Improvement of thermal stability and photoluminescence in Mg2Y2Al2Si2O12:Ce3+ by the cation substitution of Ca2+, Sr2+ and Ba2+ ions

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

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.

Remarkable structure and luminescence regulation of a Gd2LuAl5O12:Ce garnet phosphor with a Ca2+/Si4+ pair for high-quality w-WLED lighting

Doping Gd₂LuAl₅O₁₂:Ce with a Ca²⁺/Si⁴⁺ pair produced a series of Gd_1.97-xLuCa_xAl_5-xSi_xO₁₂:0.03Ce (x = 0.0-1.2) garnet new phosphors, and remarkable regulation of the crystal structure and Ce³⁺ luminescence was achieved. Rietveld refinement and spectral analysis revealed that greater incorporation of Ca²⁺/Si⁴⁺ led to a higher lattice rigidity by cell contraction, a narrower bandgap, a longer average bond length/less distortion of the [CeO₈] dodecahedron and decreased centroid shift/crystal field splitting of the Ce³⁺ 5d energy level. A blue-shifted 4f-5d₁ transition, a narrower emission band, improved thermal stability of luminescence and a shorter fluorescence lifetime were observed with increasing Ca²⁺/Si⁴⁺ content. Applying the yellowish-green-emitting Gd_0.77LuCa_1.2Al_3.8Si_1.2O₁₂:0.03Ce optimal phosphor (x = 1.2), together with a commercial CaAlSiN₃:Eu red phosphor, in 450 nm-excited LED lighting produced a low CCT of ∼3625 K and a high CRI/R9 of ∼95.2/94.8, indicating that the phosphor has application potential in high-performance w-WLED.

Site-selective and cooperative doping of Gd3Al5O12:Ce garnets for structural stabilization and warm WLED lighting of low CCT and high CRI

Synergistic doping of the metastable Gd₃Al₅O₁₂:Ce garnet with a Ca²⁺/Hf⁴⁺ pair and Sc³⁺ to form Gd_2.97-xCa_xHf_xSc_yAl₃O₁₂:0.03Ce (x = 0.5-2.0, y = 0.0-1.5, x + y = 2.0) solid solution was conducted for the structural stabilization and photoluminescence manipulation. The site selection of Ca²⁺/Hf⁴⁺/Sc³⁺ dopants and the effects of doping on the crystal structure, local coordination, band structure and Ce³⁺ luminescence were revealed in detail with the results of XRD, Rietveld refinement, TEM, and UV-Vis/photoluminescence spectroscopy. A decrease in Ca²⁺/Hf⁴⁺ and an increase in the Sc³⁺ content were observed to shrink the lattice, widen the bandgap of the garnet host, red-shift the excitation/emission wavelength, broaden the emission band and shorten the fluorescence lifetime of Ce³⁺. The spectral changes were rationalized by considering the local coordination and crystal field splitting of the Ce³⁺ 5d energy level. Application of typical Gd_0.97Ca₂Hf₂Al₃O₁₂:0.03Ce (x = 2.0, y = 0) cyan and Gd_2.47Ca_0.5Hf_0.5Sc_1.5Al₃O₁₂:0.03Ce (x = 0.5, y = 1.5) greenish-yellow phosphors in w-WLED lighting produced low correlated color temperatures of ∼3842 and 3514 K, high color rendering indices of ∼88 and 93 and favorable luminous efficacies of ∼32.9 and 14.7 lm/W under the excitation of 395 nm n-UV and 450 nm blue LED chips, respectively.