Investigation of the Solid-Solution Limit, Crystal Structure, and Thermal Quenching Mitigation of Sr-Substituted...

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 Rb₂Sr_1-CaP₂O₇: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 (Rb₂Ca_0.8Sr_0.2P₂O₇:Eu²⁺). The integrated intensity of bright orange-emitting Rb₂Ca_0.8Sr_0.2P₂O₇: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.