From Figure 12.23, the phase diagram for the MgO–Al2O3 system, it may be noted that the sp...

From Figure 12.23, 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.

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(b) The maximum nonstoichiometry on the MgOrich 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 this composition.

Figure 12.23 The magnesium oxide–aluminum oxide phase diagram; ss denotes solid solution.

(Adapted from B. Hallstedt, “Thermodynamic Assessment of the System MgO–Al2O3,” J. Am. Ceram. Soc., 75[6], 1502 (1992). Reprinted by permission of the American Ceramic Society.)