Emission-tunable phosphors Ca9MgM′ (PO4)7: Eu2+,Mn2+(M′=Li, Na, K) for white light-emitting diodes

► Ca9MgM′(PO4)7:Eu2+,Mn2+ (M′=Li, Na, K) was synthesized by solid state reactions. ► Energy transfer from Eu2+ to Mn2+ was ascribed to dipole-quadrupole interaction. ► Ca9MgM′(PO4)7:Eu2+ has great potential to be used as a commercial blue phosphor.