Energetics of formation of KF-GdF₃ binary-intermediate compounds

Enthalpies of formation were measured for KGdF₄, K₂GdF₅ and KGd₂F₇ using the transposed temperature drop calorimetry method. These compounds were synthesized by conventional solid state reaction methods using mixtures of KF and GdF₃. The measured enthalpies of formation from fluorides for KGdF₄, K₂GdF₅ and KGd₂F₇ were - 22.3, - 16.1 and - 18.3 kJ mol^-1, respectively. From these measurements, enthalpies of formation from the elements were computed to be - 2290.0, - 2852.4 and - 3985.2 kJ mol^-1 for the respective compounds. These enthalpies were compared with estimated values calculated by two methods previously reported to be effective for multicomponent oxides. Estimated values compare favorably with measured data, with deviations ranging from 0.5 to 2.9%. The errors of 0.5-2.9% correspond to a 16-85 kJ mol^-1 uncertainty range for Δ_fH⁰ of ternary fluorides from binary fluorides. One of these methods, based on the summation of Δ_fH⁰ values for the constituent binary fluorides, is recommended for the K-Gd-F compounds.