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Abstract: M4.95−y (PO4)3Cl:0.05Eu2+, yMn2+ (M=Ca, Sr) phosphors were synthesized at 1200°C under a reductive atmosphere. The energy transfer mechanism and efficiency (η) 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 η 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. [Copyright &y& Elsevier]