr₂Tb₃B_27−δO₄₆ exhibits the characteristic emission lines originating from Tb³⁺ corresponding to ⁵D₄–⁷F_{6,5,4,3,2,1,0} transitions, whereas Ba₂Eu₃B_27−δO₄₆ displays bright red emission from Eu³⁺ with four groups of emission bands associated with ⁵D₀ → ⁷F_1,2,3,4 transitions. Both phosphors show stable luminescence and color purity at elevated temperatures. As it may be expected, Sr₂Tb₃B_27−δO₄₆ and Ba₂Eu₃B_27−δO₄₆ (δ = 2/3) are promising green (Tb) and red (Eu) components for use in WLEDs.