Tailoring photoluminescence stability in double perovskite red phosphors A2BAlF6:Mn4+ (A = Rb, Cs; B = K, Rb) via neighboring-cation modulation

Abstract

Three isotypic double perovskites A₂BAlF₆:Mn⁴⁺ (A = Rb, Cs; B = K, Rb) were synthesized via a facile co-precipitation method at room temperature. XRD Rietveld refinements and Raman spectra indicate that the neighboring smaller A-site or larger B-site cation in A₂BAlF₆:Mn⁴⁺ lowers structural stability, but suppresses the [AlF₆] octahedron to increase the crystal field strength and bond strength of the Al–F bond. Cs₂KAlF₆:Mn⁴⁺ presents a sharp line red light and excellent moisture resistance, but experiences severe thermal quenching. In comparison, altering the A- or B-site cation to Rb, Rb₂KAlF₆:Mn⁴⁺ and Cs₂RbAlF₆:Mn⁴⁺ shows blue shifts in emission and declining water resistance, but enhancements in thermal stability. A combination of static and dynamic emission degradations demonstrates that the water resistance of Mn⁴⁺ emission mainly relies on the structural stability of the host matrix; meanwhile, the thermal stability highly depends on the rigidity of its local accommodation. The variations of the temperature-dependent white light-emitting diode (WLED) performances further evidence their sufficient thermal stability for LED applications. These findings suggest that neighboring-cation modulation might be a feasible guideline for exploiting stable Mn⁴⁺-activated fluoride red phosphors in warm WLED applications.