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:Mn⁴⁺ red phosphors by using chemical coprecipitation method. The prepared hexagonal <i>P</i>3̅m1 K₂GeF₆:Mn⁴⁺ exhibited efficient red emission, high color purity, good Mn⁴⁺ concentration stability, and low thermal quenching. Structural evolution from hexagonal <i>P</i>3̅<i>m</i>1 to <i>P</i>6₃mc and then <i>P</i>6₃<i>mc</i> to cubic <i>Fm</i>3<i>m</i> occurred after thermal treatment at approximately 400 and 500 °C, respectively. Hexagonal <i>P</i>6₃mc phase showed an obvious zero phonon line peak at 621 nm, whereas cubic <i>Fm</i>3<i>m</i> phase showed no red emission. Yellowish K₂GeF₆:Mn⁴⁺ with both hexagonal <i>P</i>3̅<i>m</i>1 and <i>P</i>6₃<i>mc</i> symmetries are promising commercial red phosphors for white light-emitting diodes