Site-selective occupation, optical spectrum regulation and photoluminescence property investigation of Eu2+-activated blue light-excited yellow-orange emitting phosphors

Abstract

We report yellow-orange emitting phosphors Sr_9−xCa_xMg_1.5(PO₄)₇:0.05Eu²⁺ (SC_xMPO:Eu²⁺, x = 0.5–2.5) and Sr_9−yBa_yMg_1.5(PO₄)₇:0.05Eu²⁺ (SB_yMPO:Eu²⁺, y = 0.5–3.0) with broad emission bands (450–800 nm). All these phosphors can be excited efficiently by blue light and n-UV light. Their crystal structure, photoluminescence spectra, fluorescence decay curves and thermal stability were investigated in detail. As doping concentrations of Ca²⁺ or Ba²⁺ increase, Eu²⁺ emitting centers will selectively occupy different Sr²⁺ sites, thus leading to the regulation of optical spectra of SC_xMPO:Eu²⁺ and SB_yMPO:Eu²⁺. Accordingly, the emission colors of SC_xMPO:Eu²⁺ and SB_yMPO:Eu²⁺ samples can gradually turn from yellow to orange when excited using 460 nm blue light. And the emission colors of a given sample can also be varied under different excitations because there are three kinds of emitting centers in SC_xMPO:Eu²⁺ and SB_yMPO:Eu²⁺. In addition, introducing Ca²⁺ and Ba²⁺ can enhance the thermal stability of the phosphors obviously, and overall, the thermal stability of SB_yMPO:Eu²⁺ is better than that of SC_xMPO:Eu²⁺. We chose SB_2.5MPO:zEu²⁺ as an example to further investigate its photoluminescence properties, and found that the optimal doping concentration of Eu²⁺ is 0.08, and dipole–quadrupole interaction is dominated in the concentration quenching mechanism. Furthermore, high-quality warm white light can be obtained by two ways: (a) 470 nm blue LED chip + SC_1.5MPO:Eu²⁺ [CCT = 3639 K, R_a = 82.21] and (b) 470 nm blue LED chip + SB_2.5MPO:Eu²⁺ and YAG:Ce³⁺ [CCT = 4284 K, R_a = 86.69]. The excellent performances indicate that SC_xMPO:Eu²⁺ and SB_yMPO:Eu²⁺ are attractive candidates for warm WLEDs.