Improvement of thermal stability and photoluminescence in Sr0.8Ca0.2Al2Si2O8:Eu2+ by the substitution of Si–Na # Al–Sr and Ca # Sr for structural modifications

Abstract

Blue emitting phosphor is urgently needed in commercial application of white light emitting diodes for the reason that the existing ones lack proper emission and have poor thermal stability. Herein, we report two methods for Sr_0.8Ca_0.2Al₂Si₂O₈:Eu²⁺ phosphors that were performed to optimize the luminescence properties and thermal stability based on structural reconstruction. One is partial substitution of Al³⁺–Sr²⁺ by Si⁴⁺–Na⁺ forming Sr_0.77−xNa_xCa_0.2Al_2−xSi_2+xO₈:0.03Eu²⁺ (x = 0–0.7); the other is altering the proportion of Ca²⁺ and Sr²⁺, forming Sr_0.97−yCa_yAl₂Si₂O₈:0.03Eu²⁺ (y = 0–0.97) solid solution. The phosphor's structural evolution, variation of the Eu²⁺ local environment, luminescence and thermal quenching were explored in detail. As a result, for Sr_0.77−xNa_xCa_0.2Al_2−xSi_2+xO₈:0.03Eu²⁺ (x = 0–0.7) series, (Al/Si)O₄ tetrahedrons are enlarged and Eu–O polyhedrons are compressed; the emission gets red-shifted from 418 nm to 428 nm under 350 nm excitation; the thermostability of Sr_0.97−yCa_yAl₂Si₂O₈:0.03Eu²⁺ is improved with the incorporation of Si⁴⁺–Na⁺ pair. For Sr_0.97−yCa_yAl₂Si₂O₈:0.03Eu²⁺ (y = 0–0.97) series, the emission peak shows a large shift from 420 nm to 440 nm, which is attributed to a comprehensive result of nephelauxetic effect and crystal field splitting. Furthermore, thermal stability is improved by the coexistence of Ca²⁺ and Sr²⁺. White light emitting diodes fabricated by combining the as-prepared phosphor with commercial phosphors exhibit an excellent color rendering index R_a of 90.1 and a temperature of 4459 K with CIE coordinates of (0.357, 0.356). These results indicate that the as-developed composition series are promising for commercial application.