Eu2+ activated novel eulytite Na3Bi5(PO4)6: Eu3+ phosphors: Enhanced luminescence and thermal stability for photonics application

For the first time, novel eulytite-like Eu2+/Eu3+: Na3Bi5(PO4)(6) phosphor was synthesized via high temperature solid-state reaction method in reduction environment, and the structure, luminescence performances and thermal stability were investigated and discussed using various techniques. X-ray refinement diffraction and Raman spectra revealed the around 200 nm well-crystallized eulytite-type (I43d space group) phosphors were synthesized, and a diagram of crystal structure of Na3Bi5(PO4)(6) was proposed. X-ray photoelectron spectroscopy analysis confirmed the co-existence of Eu2+ and Eu3+ ions which exhibited characteristic 4f(6)5d -> S-8(7/2) transition of Eu2+ and F-7(0)-> D-5(0,1,2,3,4) transitions of Eu3+ ions. On the other hand, due to the activation of Eu2+, samples displayed good tunability on excited and emission behaviors under different excited laser. The JO parameters, emission cross-section, branching ratio and asymmetric ratio indicated that the Eu doping increased the covalency and asymmetry of host. Thermal quenching was studied and the reasons were discussed. Through the comparison of phosphors prepared in different conditions, the thermal stability & repeatability, radiative lifetime, color purity and activation energy were remarkably superior due to the Eu doping and in particularly Eu2+ activation. Finally, the energy level and CIE chromaticity diagrams were plotted to explain the mechanism of Eu2+ activation and energy transfer between Eu2+ and Eu3+ ions. The 0.5%Eu doped Na3Bi5(PO4)(6) exhibited promising tunable red-emission performance with quantum efficiency of 92%, activation energy of 0.24 eV, red color purity of 93.74% and very low non-radiative transfer ratio 44.20 s(-1) with smaller CCT (<2200 K).