Investigation of the Solid-Solution Limit, Crystal Structure, and Thermal Quenching Mitigation of Sr-Substituted Rb2CaP2O7:Eu2+ Phosphors for White LED Applications.

Abstract

Rb2CaP2O7:Eu2+ is a bright reddish-orange-emitting phosphor, but its luminescence thermal stability is poor. In this study, we investigated the solid-solution limit and thermal quenching mitigation of Rb2CaP2O7:Eu2+ phosphors by cation substitution with Sr2+ and revisited their crystal structure. First, we carefully investigated the solid solution limit of Sr in the structure of Rb2CaP2O7. The results show that up to 80% of Ca can be substituted by Sr, whereas Ca hardly resides in the structure of Rb2SrP2O7. Consequently, the photoluminescence was fine-tuned from reddish-orange (612 nm) to yellow (580 nm) light emission by increasing the Sr2+ concentration in the solid-solution phosphors Rb2Sr1-xCaxP2O7:Eu2+ under excitation at 342 nm. The mechanism for the blue shift of the emission spectrum was discussed. With the associated modification of the local environment of the activator (as reflected by the changes in the effective coordination number, average bond length, distortion index, and quadratic elongation), the luminescence thermal quenching issue of Rb2CaP2O7:Eu2+ was mitigated by substituting 20% Sr into the Ca site (Rb2Ca0.8Sr0.2P2O7:Eu2+). The integrated intensity of bright orange-emitting Rb2Ca0.8Sr0.2P2O7:Eu2+ (603 nm) at 150 °C retained 53% of its initial value, 1.64 times that of Rb2CaP2O7:Eu2+ (32.3%). Such an enhancement could be attributed to the improved rigidity of the crystal structure due to the local structure modification as evidenced by Rietveld refinement. The cation substitution is an effective approach for mitigating the thermal quenching issue of phosphors.