Problem 4 - Entrance Test

Both Schottky and Frenkel defects are stoichiometric defects. - Schottky defects occur when an equal number of cations and anions are missing from their lattice sites, maintaining electrical neutrality. They are common in ionic compounds with high coordination numbers and similar-sized cations and anions (e.g., NaCl, KCl, CsCl). - Frenkel defects occur when an ion (usually cation) leaves its lattice site and occupies an interstitial position. They are common in ionic compounds with large differences in ionic sizes and low coordination numbers (e.g., AgCl, AgBr, AgI, ZnS). AgBr is unique because it exhibits both types of defects. The Ag+ ion is small enough to fit into interstitial sites (Frenkel defect), and it also has a relatively small size difference with Br- and a moderately high coordination number, allowing for Schottky defects to occur as well. - KCl and CsCl primarily show Schottky defects due to similar ionic sizes and high coordination numbers. - ZnS primarily shows Frenkel defects due to the large size difference between Zn²+ and S²⁻ and low coordination number (tetrahedral). Therefore, AgBr is the correct answer.