(Ca,Mg,Sr)9Y(PO4)7:Eu2+,Mn2+: Phosphors for white-light near-UV LEDs through crystal field tuning and energy transfer

Abstract

Two series of single-composition (Ca,Mg,Sr)₉Y(PO₄)₇:Eu²⁺ and (Ca_0.5Sr_0.5)₉Y(PO₄)₇:Eu²⁺,Mn²⁺ phosphors were synthesized via high-temperature solid-state reactions. Their emission colors could be tuned from blue to green and eventually to red through tuning the crystal field splitting and energy transfer. On examining the Mn²⁺ concentration-dependent photoluminescence properties, we found that co-doping with Mn²⁺ would lead to a change in Eu²⁺/Eu³⁺ ratio. Moreover, an energy transfer from Eu²⁺ to Mn²⁺ occurs because of the spectral overlap between the emission band of Eu²⁺ and the excitation band of Mn²⁺. The resonance-type energy transfer via a dipole–quadrupole interaction mechanism was supported by decay lifetime data and the critical distance of energy transfer was calculated to be 11.09 Å. A trichromatic white-light emitting diode was fabricated by integrating a 380 nm near-ultraviolet (n-UV) chip comprising yellow-emitting (Ca_0.5Sr_0.5)₉Y(PO₄)₇:0.007Eu²⁺,0.02Mn²⁺ and blue-emitting (Ca_0.5Mg_0.5)₉Y(PO₄)₇:0.007Eu²⁺ phosphors into a single package. Such a composite device emitted white light with a correlated color temperature of 6303 K, a color rendering index of 87.4, and color coordinates (0.314, 0.348) close to those of ideal white light. The results suggest that a phosphor blend of (Ca_0.5Sr_0.5)₉Y(PO₄)₇:0.007Eu²⁺,0.02Mn²⁺ and (Ca_0.5Mg_0.5)₉Y(PO₄)₇:0.007Eu²⁺ is potentially useful for white n-UV light-emitting diodes (LEDs).