Stable narrowband red phosphor K3GaF6:Mn4+ derived from hydrous K2GaF5(H2O) and K2MnF6

Abstract

A novel hydrous fluoride K₂GaF₅(H₂O) with a special [GaF₅(H₂O)] group has been discovered. Interestingly, further rapidly adding K₂MnF₆ into the co-precipitation reaction system of K₂GaF₅(H₂O) results in stable anhydrous cryolite phosphor K₃GaF₆:Mn⁴⁺. The investigation of a possible formation mechanism unravels that hydrous K₂GaF₅(H₂O) can act as an intermediate to transform into anhydrous K₃GaF₆:Mn⁴⁺ induced by K₂MnF₆. Detailed characterizations, such as phase identification, morphology, elemental composition analysis, and IR spectra, record the time-dependent evolution process. Upon blue light excitation, phosphor K₃GaF₆:Mn⁴⁺ shows sharp line red emission at ∼626 nm. It presents good moisture tolerance (50% of initial luminous efficacy at 48 h), which might, due to the absence of coordinated water molecules, make the host matrix less hydrophilic. This performance, much better than those of Na₃GaF₆:Mn⁴⁺, K₃AlF₆:Mn⁴⁺, and Rb₂GeF₆:Mn⁴⁺, illustrates that K₃GaF₆ is a suitable host for Mn⁴⁺ doping. Besides, a good recovery in luminescence properties during thermal cycling (298–473 K) further demonstrates the high stability of anhydrous K₃GaF₆:Mn⁴⁺. By incorporating K₃GaF₆:Mn⁴⁺ as a red light component, a fabricated warm white light-emitting diode (WLED) presents low correlated color temperature (CCT = 3691 K), high color rendering index (R_a = 87.2 and R₉ = 50.2), and a luminous efficacy of 92.1 lm W⁻¹, and further exhibits good resistance in photoelectric properties to thermal impact with temperature ranging from 293 to 353 K, showing potential in warm WLED applications. Herein, the inductive transition from hydrous K₂GaF₅(H₂O) to moisture-proof K₃GaF₆:Mn⁴⁺ is helpful for the investigation of the moisture resistance of Mn⁴⁺ activated fluorides. Moreover, these results probably propose a feasible strategy to exploit stable Mn⁴⁺-activated fluoride phosphors for warm WLED applications.