Enriching the Deep-Red Emission in (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) Compositions for Light-Emitting Diodes

Red emission from Mn 4+ -containing oxides inspired the development of high color rendering and cost-effective white-light-emitting diodes (WLEDs). Aiming at this fact, a series of new crystallographic site modified (Mg, Ba) 3 M 2 GeO 8: Mn 4+ (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The Mg 3 Al 2 GeO 8 host is composed of three phases: orthorhombic-Mg 3 Ga 2 GeO 8, orthorhombic-Mg 2 GeO 4, and cubic-MgAl 2 O 4 . However, Mg 3 Ga 2 GeO 8 secured an orthorhombic crystal structure. Interestingly, Mg 3 Al 2 GeO 8: Mn 4+ showed a 13-fold more intense emission than Mg 3 Ga 2 GeO 8: Mn 4+ since Mn 4+ occupancy was preferable to [AlO 6 ] sites compared to [GaO 6 ]. The coexisting phases of MgAl 2 O 4 and Mg 2 GeO 4 in Mg 3 Al 2 GeO 8: Mn 4+ contributed to Mn 4+ luminescence by providing additional [AlO 6 ] and [MgO 6 ] octahedrons for Mn 4+ occupancy. Further, these sites reduced the natural reduction probability of Mn 4+ to Mn 2+ in [AlO 4 ] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on Mg 3 M 2 GeO 8: Mn 4+ to improve the luminescence, and Mg 3– x Ba x M 2 GeO 8: Mn 4+ (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba 2+ ions in Mg 2+ sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned Mg 2.73 Ba 0.27 Al 1.993 GeO 8: 0.005Mn 4+ exhibited a 35-fold higher emission than that of Mg 3 Ga 1.993 GeO 8: 0.005Mn 4+ . Additionally, this could retain 70% of its ambient emission intensity at 453 K. A warm WLED with a correlated color temperature (CCT) of 3730 K and a CRI of 89 was fabricated by combining the optimized red component with Y 3 Al 5 O 12: Ce 3+ and 410 nm blue LED. By tuning the ratio of blue (BaMgAl 10 O 17: Eu 2+ ), green (Ce 0.63 Tb 0.37 MgAl 11 O 19 ), and red (Mg 2.73 Ba 0.27 Al 2 GeO 8: 0.005Mn 4+ ) phosphors, another WLED was developed using a 280 nm UV-LED chip. This showed natural white emission with a CRI of 79 and a CCT of 5306 K. Meanwhile, three red LEDs were also fabricated using the Mg 2.73 Ba 0.27 Al 1.993 GeO 8: 0.005Mn 4+ phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.