Thermochemistry of sodium rare earth ternary fluorides, NaREF₄

Sodium rare earth fluorides, NaREF₄ (RE = rare earth), are used as luminescent materials for light emission and biomedical applications and are important compositions for extracting and separating RE elements. Solution calorimetric measurements of a series of β-structured NaREF₄ (Na_1.5RE_1.5F₆) phases with various RE elements determined their heats of formation. Though the lattice contracts from light to heavy RE elements, NaREF₄ compounds show more exothermic enthalpies of formation from binary components with the decrease of RE³⁺ radii, contrary to behavior seen in most RE oxide ternary compounds. By constructing Born–Haber cycles, the different slopes of lines relating lattice energies to lattice parameters in binary and ternary fluorides appear to be the reason for this reverse trend, which can be associated with changes in the coordination number of RE cations. These trends and the metastability of sodium light RE fluorides not only reveal the key role of ionic radius in RE compound stability, but also are significant for the design and synthesis of new materials and motivate the more effective utilization of RE.