Redefinition of Crystal Structure and Bi³⁺ Yellow Luminescence with Strong Near-Ultraviolet Excitation in La₃BWO₉:Bi³⁺ Phosphor for White Light-Emitting Diodes

Abstract

Bi³⁺-activated photonic materials have received increased interest recently because they can be excited effectively with near-ultraviolet (NUV) but not visible light, thereby avoiding the reabsorption among phosphors, which cannot be solved intrinsically by traditional rare earth (e.g., Eu²⁺, Ce³⁺) phosphors. Such unique property suggests their potential application in NUV chip-based WLEDs. However, few Bi³⁺ phosphors exhibit strong excitation peak in NUV, though the excitation tail of some can extend to NUV. Herein, we report a novel yellow-emitting La₃BWO₉:Bi³⁺ (LBW:Bi³⁺) phosphor with strong NUV excitation. The photoluminescence (PL) spectroscopy analysis indicates that there are two Bi³⁺ luminescent centers in LBW:Bi³⁺ phosphor, which is clearly in contradiction with the established hexagonal structure of La₃BWO₉ with P6₃ space group because only one La site in this structure can accommodate Bi³⁺ ions. Combining the luminescent properties of Bi³⁺ with Rietveld refinement, La₃BWO₉ was redefined as a trigonal structure with the lower space group of P3 in which there are two independent crystallographic La sites. In addition, the rationalization of P3 space group was further confirmed by the finding of the reflection (0001) according to the extinction rule. Therefore, the PL behavior of Bi³⁺ can act as a complementary tool to determinate the real crystal structure especially when it is hard to distinguish by conventional X-ray diffraction techniques.