New Mg²⁺/Ge⁴⁺-Stabilized Gd₃MgGeAl_5-2O₁₂:Ce Garnet Phosphor with Orange-Yellow Emission for Warm-White LEDs (x =...

By stabilization of the Gd₃Al₅O₁₂ garnet by replacing 80% or more of Al³⁺ with Mg²⁺/Ge⁴⁺ pairs, a series of new orange-yellow-emitting Gd₃MgGeAl_5-2O₁₂:Ce (x = 2.0-2.5) phosphors were successfully developed for potential application in warm-white-light-emitting diodes (WLEDs). Rietveld structure refinement proved that Mg²⁺ first substitutes the octahedral Al³⁺ ion, followed by the replacement of the tetrahedral Al³⁺ together with Ge⁴⁺. The band structure of the x = 2.0 typical garnet was analyzed via density functional theory (DFT) calculations. The incorporation of an increasing content of Mg²⁺/Ge⁴⁺ was experimentally shown to narrow the band gap and expand the unit cell of the garnet host and blue shift the emission/excitation wavelength and shorten the fluorescence lifetime of Ce³⁺. The photoluminescence behaviors were rationalized by considering the influence of Mg²⁺/Ge⁴⁺ on the crystal structure, band structure, and local coordination. An LED lamp fabricated by combining the (Gd_2.97Ce_0.03)Mg₂Ge₂AlO₁₂ optimal phosphor with a 450 nm-emitting InGaN blue LED chip exhibited a color-rendering index of 71.6, luminous efficacy of 16.1 lm/W, and a low correlated color temperature of 2201 K under a driving current of 20 mA, indicating that phosphor may have potential application in warm WLEDs.