Highly Luminous and Thermally Stable Mg-Substituted Ca_2-xMg_xSiO₄:Ce (0 ≤ x ≤ 1) Phosphor for NUV-LEDs

Blue-emitting Ca_2-xMg_xSiO₄: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 Ca_2-xMg_xSiO₄ 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 Ca_2-xMg_xSiO₄: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 Ca_2-xMg_xSiO₄: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.