Abstract:
Red emission from Mn4+-containing oxides inspired
the development of high color rendering and cost-effective whitelight-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 Mg3−xBaxM2GeO8: 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 Mg2.73Ba0.27Al1.993GeO8: 0.005Mn4+ exhibited a 35-fold higher
emission than that of Mg3Ga1.993GeO8: 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
(Ce0.63Tb0.37MgAl11O19), and red (Mg2.73Ba0.27Al2GeO8: 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 Mg2.73Ba0.27Al1.993GeO8: 0.005Mn4+ phosphor with commercial sources. These could be potential pc-LEDs for
plant growth applications.
