Enhanced luminescence, crystal field optimization and stability of a novel Mg2+ and Mn4+ co-doped K2NaInF6 red phosphor for application in high-performance warm WLEDs

Abstract

Mn⁴⁺-activated fluoride red phosphors hold great promise for enhancing the light quality of white light-emitting diodes (WLEDs). However, their practical application is severely hampered by relatively low fluorescence intensity. To solve this problem, this study presents an innovative crystal field optimization strategy based on the ion-pair substitution mechanism (Mn⁴⁺ + Mg²⁺ → In³⁺ + In³⁺) to improve the photoluminescence performance of K₂NaInF₆:Mn⁴⁺ red phosphor. Given the purity of the crystal phase, an effective water bath co-precipitation method is used to synthesize a series of K₂NaInF₆:Mn⁴⁺,Mg²⁺ red phosphors. The comprehensive characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), are employed to systematically analyze the crystal structure, morphology and composition. Under blue light excitation, the as-prepared phosphors exhibit outstanding optical performance, characterized by a low correlated color temperature and high color purity. The optimal amount of HF used and the doping concentrations of Mg²⁺ or Mn⁴⁺ ions have been verified through extensive experiments. Theoretical calculations are used to determine concentration quenching mechanism, crystal field strength and thermal quenching mechanism. Thermal stability measurements reveal that the photoluminescence intensity of the phosphors can maintain 44 % of its initial value at 303 K even at an elevated temperature of 423 K. Additionally, a series of optical parameters, such as excitation energy, chromaticity shift, and chromaticity variation, are systematically calculated. Surprisingly, after 90 min of immersion in water, the luminescence intensity of the K₂NaInF₆:Mn⁴⁺,Mg²⁺ phosphor can be maintained at 91.55 % of its initial value. Most significantly, a high-performance warm WLED is successfully fabricated using K₂NaInF₆:Mn⁴⁺,Mg²⁺ as the red phosphor component. Notably, the warm WLED can maintain stable luminous performance under high driving currents, highlighting its potential for practical applications. Overall, this study provides valuable insights into the rational design of novel and highly efficient red-emitting fluoride phosphors, which are essential for advancing the development of high-quality warm WLEDs.