Highly Luminous and Thermally Stable Mg-Substituted Ca2–xMgxSiO4:Ce (0 ≤ x ≤ 1) Phosphor for NUV-LEDs

Blue-emitting Ca2–xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphors were successfully synthesized and characterized. Rietveld refinement revealed that four main phases exist within the solid-solution range of CaO-MgO-SiO2, namely, β-Ca2SiO4 (Mg (x) = 0.0), Ca14Mg2(SiO4)8 (Mg (x) = 0.25), Ca3Mg(SiO4)2 (Mg (x) = 0.5), and CaMgSiO4 (Mg (x) = 1.0). The variation of the IR modes was more prominent with increasing Mg2+ content in the Ca2–xMgxSiO4 materials. The sharing of O atoms of the SiO4-tetrahedra by the MgO6-octahedra induced weakening of the Si–O bonds, which resulted in the red shift of the [SiO4] internal modes and appearance of a Mg–O stretching vibration at ∼418 cm–1. Raman measurement revealed that the change of the Ca–O bond lengths because of the Mg2+-substitution directly reflected the frequency shift of the Si–O stretching-Raman modes. Notably, the thermal stability of Ca2–xMgxSiO4:Ce (Mg (x) > 0.0) phosphors was superior to that of β-Ca2SiO4:Ce (Mg (x) = 0.0) as confirmed by temperature-dependent photoluminescence (PL) measurements. This indicated that Mg2+ ions play an important role in enhancement of the thermal stability. In combination with the results from PL and electroluminescence (EL), it was elucidated that the luminous efficiency of Ca2–xMgxSiO4:Ce (Mg (x) = 0.1) was approximately twice as much as β-Ca2SiO4:Ce (Mg (x) = 0.00), directly indicating a "Mg2+-substitution effect". The large enhancements of PL, EL, and thermal stability because of Mg2+-substitution may provide a platform in the discovery of more efficient phosphors for NUV-LEDs.