Ultra-narrow band blue emission of Eu²⁺ in halogenated (Alumino)borate systems based on high lattice symmetry

Phosphor materials with ultra-high color purity are highly desired in backlit display and WLEDs. How to achieve high-purity three-primary emission in rare earth ions activated inorganic phosphors has become a hot topic. Herein, we reported ultra-narrow band and highly efficient blue-violet-emitting Eu²⁺-doped Ba₂B₅O₉X (fwhm = 31 nm) and NaBa₄(AlB₄O₉)₂X₃ (X = Cl, Br) (fwhm = 43 nm) phosphors with peak positions around 424-437 nm. Especially, the color purity of Ba₂B₅O₉Cl:Eu sample even exceeded 97%, its internal quantum efficiency could achieve 87%. The EXANES analysis revealed that the Eu mainly existed in the form of +2. According to the Rietveld structural refinement, extraordinarily narrow band emission should be attributed to the highly symmetric lattice structures with the flower-like polyhedrons in the studied (alumino)borate matrix. Significantly, the color gamut of as-prepared blue phosphor combined with the standard green and red phosphors was almost close to that of Rec. 2020 display standard. In addition, cation substitution strategy in NaBa₄(Al_xB_{5- x}O₉)₂Cl₃ (x = 0-4) and NaBa₄(Ga_yB_{5- y}O₉)₂Cl₃ (y = 0-3) samples successfully achieved spectra adjustment, and the underlying mechanism was proposed. All these results demonstrate that the as-prepared phosphors could be superior blue-emitting candidates for backlit display as well as WLEDs.