The sodium fluoride-lanthanide trifluoride systems

Article abstract of DOI:10.1021/ic50041a032

Phase diagrams are reported for the NaF-LnF₃ binary systems. Hexagonal phases of the formula NaF·LnF₃ are formed in all NaF-LnF₃ systems. Above 700° the hexagonal phases NaF·PrF₃ to NaF·LuF₃ become converted to disordered fluorite-like cubic phases of variable composition. The composition limits of the cubic phase region are extended in proportion to the difference in ion size between Na⁺ and Ln³⁺ and vary in breadth ranging from 9 mole % (NaF-PrF₃) to 25 mole % (NaF-LuF₃). In each case, the LnF₃-rich phase boundary was found empirically to be 5NaF·OLnF₃ as the interstitial positions for allions become filled. Values of the fluorite unit-cell constants were calculated from ionic radii and found to be in good agreement with measured values. Unit-cube dimensions, a₀, range from a maximum value of 5.720 A for the LnF₃-rich phase in the system NaF-PrF₃ to 5.425 A for the NaF-rich phase in the system NaF-LuF₃. Lattice constants for the hexagonal phases NaF·LaF₃ to NaF·LuF₃ vary for a₀ from 6.157 to 5.907 A and for C₀ from 3.822 to 3.523 A. At high temperatures the 5NaF·9LnF₃ phase displays disordering characteristic of the fluorite-like solid solutions; at lower temperatures cationic ordering takes place in the 5NaF·OLnF₃ phases where Ln = Dy to Lu, transforming the cubic phase to an orthorhombic phase. Hexagonal NaF·LnF₃ and orthorhombic 5NaF·OLnF₃ compounds are partially miscible in the solid state. In the series Dy to Lu, the hexagonal phase is increasingly soluble in 5NaF·OLnF₃, stabilizing the 5:9 phase to the extent that the lutetium compound is stable at temperatures below 300°, the lower temperature limit of this study. Optical properties and refractive indices of the intermediate crystalline phases in the NaF-LnF₃ systems were determined as part of this investigation. Minimum liquidus temperatures in the NaF-LnF₃ systems are associated with an NaF-NaF·LnF₃ eutectic which varies in composition only from 25 to 29 mole % LnF₃ and in temperature from 733 to 595°.