MgAl2O4-Al8/3O4 spinels: Formulation and calibration of the low-pressure thermodynamics of mixing

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ID: 304987
2014
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Abstract
A thermodynamic model for the mixing properties of MgAl~2~O~4~--Al~8/3~O~4~ spinels is formulated, calibrated, and used to develop a closed-form polynomial for the excess Gibbs energy of mixing. The model is formulated on the assumptions that Mg, Al and vacancies exhibit long-range-ordering (LRO) between tetrahedral and octahedral sites, and that the vibrational Gibbs energy may be described by a Taylor expansion of second degree in the composition variable X ≡ $$\frac{3}{2}$$(2X~Al~^OCT^ + X~Al~^TET^ − 2) = (3X~Vac~^TET^ + 6X~Vac~^OCT^) and in LRO variables s~1~ ≡ X~Al~^OCT^ − $$\frac{1}{2}$$X~Al~^TET^ and s~2~ ≡ 3X~Vac~^TET^ − 6X~Vac~^OCT^. Calibration of the parameters produces a model that, in addition to being consistent with spectroscopic constraints on Mg-Al ordering in MgAl~2~O~4~-rich spinels, successfully reproduces the brackets on the compositions of MgAl~2~O~4~--Al~8/3~O~4~ spinels coexisting with corundum over the temperature range 1400 to 2200 K, employing only a few mixing parameters and making physically plausible predictions, such as random distribution of vacancies and Al between octahedral and tetrahedral sites in the metastable, cubic γ-Al~8/3~O~4~ endmember. The resulting activity--composition relations for MgAl~2~O~4~--Al~8/3~O~4~ spinels are extremely simple and show minimal (Al~8/3~O~4~) to negligible (MgAl~2~O~4~) temperature dependence.
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Authors Richard O. Sack
Journal american journal of science
Year 2014
DOI
10.2475/04.2014.02
URL
https://ajs.scholasticahq.com/article/65528.pdf
Keywords Keywords not found

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