Phosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr_0.99-xBa_x)₂P₂O₇:0.02Eu²⁺ (SB_xPE x = 0–0.5) are developed. Upon excitation at 350 nm, the optimized SB_0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB_0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB_0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications. Inorganic Materials; Materials Application; Optical Materials