Phase Evolution and Luminescence of Yb<sup>3+</sup>/Tb<sup>3+</sup>-Doped Lanthanum Fluoride and Lanthanum Oxyfluoride as a Function of Annealing Conditions

In this study, LaF3–LaOF:Yb3+/Tb3+ upconversion phosphors were synthesized using the solid-phase reaction method, and the effects of the annealing conditions (temperature and time) on their luminescence characteristics were analyzed to examine the conversion mechanism. The sample synthesized at 1150°C for 1 h was found to consist of LaF3:Tb3+ and LaOF:Yb3+/Tb3+. The prepared phosphor showed photoluminescence emission peaks at wavelengths of 486 nm (5D4 → 7F6), 541 nm (5D4 → 7F5), 583 nm (5D4 → 7F4), and 620 nm (5D4 → 7F3). Additionally, the slope of photoluminescence intensity against pump power was found to be approximately 2.0, indicating that emission by this phosphor is a two-photon process.