Ca2Na2La6(SiO4)4(PO4)2O: Eu2+/Eu3+: A visual dual-emitting fluorescent ratiometric temperature sensor

A novel apatite-based UV-excited dual-emitting Ca2Na2La6(SiO4)(4)(PO4)(2)O: Eu2+/Eu3+ phosphor (CNL: Eu2+/Eu3+) was designed and successfully synthesized by a solid-state reaction. Compared with previous reports on this family of materials, a structural study based on DFT calculation exhibited a new consequence that the monovalent ions in this system are more inclined to occupy the seven-coordinate cationic sites rather than the nine-coordinate sites. This result was confirmed by the structural refinement and high-resolution transmission electron microscopy (HRTEM) data. Due to the coexistence of Eu2+ and Eu3+ dopants in the material, under 345 or 392nm excitation, CNL: 0.02Eu(2+)/Eu3+ exhibited a green Eu2+ emission band (528nm) and red Eu3+ emission peaks (around 618nm). The application potential of CNL:0.02Eu(2+)/Eu3+ in luminescent thermometry was studied by exploiting the temperature sensitivity of the fluorescent intensity ratio (green/red) at different temperatures. It was found that, under 345nm excitation, the fluorescent intensity ratio of CNL: 0.02Eu(2+)/Eu3+ displayed linear correlation over the temperature range of 298 to 473K with a high sensitivity of 2.82%K-1. Additionally, the emission color of the CNL: 0.02Eu(2+)/Eu3+ sample under UV lamp (254 and 365nm) excitation showed an obvious change (from green to red) as the temperature increased from 298 to 473K (from green to red). These results indicated that CNL: Eu2+/Eu3+ can serve as an excellent visual luminescent ratiometric thermometer. Furthermore, this work provides a novel reference for developing high-performance luminescence temperature-sensing materials.