Crystal field optimization and fluorescence enhancement of a Mn⁴⁺-doped fluoride red phosphor with excellent stability induced by double-site metal ion replacement for warm WLED

The effective double-site metal ion replacement strategy was adopted to optimize the crystal field environment of a Mn⁴⁺-activated fluoride phosphor. In this study, a series of K_2yBa_1−ySi_1−xGe_xF₆:Mn⁴⁺ phosphors with optimized fluorescence intensity, excellent water resistance, and outstanding thermal stability was synthesized. The composition adjustment includes two different types of ion substitution based on the BaSiF₆:Mn⁴⁺ red phosphor: [Ge⁴⁺ → Si⁴⁺] and [K⁺ → Ba²⁺]. X-ray diffraction and theoretical analysis revealed that Ge⁴⁺ and K⁺ could be successfully introduced into BaSiF₆:Mn⁴⁺ to form new solid solution K_2yBa_1−ySi_1−xGe_xF₆:Mn⁴⁺ phosphors. The emission intensity enhancement and slight wavelength shift were detected in different cation replacement procedures. Furthermore, K_0.6Ba_0.7Si_0.5Ge_0.5F₆:Mn⁴⁺ with superior color stability performance possessed a negative thermal quenching phenomenon. Excellent water resistance was also found, which was more reliable than K₂SiF₆:Mn⁴⁺ commercial phosphor. A warm WLED with low correlated color temperature (CCT = 4000 K) and high color rendering index (R_a = 90.6) was successfully packaged by using K_0.6Ba_0.7Si_0.5Ge_0.5F₆:Mn⁴⁺ as the red light component, and it also exhibited high stability for different currents. These findings demonstrate that the effective double-site metal ion replacement strategy can open up a new avenue for designing new Mn⁴⁺-doped fluoride phosphors to improve the optical properties of WLEDs.