Insights into the high brightness, color purity, narrowband red-emission and luminous mechanism of Eu-doped CaY2Ge4O12†

In this study, Eu³⁺-doped CaY₂Ge₄O₁₂, capable of emitting narrow-band red light (full width at half maximum of the strong emission peaks was less than 1.5 nm), was first prepared using the high-temperature solid-phase method at different temperatures. But the sintering temperature does not significantly affect the grain size of CaY₂Ge₄O₁₂:Eu³⁺ in this study. Under 395 nm excitation, CaY_2−xGe₄O₁₂:xEu³⁺ exhibited extremely high brightness, color purity exceeding 97%, an average correlated color temperature (CCT) of 2200 K, and quantum yield (QY) close to 100%. The emission intensity at 150 °C remained above 85% of that at 27 °C. The excitation transitions of CaY₂Ge₄O₁₂:Eu³⁺ were of two types: one was the 4f–4f upward transition of Eu³⁺ caused by narrowband excitation at 288, 299, 320, 363, 386, and 395 nm; the other was the valence band (VB) to conduction band (CB) transition caused by 241 nm excitation. Emissions were observed at 580, 592, 613, 652, and 703 nm. Doping concentration is the main factor affecting the grain size and luminescence intensity of CaY₂Ge₄O₁₂:Eu³⁺. Due to Eu ion doping, the crystal field symmetry decreased, lifting the forbidden transition of ⁵D₀ → ⁷F_j (j = 0, 2, 3, 4). Surprisingly, after the forbidden transition ⁵D₀ → ⁷F₄ (703 nm) was lifted, it showed anomalous enhancement (much higher than the typically normally strongest transition, ⁵D₀ → ⁷F₄ at 613 nm), greatly increasing the red-light brightness to a level comparable to commercial blue and green light samples. Finally, the luminescence and thermal stability mechanisms of CaY_2−xGe₄O₁₂:xEu³⁺ were detailed.