Ce ³⁺ -Sensitized Single-Phase White Emission from Dy ³⁺ in LiSrY ₂ (BO ₃ ) ₃ with High Thermal Stability under Near-UV Excitation

Among rare-earth ions, Dy 3+ uniquely enables intrinsic white-light emission and is therefore considered a promising activator for solid-state lighting. However, its parity-forbidden 4f–4f transitions lead to intrinsically weak absorption and limited emission intensity. Ce 3+ was identified as an efficient sensitizer in the host LiSrY 2 (BO 3 ) 3 (LSYB) to enhance Dy 3+ emission through energy transfer. A series of Ce 3+ -singly doped, Dy 3+ -singly doped, and Ce 3+ /Dy 3+ codoped phosphors were successfully synthesized via a high-temperature solid-state reaction. Under 347 nm excitation, Dy 3+ -doped samples exhibit pale yellow emission with yellow-to-blue intensity ratios of 1.15–1.26, indicating that Dy 3+ occupies noncentrosymmetric sites. The Dy 3+ -only phosphor shows relatively low internal quantum efficiency (IQE = 15.47%) and external quantum efficiency (EQE = 1.29%). Upon Ce 3+ sensitization, the emission intensity of Dy 3+ is significantly enhanced. The optimized codoped phosphor LiSr(Y 0.97 Ce 0.01 Dy 0.02 ) 2 (BO 3 ) 3 achieves a markedly improved EQE of 6.10% under 335 nm excitation, with the Dy 3+ contribution reaching 3.07%. The Ce 3+ emission lifetime decreases with increasing Dy 3+ concentration, confirming the occurrence of Ce 3+ → Dy 3+ energy transfer, which is dominated by an electric dipole–dipole interaction mechanism. LiSr(Y 0.96 Ce 0.01 Dy 0.03 ) 2 (BO 3 ) 3 exhibits near-ideal white emission with CIE coordinates of (0.3288, 0.3479), CCT = 5654 K, R a = 67. Furthermore, the optimized white-emitting phosphor exhibits remarkable thermal stability, maintaining 93.88% of its emission intensity at 423 K relative to that at room temperature.