Composition Screening in Blue-Emitting Li₄Sr_1+<i>x</i>Ca_{0.97–<i>x</i>}(SiO₄)₂:Ce³⁺ Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence

Abstract

Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). Li₄Sr_1+xCa_0.97-x(SiO₄)₂:0.03Ce³⁺ (-0.7 ≤ x ≤ 1.0) phosphors were designed from the initial model of Li₄SrCa(SiO₄)₂:Ce³⁺, and their single-phased crystal structures were found to be located in the composition range of -0.4 ≤ x ≤ 0.7. Depending on the substitution of Sr²⁺ for Ca²⁺ ions, the absolute QE value of blue-emitting composition-optimized Li₄Sr_1.4Ca_0.57(SiO₄)₂:0.03Ce³⁺ reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO₆, and decrease of vibrational frequency by increasing Sr²⁺ content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.