Mixing the valence control of Eu2+/Eu3+ and energy transfer construction of Eu2+/Mn2+ in the solid solution (1 − x)Ca3(PO4)2–xCa9Y(PO4)7 for multichannel photoluminescence tuning

Abstract

Recently, controllable photoluminescence tuning by devising a solid solution framework, adjusting the valence mixing of Eu²⁺/Eu³⁺ and designing efficient energy transfer between activator ions has been extensively investigated and reported due to its significant advantages in the improvement and regulation of the luminescence performances of white light-emitting diodes (W-LEDs). In this study, we designed a series of novel Eu²⁺-doped (1 − x)Ca₃(PO₄)₂–xCa₉Y(PO₄)₇ (x = 0–1.0) isostructural solid solution phosphors with a β-Ca₃(PO₄)₂-type structure, and powder samples were prepared via the traditional high-temperature solid-state reaction process. The crystal field variation around the Eu²⁺ ions causes superposition of linear luminescence and induces optical property tuning with a change in the solid solution ratio, x, values. Besides the high-energy emission peak at 418 nm in Ca₃(PO₄)₂:0.03Eu²⁺, another low-energy emission peak at 486 nm was observed with the formation of the solid solution. Moreover, the corresponding high-energy emission peaks shifted from 418 to 430 nm, and the luminescence intensity of the low-energy emission at 486 nm increased with an increase in x, which was attributed to the combined effect of crystal field splitting of the local lattice and superposition of linear luminescence. In addition, tunable emission across the whole white light region could be realized by constructing a solid solution and adjusting the overall Eu concentration in (1 − x)Ca₃(PO₄)₂–xCa₉Y(PO₄)₇:Eu (x = 0.5, 0.7, 0.9, and 1.0), and the corresponding luminescence mechanisms have been proposed and discussed in detail. In the pursuit of precise color tuning, we also tested the Eu²⁺ → Mn²⁺ energy transfer in different compositions (x = 0.2, 0.5, 0.7, and 0.9) of the solid solution phosphors, and successive emission tuning from the blue and cyan range and then the red range was successfully achieved. Finally, the temperature-dependent photoluminescence and decay time were revealed systematically, and the corresponding mechanisms for the thermal quenching behavior have been discussed. This study provides a new perspective for color tuning originating from the simultaneous isostructural solid solution, valence mixing of Eu²⁺/Eu³⁺ and efficient energy transfer.