Thermochemistry of MgAl₂O₄‐Al_8/3O₄ Defect Spinels

Solution calorimetry of MgAl₂O₄‐Al_8/3O₄ solid solutions was performed in a molten 2PbO · B₂O₃ solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end‐members. These data were combined with information on the energetics of the α‐γ transition in Al₂O₃ and on the MgAl₂O₄‐Al_8/3O₄ (MgO‐Al₂O₃) subsolidus phase relations to estimate the partial molar entropy of mixing of γ‐Al_8/3O₄ in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values.