Hydrolysis-driven degradation of K2SiF6:Mn4+ phosphor: Dissolution dynamics, MnO2 crystallization, and wavelength-dependent degradation

Mn 4+ -activated fluoride phosphors, exemplified by K 2 SiF 6 :Mn 4+ , exhibit exceptional narrow-band red emission but suffer from severe hydrolysis-driven degradation, significantly limiting their operational longevity. This study systematically investigates the stepwise degradation of K 2 SiF 6 :Mn 4+ upon water exposure, uncovering new insights into its hydrolytic behaviour. Upon immersion, the phosphor suspension undergoes a browning-clearing transition, attributed to the hydrolysis of [MnF 6 ] 2– which generates black suspended MnO 2 that temporarily darkens the solution before sedimentation. The degradation process involves two sequential stages: an initial rapid dissolution-dissociation of K 2 SiF 6 :Mn 4+ , accompanied by a sharp pH decrease, followed by relatively slower hydrolysis of [MnF 6 ] 2– and [SiF 6 ] 2– yielding crystalline MnO 2 and amorphous H 2 SiO 3 . Besides, upon reaching dissolution-ionization equilibrium, the released K + , [SiF 6 ] 2– , and [MnF 6 ] 2– ions would nucleate and precipitate, forming crystalline K 2 SiF 6 and K 2 SiF 6 :Mn 4+ particles with much smaller size compared to the original K 2 SiF 6 :Mn 4+ . Notably, this study reveals that the degradation of K 2 SiF 6 :Mn 4+ is significantly accelerated under both blue and UV light illumination, with UV light inducing faster degradation. These findings provide fundamental insights into the hydrolysis-driven degradation mechanism of Mn 4+ -activated fluoride phosphors, offering a scientific basis for developing strategies to enhance their stability and extend their service life.