Thermal Stability of Mixed Fluorides of 3d Elements

Literature data on the existence of mixed fluorides of 3d elements M(II)M(III)F5 (M = Mn, Fe, Co, Ni) were analyzed. Heterogeneous equilibria in MF2–MF3 systems were considered, and equilibrium constants and enthalpies were calculated for the reactions with participation of M2F5(c). The FeF2 sublimation enthalpy of 268.7 ± 1.9 kJ/mol was confirmed, and the FeF3(c) sublimation enthalpy was essentially refined, equal to 264.5 ± 2.2 kJ/mol. For crystalline fluorides Mn2F5, Fe2F5, Co2F5, and Ni2F5, the standard formation enthalpies were determined as equal to –1909.0 ± 7.8, –1716.3 ± 4.4, ≥–1554.2 ± 11, and –1370.5 ± 8.0 kJ/mol, respectively, at 0 K. The enthalpies and Gibbs energies of formation of these compounds from crystalline MF2 and MF3 were within a range of values typical for the solid solutions with weak interaction.