Energy transfer from Eu2+ to Mn2+ in M5(PO4)3Cl (M = Ca, Sr)

M4.95-y(PO4)(3)Cl:0.05Eu(2+), yMn(2+) (m = Ca, Sr) phosphors were synthesized at 1200 degrees C under a reductive atmosphere. The energy transfer mechanism and efficiency (eta) from Eu2+ to Mn2+ as well as the quantum efficiency (Q) of the Mn2+ emission were analyzed using PL spectra and decay curves. Least squares fittings of the Eu2+ decay curves indicate that the energy transfer from Eu2+ to Mn2+ follows the exchange mechanism in the Sr system, while besides exchange mechanism other complicated quenching processes are involved in the Ca system. Superficially the eta is high but the Q is low, which implies some energy loss. Two quenching models representative of an "inverse bottleneck process", which is related to the lifetime mismatch between Eu2+ and Mn2+, and a "charge transfer process" in the Eu2+-Mn2+ clusters, are proposed. The results may provide some clues for designing excellent sensitizer and activator co-doped phosphors in our further work. (C) 2013 Elsevier B.V. All rights reserved.