Thermodynamics of the system NaF-AlF 3 : Part VII. Non-stoichiometric solid cryolite
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THIS note extends and updates the interpretation of data, originally published in 1978,[1] on the homogeneous field of nonstoichiometric solid Na3AlF6. The reason for the renewed interest is a study[2] on the cryoscopic method of elucidating the chemical state of dissolved material, a method much used on cryolite melts. Although it has been known since 1972[3] that the solid cryolite separating is nonstoichiometric, no analysis has previously been made of the implications of the fact. The analysis reveals the need for a model linking the temperature and composition of the nonstoichiometric material with the activities of NaF and AlF3. Such a model was derived earlier,[1] but it can be refined. The original experimental data consist of measurements of the electrical conductivity of solid material of various compositions, both within the homogeneous region and in the two-phase regions with excesses of either NaF or AlF3. An expression was derived relating conductivity in the homogeneous region to composition and temperature; that expression was then applied to conductivities of mixtures outside the homogeneous region to deduce the composition at the phase boundaries. None of that work needs revision. The model used to interpret the results was that the nonstoichiometry involved cation (Na+) vacancies (which give rise to the observed electrical conductance) and (AlF4)2 ions replacing (AlF6)32. This implies that two anion vacancies are located on an (AlF6)32. (The possibility of the formation of (AlF5)22 from single anion vacancies is discussed later.) The equation is[1] 3Na+ 1 (AlF6)32 5 Na+ 1 2 M 1 (AlF4)2 1 2NaF cryolite lattice cation sites anion site removed
[1]
where ▫ represents a cation vacancy. If x is the deficit of NaF (i.e., the composition of the material is (3 2 x)NaF z AlF3), then the equilibrium constant for Reaction [1] is ERNEST W. DEWING, 648, Pimlico Pl., Kingston, ON, Canada K7M 5T8, is retired. Manuscript submitted June 14, 1996.
METALLURGICAL AND MATERIALS TRANSACTIONS B
K5
[M ]2 z [AlF42] z (aNaF)2 [Na+]2 z [AlF632 ]
5 x3 z (aNaF)2 /[18(1 2 x/ 3)2 z (1 2 x/2)]
[2] [3]
It is implicit in the model that the activity of cryolite, referred to the stoichiometric solid as standard state, is aNa3AlF6 5 [Na+]3 z [AlF632 ] 5 (1 2 x/3)3 z (1 2 x/2)
[4] [5]
The values for the activities of NaF and AlF3 in liquid mixtures, and hence along the phase boundaries, have been improved[4] since the original calculations were done, and these new values have been used to recalculate K as a function of temperature along the boundaries on both the NaF and AlF3 sides. Some important additional points can also be added. Solheim et al.[5,6] have made very precise measurements of the liquidus curve, and place its maximum at NAlF3 5 0.2571, or x 5 0.1105. Holm and Grønvold[7] made measurements of the enthalpy of cryolite, and it was shown earlier[1] that the ‘‘premelting’’ they observed (i.e., an upward trend in the enthalpies before the melting point was reached) was consistent with the proposed nonstoichiometry.
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