1 research outputs found
Highly Luminous and Thermally Stable Mg-Substituted Ca<sub>2–<i>x</i></sub>Mg<sub><i>x</i></sub>SiO<sub>4</sub>:Ce (0 ≤ <i>x</i> ≤ 1) Phosphor for NUV-LEDs
Blue-emitting Ca<sub>2–<i>x</i></sub>Mg<sub><i>x</i></sub>SiO<sub>4</sub>:Ce (0.0 ≤ <i>x</i> ≤ 1.0) phosphors
were successfully synthesized and characterized. Rietveld refinement
revealed that four main phases exist within the solid-solution range
of CaO-MgO-SiO<sub>2</sub>, namely, β-Ca<sub>2</sub>SiO<sub>4</sub> (Mg (<i>x</i>) = 0.0), Ca<sub>14</sub>Mg<sub>2</sub>(SiO<sub>4</sub>)<sub>8</sub> (Mg (<i>x</i>) = 0.25), Ca<sub>3</sub>Mg(SiO<sub>4</sub>)<sub>2</sub> (Mg (<i>x</i>) =
0.5), and CaMgSiO<sub>4</sub> (Mg (<i>x</i>) = 1.0). The
variation of the IR modes was more prominent with increasing Mg<sup>2+</sup> content in the Ca<sub>2<i>–x</i></sub>Mg<sub><i>x</i></sub>SiO<sub>4</sub> materials. The sharing of
O atoms of the SiO<sub>4</sub>-tetrahedra by the MgO<sub>6</sub>-octahedra
induced weakening of the Si–O bonds, which resulted in the
red shift of the [SiO<sub>4</sub>] internal modes and appearance of
a Mg–O stretching vibration at ∼418 cm<sup>–1</sup>. Raman measurement revealed that the change of the Ca–O bond
lengths because of the Mg<sup>2+</sup>-substitution directly reflected
the frequency shift of the Si–O stretching-Raman modes. Notably,
the thermal stability of Ca<sub>2<i>–x</i></sub>Mg<sub><i>x</i></sub>SiO<sub>4</sub>:Ce (Mg (<i>x</i>) > 0.0) phosphors was superior to that of β-Ca<sub>2</sub>SiO<sub>4</sub>:Ce (Mg (<i>x</i>) = 0.0) as confirmed by
temperature-dependent photoluminescence (PL) measurements. This indicated
that Mg<sup>2+</sup> 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<sub>2–<i>x</i></sub>Mg<sub><i>x</i></sub>SiO<sub>4</sub>:Ce (Mg (<i>x</i>) = 0.1) was approximately
twice as much as β-Ca<sub>2</sub>SiO<sub>4</sub>:Ce (Mg (<i>x</i>) = 0.00), directly indicating a “Mg<sup>2+</sup>-substitution effect”. The large enhancements of PL, EL, and
thermal stability because of Mg<sup>2+</sup>-substitution may provide
a platform in the discovery of more efficient phosphors for NUV-LEDs