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

Two new alkali-earth and rare-earth metal borates, Sr2Tb3B27-δO46 and Ba2Eu3B27-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) Å3, Z = 3 and a = 7.7964(1) Å, c = 46.5086(6) Å, V = 2448.23(7) Å3, Z = 3, respectively, which further confirms that "LnMB9O16" has the right formula of M2Ln3B27-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, Sr2Tb3B27-δO46 exhibits the characteristic emission lines originating from Tb3+ corresponding to 5D4-7F6,5,4,3,2,1,0 transitions, whereas Ba2Eu3B27-δO46 displays bright red emission from Eu3+ with four groups of emission bands associated with 5D0 → 7F1,2,3,4 transitions. Both phosphors show stable luminescence and color purity at elevated temperatures. As it may be expected, Sr2Tb3B27-δO46 and Ba2Eu3B27-δO46 (δ = 2/3) are promising green (Tb) and red (Eu) components for use in WLEDs.