Red emission from Mn⁴⁺-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)₃M₂GeO₈: Mn⁴⁺ (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The Mg₃Al₂GeO₈ host is composed of three phases: orthorhombic-Mg₃Ga₂GeO₈, orthorhombic-Mg₂GeO₄, and cubic-MgAl₂O₄. However, Mg₃Ga₂GeO₈ secured an orthorhombic crystal structure. Interestingly, Mg₃Al₂GeO₈: Mn⁴⁺ showed a 13-fold more intense emission than Mg₃Ga₂GeO₈: Mn⁴⁺ since Mn⁴⁺ occupancy was preferable to [AlO₆] sites compared to [GaO₆]. The coexisting phases of MgAl₂O₄ and Mg₂GeO₄ in Mg₃Al₂GeO₈: Mn⁴⁺ contributed to Mn⁴⁺ luminescence by providing additional [AlO₆] and [MgO₆] octahedrons for Mn⁴⁺ occupancy. Further, these sites reduced the natural reduction probability of Mn⁴⁺ to Mn²⁺ in [AlO₄] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on Mg₃M₂GeO₈: Mn⁴⁺ to improve the luminescence, and Mg_3-xBa_xM₂GeO₈: Mn⁴⁺ (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba²⁺ ions in Mg²⁺ sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned Mg_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ exhibited a 35-fold higher emission than that of Mg₃Ga_1.993GeO₈: 0.005Mn⁴⁺. 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 Y₃Al₅O₁₂: Ce³⁺ and 410 nm blue LED. By tuning the ratio of blue (BaMgAl₁₀O₁₇: Eu²⁺), green (Ce_0.63Tb_0.37MgAl₁₁O₁₉), and red (Mg_2.73Ba_0.27Al₂GeO₈: 0.005Mn⁴⁺) 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_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.

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