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.

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Fig. 1.


  1. 1.

    Except the specified cases, the calculations given in the text were performed by the third law of thermodynamics. The thermodynamic functions and formation enthalpies of reaction participants were taken from the papers [13, 14]. For the compounds M2F5(c), it was assumed that \(\Phi _{T}^{^\circ }\) = \(\Phi _{T}^{^\circ }\)(MF2(c)) + \(\Phi _{T}^{^\circ }\)(MF3(c)). The letter “c” in brackets means that the thermodynamic properties of a crystal are only used in calculations. The errors in the enthalpies of reactions (3)–(5) are equal to a 95-% confidence range of reproducibility. The formation enthalpies of М2F5(c) incorporates the errors of the formation enthalpies of binary fluorides from the original works.


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This work was performed within the state task to the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences in the field of fundamental studies.

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Correspondence to A. S. Alikhanyan.

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Translated by E. Glushachenkova

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Nikitin, M.I., Chilingarov, N.S. & Alikhanyan, A.S. Thermal Stability of Mixed Fluorides of 3d Elements. Russ. J. Inorg. Chem. 66, 89–95 (2021).

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  • heterogeneous equilibria
  • mass spectrometry
  • formation enthalpies of compounds