Final answer:
The correct point defect created by Al2O3 as an impurity in MgO is one Mg2+ vacancy for every two Al3+ ions added. Other options would result in a charge imbalance within the crystal structure.
Step-by-step explanation:
When Al2O3 is introduced as an impurity into MgO, it creates point defects due to charge and size differences between Mg2+ and Al3+ ions. Each aluminum ion has a 3+ charge compared to the 2+ charge of magnesium ions. Therefore, to maintain electrical neutrality, for every two Al3+ ions added, one magnesium ion must be replaced while also compensating for the difference in charge balance. The acceptable point defects are:
- a - One Mg2+ vacancy for every two Al3+ ions added: This compensation maintains charge neutrality by removing one Mg2+ for every two Al3+ ions since two Al3+ introduce six positive charges, which can be balanced by removing three Mg2+ ions (six positive charges).
- b - Three Mg2+ vacancies for every two Al3+ ions added: This option would result in a charge imbalance, with more positive charges removed than introduced, and is not a correct choice.
- c - One O2- vacancy for every two Al3+ ions added: This approach does not compensate for the additional positive charges introduced by Al3+ ions.
- d - Three O2- interstitials for every two Al3+ ions added: This would add extra negative charges rather than compensating for additional positive charges, leading to a charge imbalance.
The correct response is option (a) - One Mg2+ vacancy for every two Al3+ ions added. This is required to maintain the electrical neutrality of the crystal given Al3+ has a higher charge than Mg2+.