Near ultraviolet excitable cyan-green phosphors of Ba6La2Al3ScO15:Ce3+ and Ba6La2Al3ScO15:Ce3+,Tb3+: investigations on the crystal structure, site assignment of Ce3+ and Tb3+ ions, and an energy transfer process from Ce3+ to Tb3+

Abstract

An energy transfer from Ce³⁺ to Tb³⁺ is feasible in energy transition processes for the development of optical devices, especially phosphor-converted white LEDs (pc-wLEDs). Most of the energy transfer phosphors co-doped with Ce³⁺ and Tb³⁺, unfortunately, have weak absorption under a near-ultraviolet light of around 405 nm and blue light excitation, making them incapable for use as near ultraviolet or blue LEDs. In this study, novel energy transfer luminescent materials, Ba₆La₂Al₃ScO₁₅:Ce³⁺,Tb³⁺, isostructural with existing Ba₆La₂(Al,Fe)₄O₁₅, were successfully synthesized through a conventional solid-state reaction as a single phase. The phosphors showed a broad cyan emission of Ce³⁺ and narrow green emissions of Tb³⁺ under a near-ultraviolet light of 405 nm, which was nearly located at an emission wavelength of near-ultraviolet LEDs. The occurrence of energy transfer from Ce³⁺ to Tb³⁺ was evidenced by emission lifetime measurements. The lifetime analysis based on formulated energy transfer models, such as Inokuti–Hirayama, Yokota–Tanimota and Martin models, revealed that an energy transfer process from Ce³⁺ to Tb³⁺ took place dominantly by a dipole–dipole interaction with low migration among the donors of Ce³⁺.