An efficient and thermally stable red emitting Eu2+doped oxide for rechargeable portable flashlight

The pursuit of efficient red emitting Eu2+ doped oxide is endless, aiming to replace the state-of-the-art CaAlSiN3: Eu2+ nitride counterpart to achieve cheaper and more stable warm illumination. Herein, we identify a hitherto unreported CaLuGaO4 olivine compound. The crystal structure of the compound is belong to orthorhombic Pnma. Phonon dispersion relations have no imaginary frequency, verifying thermodynamic stability of the crystal structure. It structurally contains an octahedral Ca2+ site that definitely has strong crystal field splitting energy, which motivates us to incorporate it with Eu2+ to realize red emission through 4f65d1 -> 4f7 transition of the octhdedral Eu2+ ion. This goal is achieved as found the divalent Eu2+ is necessarily stabilized in this site under CO sintering atmosphere rather than H2/N2. The as-designed CaLuGaO4:Eu2+ exhibits impressively appealing properties, i.e., intense red emission at 650 nm (lambda ex = 450 nm), with high PLQY up to 86 % and good thermal stability in temperature range 10-500 K, marking it as a promising and formidable red phosphor in warm illumination. The excellent emission properties are theoretically revealed to associate with the closed-packed dimensional crystal structure with high Debye temperature equals 440 K, which reduces the non-radiative recombination loss. The investigations in electron-vibrational interaction in the Eu2+:5d state disclosing the interaction is intermediate coupling regime with Huang-Rhys factor of 4.3. As a proof-of-concept experiment of the red phosphor for illumination, a prototype WLED possession of low CCT = 3917 K, high Ra = 92.2, and excellent luminous efficiency = 110 lm/W is constructed, and a portable rechargeable LED flashlight is assembled, by utilizing CaLuGaO4:Eu2+ phosphor as a key red convector. Moreover, we have also demonstrated the phosphor synthesized under H2/N2 can apply for latent fingerprint visualization.