Unraveling the Eu2+ → Mn2+ Energy Transfer Mechanism in w-LED Phosphors

Recent research on white light LED (w-LED) phosphors has focused on narrow-band green and red luminescent materials to improve the efficacy of w-LEDs and to widen the color gamut of w-LED-based displays. Mn2+ is a promising emitter capable of narrow-band emission, either green or red, depending on the local coordination. However, the extremely low absorption coefficients for the spin- and parity-forbidden d-d transitions in Mn2+ form a serious drawback and require addition of a sensitizer ion such as Ce3+ or Eu2+, with strong absorption in the blue. The performance of the codoped phosphor then critically depends on efficient energy transfer. Despite extensive research, a clear understanding of the Eu2+ -> Mn2+ and Ce3+ -> Mn2+ transfer mechanism is lacking. Typically, Dexter exchange interaction or electric dipole-quadrupole coupling are considered. Here we investigate Eu2+ -> Mn2+ energy transfer in Ba2MgSi2O7 and show that the most probable mechanism is exchange interaction with fast (nanoseconds) energy transfer from Eu2+ to nearest-neighbor Mn2+ and much slower (>100 ns) transfer to next-nearest neighbors, as expected for exchange interaction. We critically evaluate previous studies where the assignment of dipole-quadrupole interaction was erroneously based on C-Mn(8/3) concentration dependence of energy transfer efficiencies. Preferential Eu2+-Mn2+ pair formation is suggested as a mechanism that enhances energy transfer efficiencies.