Thermochemistry of MgAl ₂ O ₄ ‐Al _8/3 O ₄ Defect Spinels

Abstract

Solution calorimetry of MgAl 2 O 4 ‐Al 8/3 O 4 solid solutions was performed in a molten 2PbO · B 2 O 3 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 Al 2 O 3 and on the MgAl 2 O 4 ‐Al 8/3 O 4 (MgO‐Al 2 O 3 ) subsolidus phase relations to estimate the partial molar entropy of mixing of γ‐Al 8/3 O 4 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.