“…According to the phase diagram of NaF-YF 3 and YF 3 -GdF 3 systems [38], the crystallization of the following fluorides could be achieved in function of temperature, NaYF 4 in two forms (cubic α phase, Fm-3m; and hexagonal β phase, P63/m), and three crystal structures of the Y 1−x Gd x F 3 phases (orthorhombic β phase, Pnma; hexagonal, P63cm; and trigonal, P-3c1 phase). Contrary to our previous work, where we obtained the β-NaYF 4 :Yb,Er phase under identical processing conditions [39], since the YF 3 and GdF 3 phases show complete miscibility, all samples synthesized in this study, independent of stoichiometry, have the same orthorhombic crystal structure. The XRPD analysis revealed a change in the unit cell parameters due to the substitution of ytterbium by gadolinium and the coexistence of two different particle populations in all samples, which is associated with the formation of monocrystals.…”