Calculation Method: The first-principles calculations were performed on the basis of the DFT method,[6] as implemented in the Vienna ab initio simulation package (VASP), employing the generalized gradient approximations (GGA)[45] of Perdew–Burke–Ernzerhof for the exchange correlation functional. The electron cloud density and distribution of Ln3+ (Ln = Y, Yb, and Pr) in both hexagonal and cubic NaLnF4 structure were described by using augmented-plane-wave method.[46] An energy cutoff of 400 eV was adopted, and k points via a 3 × 3 × 2 MonkhorstPack scheme were sampled.