Redistribution of Activator Tuning of Photoluminescence by Isovalent and Aliovalent Cation Substitutions in Whitlockite Phosphors

Abstract

Many strategies, including double substitution, addition of charge compensation, cation-size-mismatch and neighboring-cation substitution, have contributed to tuning photoluminescence of phosphors for white light-emitting diodes. These strategies generally involve modification of a certain special site where the activator occupies; tuning strategy based on multiple cation sites is very rare and desirable. Here we report that isovalent (Sr²⁺) and aliovalent (Gd³⁺) substitutions for Ca²⁺ tune the photoluminescence from one band to multiple bands in whitlockite β-Ca_3–<i>x</i>Sr_<i>x</i>(PO₄)₂:Eu²⁺ and β-Ca_{3–3<i>y</i>/7}Gd_2<i>y</i>/7(PO₄)₂:Eu²⁺ phosphors. The saltatory variation of the emission spectra is caused by the removal of Eu²⁺ from the site M(4) to other sites. Moreover, we found the mechanisms of dopant redistribution tuning the luminescence are different. The incorporation of Gd³⁺ makes the site M(4) empty according to the scheme 3Ca²⁺ = 2Gd³⁺ + □, while Sr²⁺ substitution causes the cation sites to be enlarged due to cation size mismatch. Additionally, the influence of the cation substitutions on the photoluminescence thermal stability of phosphors is researched. The strategies, emptying and enlarging sites, developed herein are expected to provide a general route for tuning luminescence of phosphors with multiple sites in the future