Site occupancy preference, enhancement mechanism, and thermal resistance of Mn⁴⁺ red luminescence in Sr₄Al₁₄O₂₅: Mn⁴⁺ for warm WLEDs

Red phosphors play an indispensable role in phosphor-based warm white light-emitting diodes (WLEDs). We demonstrated recently that the non-rare-earth phosphor Sr₄Al₁₄O₂₅:Mn⁴⁺ exhibits red luminescence even more intensely than the commercial Mn⁴⁺ phosphor 3.5MgO.0.5MgF₂.GeO₂:Mn⁴⁺ upon blue excitation. Herein, on the basis of crystal field calculations employing the exchange charge model, we identify the energy levels of three types of Mn⁴⁺ ions situated at Al³⁺ sites in the Sr₄Al₁₄O₂₅ crystal lattice and find that the doped manganese ions occupy preferentially the Al4 and Al5 more covalent sites rather than the Al6 site. We report that the Mn⁴⁺ luminescence can be enhanced upon the inclusion of Mg²⁺ in the synthesis reaction. The mechanisms for this effect comprise the lower nonradiative decay rate from the ²E_g state because of the reduction in energy migration along Mn⁴⁺ ions to killer sites and the morphology evolution from orderly layered smooth nanosheets to irregular nanoparticles disorderly compacted in porous bundles. Interestingly, various other phases are formed upon the addition of Mg²⁺. The resistance of Mn⁴⁺ photoluminescence in the phosphor to thermal impact has also been studied and no obvious thermal degradation after a cycle experiment by heating and cooling the sample between 25 and 300°C was found. As proof of concept, a warm perception WLED has been made when the phosphor was applied to the package of a blue LED chip and YAG:Ce.