Synthesis of (La_0.8Y_0.2)PO₄: Sm³⁺, Eu³⁺, Na⁺ and kinetics mechanism study with Z(α) master plots method for thermal process of its precursor

Because luminescence can be enhanced by energy transfer from Sm³⁺ to Eu³⁺ ions, Eu³⁺, Sm³⁺-codoped phosphors have attracted much attention recently. In addition, Na⁺ ions can enhance the emission of Eu³⁺ in some Eu³⁺-doped phosphors. In order to obtain a red phosphor that can be efficiently excited by near-ultraviolet light, a series of triple-metal ions-codoped phosphors, (La_0.8Y_0.2)PO₄: Sm³⁺, Eu³⁺, Na⁺, were obtained by high-temperature solid-state method. The products were characterized by XRD, SEM, PLE&PL and TG/DTG. The thermal decomposition process of the precursor was studied by non-isothermal method. Then, the kinetics of the thermal decomposition reaction process and the formation mechanism of the phosphor during calcination were obtained with Z(α) master plots method. The results demonstrate that Na⁺-doping effect on R (the intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁) and PL intensity (591 nm) is exactly opposite, of which the former is a minor negative effect, while the latter is a major positive effect. Furthermore, the results indicate that the thermal decomposition of the precursor is a single-step kinetic process, and the most probable mechanism function is g(α) = [(1−α)^(−1/3)−1]^2, which belongs to the mechanism of three-dimensional diffusion. The optimal sample is (La_0.8Y_0.2)PO₄: 0.012Sm³⁺, 0.03Eu³⁺, 0.036Na⁺, and its PL intensity (591 nm) is as high as 1.84 times of that without Na⁺, indicating the optimal sample is a promising red-emitting phosphor for WLEDs.