Phase equilibria in the system sodium fluoride-yttrium fluoride

Article abstract of DOI:10.1021/ic50009a030

A phase diagram of the condensed system NaF-YF₃ was constructed from data obtained in cooling-curve and quenching experiments. The number and identity of phases co-existing at equilibrium were determined by use of the X-ray diffractometer and the petrographic microscope. Two compounds, NaF-YF₃ and 5NaF-9YF₃, were formed from the components. Each compound exists in two polymorphic forms, the high-temperature form in both cases crystallizing from molten mixtures as fluorite-like cubic crystals. The two cubic phases form a continuous solid solution with a maximum melting temperature of 975° at the composition 5NaF-9YF₃. Lattice parameters and refractive indices of the solid solution appear to be linear functions of the YF₃ content, a = 5.447-5.530 A?., refractive index 1.430-1.470. Pure crystals of NaF·YF₃ invert from the fluorite cubic form, on cooling below 691°, to a hexagonal form which is isostructural with β₂-Na₂ThF₆, with lattice constants a = 5.95, c = 3.52 A?. The five primary phase fields below the liquidus were found to be NaF, hexagonal NaF·YF₃, NaF·YF₃-5NaF·9YF₃ ss, and two forms of YF₃. Two eutectics occur in association with these primary phases, at 29 and 75 mole% YF₃ and at 638 and 947°, respectively. Because of the isomorphism of YF₃ with the trifluorides of the rare earths samarium-lutetium and the similarity of their cation sizes, the system NaF-YF₃ is predicted to be an approximate model for each of the binary systems SmF₃-LuF₃ with NaF.