The crystal structure and luminescence properties of a novel green-yellow emitting Ca1.5Mg0.5Si1-xLixO4-delta:Ce3+ phosphor with high quantum efficiency and thermal stability

A novel green-yellow emitting Ca_1.5Mg_0.5Si_1−xLi_xO_4−δ:Ce³⁺ phosphor with high quantum efficiency and thermal stability was discovered for applications in near ultraviolet pumped white light-emitting diodes. Its crystal structure was determined with a single-particle diagnosis approach. The Si sites in the SiO₄ tetrahedra are reported for the first time to accommodate Li⁺ ions. This substitution, confirmed by ⁶Li solid-state NMR and T.O.F. neutron powder diffraction, causes a disordered occupation of Ca/Mg in the Ca₃MgSi₂O₈ host and favors a phase transformation at ∼330 °C, which results in the formation of the novel phosphor. The produced phosphor was efficiently excited by near UV light peaking at 365 and 410 nm and produced broad green-yellow emission with peaks at 500 and 560 nm, respectively. Its quantum efficiency reached 88.4% (internal) and 55.7% (external) under excitation at 365 nm, and 80.5% (internal) and 42.7% (external) under excitation at 410 nm, while the decrease of luminescence intensity at 200 °C was small (∼26%). A WLED lamp with a high color rendering index of R_a = 92.8 was produced with the combination of a 365 nm emitting chip with blue emitting BaMgAl₁₀O₁₇:Eu²⁺, green-yellow emitting CMSL:0.01Ce, and red emitting Sr₂Si₅N₈:Eu²⁺ phosphors.