From Figure, the phase diagram for the MgO–Al2O3 system,

From Figure, the phase diagram for the MgO–Al2O3 system, it may be noted that the spinel solid solution exists over a range of compositions, which means that it is nonstoichiometric at compositions other than 50 mol% MgO–50 mol% Al2O3.

(a) The maximum nonstoichiometry on the Al2O3-rich side of the spinel phase field exists at about 2000°C (3630°F) corresponding to approximately 82 mol% (92 wt%) Al2O3. Determine the type of vacancy defect that is produced and the percentage of vacancies that exist at this composition.
(b) The maximum nonstoichiometry on the MgO-rich side of the spinel phase field exists at about 2000°C (3630°F) corresponding to approximately 39 mol% (62 wt%) Al2O3. Determine the type of vacancy defect that is produced and the percentage of vacancies that exist at thiscomposition.