Enhanced Red Light Emission of OH^–-Substituted Sr₅(PO₄)₃Cl:Eu³⁺ Nanophosphors

Oxide red nanophosphors convert high-energy photons to red light and have potential applications in photonic devices, including fluorescent lamps, light-emitting devices, and biosensors. In this study, a series of OH–-substituted Sr5(PO4)3Cl:Eu3+ nanophosphors emitting red light were successfully synthesized via a microwave reaction method. The effects of OH– substitution on a change in the crystal structure and photoluminescence of Eu3+ in Sr5(PO4)3Cl1–xOHx were investigated. The ratio of Cl– to OH– ions was controlled by changing the Sr/Cl ratio in a precursor solution. Though some Cl– ions were substituted with OH– ions, the emission intensity ratio of the 5D0 → 7F2 transition to the 5D0 → 7F1 transition (I618/I593) did not change. This result indicated that Eu3+ ions preferentially occupy the MI site of an apatite structure in Sr5(PO4)3Cl and Sr5(PO4)3Cl1–xOHx. In addition, the intensity of the red emission from Sr5(PO4)3Cl1–xOHx:Eu3+ increased 4-fold as OH– replaced 20% of Cl–. Decay lifetime measurement and its analysis by Judd–Ofelt theory showed that the addition of OH– ions did not change the dipole transition strength but decreased the number of nonradiative trapping sites such as point defects. This led to the improved emission quantum efficiency (η). However, an excessive amount of OH– randomized the atomic arrangement and produced local strain. This, in turn, caused nonradiative relaxation of excited electrons and decreased the intensity of the red emission.