Calorimetric investigations of some phases in the system sodium fluoride-aluminium fluoride

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: 5NaAlF(4(s))=Na5Al3F14(s)+2AlF(3(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 alpha-Na3AlF6 --> beta-Na3AlF6 and alpha-AlF3 --> beta-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 Delta H-900(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 Delta H-f(o) = -7559.2 kJ mol(-1).