Thermodynamic study of NaLaF4 in NaF-LaF3 system using solid state electrochemical cell method

Highlights: • A binary mixture of {LiF-NaF} is an excellent suitable solvent for the trivalent actinide fluorides and lanthanide fluoride. • Interaction of NaF with LaF₃ leads to form single ternary compound, NaLaF₄ (s). • The heat capacity of NaLaF₄(s) has been measured with Differential Scanning Calorimeter. • The Gibbs energy of formation of NaLaF₄(s) was experimentally measured using Solid Electrolyte Galvanic Cell. • The binary phase diagram of NaF (s) - LaF₃ (s) system has been calculated using experaimental data. • Ternary phase diagram of Na-La-F system has also been calculated. -- Abstract: The NaF-LaF₃ system is characterized with single ternary fluoride compound, NaLaF₄. This compound has been synthesized in solid state route and characterized with X-ray diffraction technique. The purity has been checked with ICPMS technique. The heat capacity of NaLaF₄ (s) has been experimentally measured with differential Scanning Calorimeter and the variation of Cp as a function of temperature can be expressed as. C_p /J.mol^-1K^-1 = (171.2±0.1)+(0.00089±0.0002)×T/K - (3.235 × 10⁶ ± 9204) × (T/K)^-2 The Gibbs energy of formation of NaLaF₄ (s) has been determined using Solid Electrolyte Galvanic Cell method with calcium fluoride as solid electrolyte. The Δ_fG of NaLaF₄ (s) as a function of temperature can be given as. Δ_fG (NaLaF₄, s, T) (kJ mol ^-1) = (- 2220.7±0.7)+(0.3123±0.0032) · (T/K) Using the experimentally obtained thermodynamic data, the binary phase diagram of NaF (s) - LaF₃ (s) system has been calculated. To study the stability domain and coexisting phases of NaLaF₄(s), the chemical potential diagram of Na-La-F-O system and ternary phase diagram of Na-La-F system has been calculated.