What defect type is most likely when Ca2+ substitutes for Na+ in a NaCl lattice, and what is the resulting defect ratio?

• A. Anion vacancies form to balance charge; no cation vacancy
• B. Interstitial Ca2+ ions form in the lattice
• C. No defects form; the lattice remains perfect
• D. Cation vacancies form to balance charge; Na+ are removed for each Ca2+ substitution

Answer: (D)

When a Ca2+ ion takes the place of a Na+ in NaCl, the lattice gains extra positive charge (+1 per substitution). To keep the crystal neutral, the structure compensates by creating cation vacancies on the Na sublattice, effectively removing Na+ ions. For every Ca2+ substituting a Na+ site, two Na+ vacancies are formed, giving a 2:1 vacancy-to-substitution ratio. This makes cation vacancies the most likely defect type. Interstitial Ca2+ would introduce an extra ion into the lattice, which is energetically unfavorable; no defects would leave the lattice charged; and forming anion vacancies wouldn’t efficiently balance the excess positive charge in this aliovalent substitution scenario.