Influence of alkaline ions on the luminescent properties of Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors

Mn4+-doped MGe4O9 (M = Li-2, LiNa and K-2) red-emitting phosphors were successfully prepared and characterized. Owing to the different ionic radii and electron configurations among the Li, Na and K ions, the MGe4O9 (M = Li-2, LiNa and K-2) compounds have different space group and crystal structure, such as different bond lengths, bond angles, crystal fields and nephelauxetic effect. Under the excitation of ultraviolet and blue light, the studied samples emit visible red emissions. The thermal stabilities of phosphors are greatly affected by the alkaline ions, especially when M = LiNa and K-2, the thermal stability is significantly enhanced. Furthermore, the crystal field strength (DO and Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ ions in MGe4O9 (M = Li-2, LiNa and K-2) host lattices. The results indicate that the alkaline ions can influence the red emissions of Mn4+ ions in MGe4O9 (M = Li-2, LiNa and K-2) phosphors and these resultant phosphors may have potential applications for solid state lighting.