*2.4. Study of the Proportion of LaPO4:Ce3*<sup>+</sup>*, Tb3*<sup>+</sup>

To explore the proportion of product, we dissolved an appropriate amount of the precursor (preparation conditions: 1 mol phosphoric acid solution, rare earth sulfate solution, and [C4mim][Tf2N] functional membrane) in a moderate amount of hydrochloric acid at 60 ◦C for 30 min, diluted the sample to the right concentration, and then tested the rare earth ion concentration by ICP. Table 1 shows the molar ratio of the mixed solution of rare earth elements from the rare earth supply phase and the molar ratio of the precursor. A clear difference between the solution proportion and precursor proportion can be seen in Table 1 because the rare earth ions do not completely precipitate. Then, we obtained the molar ratio (a fitting degree of greater than 99%) commonly used in the production of phosphate (La:Ce:Tb = 55:30:15) by simply adjusting the molar ratio of the rare earth ions in solution.

**Table 1.** The molar ratio of a mixed solution of the rare earth elements and the molar ratio of the precursor.

