NaF–RF3 systems, which are composed of sodium fluorides and rare earth trifluorides, are sources of many functional materials. Data on phase formation and phase equilibria in these systems were analyzed. The polymorphism and morphotropy of rare earth fluorides were considered taking into account the influence of pyrohydrolysis. A summary series of NaF–RF3 phase diagrams are presented and the coordinates of invariant equilibria are tabulated. The data of research by Thoma et al., performed in the sixties of the twentieth century, are now only of historical interest. In these systems, a-Na0.5–xR0.5+xF2+2x (cubic, R = Pr-Lu, Y) and b-Na3xR2–xF6 (hexagonal, R = La–Lu,Y) phases of variable composition with fluorite and gagarinite structures, respectively, are formed. In addition, solid solutions based on rare earth trifluorides with the LaF3–tysonite (R = La–Gd) structure and the berthollide phase of such a structure in the system with TbF3 were identified. Data was presented on the concentration dependence of the lattice parameters of fluorite phases. High temperature a-phases with maxima on the melting curves allow growing single crystals from the melt. A complex pattern of ordering of these phases with decreasing temperature was observed. Low-temperature syntheses of intermediate phases in these systems led, in accordance with the Ostwald’s step rule, to the initial formation of nonequilibrium phases of a fluorite structure, usually designated as “a-NaRF4”, which were then replaced by equilibrium low-temperature hexagonal phases of “b-NaRF4”. The hexagonal phase in the NaF–YF3 system, doped with ytterbium and erbium (“b-NaYF4:Yb,Er”), is one of the most well-known, efficient up-conversion phosphors

Pavel P. Fedorov

Sodium fluoride and rare earth trifluorides systems. Review

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