Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K₂GeF₆:Mn⁴⁺.

This study explored optimal preparation conditions for K2GeF6:Mn(4+) red phosphors by using chemical coprecipitation method. The prepared hexagonal P3̅m1 K2GeF6:Mn(4+) exhibited efficient red emission, high color purity, good Mn(4+) concentration stability, and low thermal quenching. Structural evolution from hexagonal P3̅m1 to P63mc and then P63mc to cubic Fm3m occurred after thermal treatment at approximately 400 and 500 °C, respectively. Hexagonal P63mc phase showed an obvious zero phonon line peak at 621 nm, whereas cubic Fm3m phase showed no red emission. Yellowish K2GeF6:Mn(4+) with both hexagonal P3̅m1 and P63mc symmetries are promising commercial red phosphors for white light-emitting diodes.