Enhancement of PL intensity and formation of core-shell structure in annealed Ca2-x/2(Si1-xPx)O4:Eu2+ phosphor

• Annealing treatments effectively influenced crystal structure and PL intensity.
• Annealing at 1423 K improved the PL intensity of P-doped C2S green phosphor.
• The co-existence of α’L-phase and β-phase caused an overall improvement.
• At 1723 K, a unique structure composed of red and green phosphor formed.

A series of (Ca2-x/2-yEuy□x/2)(Si1-xPx)O4 (0 ≤ x ≤ 0.20 and 0.02 ≤ y ≤ 0.05) green-emitting phosphors were synthesized, and characterized by photoluminescence (PL) spectroscopy and X-ray powder diffraction. We compared the PL intensities between the phosphors that were annealed at 1373–1773 K and those of non-annealed ones. Actually, the (Ca1.95Eu0.02□0.03)(Si0.94P0.06)O4 phosphor, when annealed at 1473 K for 4 h, exhibited the highest PL intensity, which was 1.6 times higher than that of the non-annealed (Ca1.96Eu0.02□0.03)(Si0.96P0.04)O4 phosphor. The coexistence of the α’L- and β-phases would be essentially important for the enhancement of PL intensity and eventually provide a favorable environment for the efficient PL emission of Eu2+ ion. The disc-shaped sintered phosphor that was annealed at 1773 K clearly showed the core-shell structure, consisting of the red-emitting phosphor core and the green-emitting phosphor as shell. This unique structure in the annealed phosphor would be formed during the phase transitions.

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