Multispectral tunability in single Eu2+-doped (Ba,Sr)5(PO4)3Br phosphor

Abstract

Spectral modification is crucial for luminescent materials. A variety of strategies have been proposed in the literature, including the selection of activator ions with characteristic electron configurations, the modification of the local crystal field, and the design of an emissive center pair and energy transfer. Eu²⁺-activated haloapatites are considered as candidates for phosphor converted light emitting devices due to their excellent stabilities, easy preparation, and high efficiency. In this work, single-phased, single-doped (Ba,Sr)₅(PO₄)₃Br:Eu²⁺ is selected and multiple approaches are applied, leading to multicolor tunabilities of the phosphors, including a crystal field strength controlled spectral shift, occupancy selectivity induced spectral saltation, defect-driven color tunability, and irradiation dependent dynamic chromaticity. In the high Eu²⁺ concentration doped Ba_5−mSr_m(PO₄)₃Br phosphor, the main emission band shifts from 448 to 452 nm with increasing Sr²⁺/Ba²⁺ ratio, and a second emission band around the green region uplifts in the binary metal solid solution phosphors, leading to variable emission colors from blue to cyan under near UV excitation. Moreover, oxygen vacancy (V_O) defects formed under synthetic reducing conditions endow the bromoapatites with bright self-activated luminescence. In the low Eu²⁺ concentration doped Ba₂Sr₃(PO₄)₃Br, the combination of the broad yellow-green emission from V_O and bluish emission from Eu²⁺ contributes to a full spectrum feature. The gradual increase of Eu²⁺ concentrations regulates the tunable emission from yellow-green, through white, to blue under short UV excitation. Furthermore, due to the different absorption preferences of these two centers, Ba₂Sr₃(PO₄)₃Br:0.01Eu²⁺ shows white light emission under 254 nm irradiation while blue emission when exposed to 365 nm irradiation. The materials with multicolor luminescence show promising versatile applications in lighting, biological detection and anti-counterfeiting. The cooperative effects of multiple strategies provide a promising approach to regulate the optical properties of inorganic phosphors.