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Calorimetric Investigations of Some Phases in the System Sodium Fluoride - Aluminium Fluoride

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Abstract

Beside the two well-known minerals cryolite, Na3AlF6, and chiolite, Na5Al3F14, the binary system NaF-AlF3 also contains a third compound, NaAlF4, sodium tetrafluoroaluminate. Solid NaAlF4 has been prepared from its vapour under controlled conditions. The stability of NaAlF4 has been investigated by differential scanning calorimetry. It is shown that the disproportionation of the compound: 5NaAlF4(s)=Na5Al3F14(s)+2AlF3(s) takes place at considerable rate between 700 and 900 K. The enthalpy of this reaction is calculated and found to be -66.9 kJ. Enthalpies of the two solid state transitions α-Na3AlF6 → β-Na3AlF6 and α-AlF3 → β-AlF3 have also been measured and new values are reported.

The enthalpy of formation of chiolite, Na5Al3F14, at 900 K has been recalculated from enthalpy increment data obtained by drop calorimetry. A value of ΔH900 o = -7513.6±12.0 kJ mol-1 has been obtained. This value is in disagreement with the recommended value given in JANAF Thermochemical Tables given at 900 K ΔHf o = -7559.2 kJ mol-1.

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Authors and Affiliations

  1. NTNU, Department of Inorganic Chemistry, N-7034, Trondheim

    M. Bruno & J. L. Holm

  2. Department of Physical Chemistry, NTNU, N-7034, Trondheim, Norway

    O. Herstad

Authors
  1. M. Bruno
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  2. O. Herstad
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  3. J. L. Holm
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Bruno, M., Herstad, O. & Holm, J.L. Calorimetric Investigations of Some Phases in the System Sodium Fluoride - Aluminium Fluoride. Journal of Thermal Analysis and Calorimetry 56, 51–57 (1999). https://doi.org/10.1023/A:1010175006354

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  • DOI: https://doi.org/10.1023/A:1010175006354

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