Problem 12 - Entrance Test

F-centers are a type of point defect, specifically a metal excess defect, found in ionic crystals. Let's analyze each statement: - **A. They are anionic vacancies.** This is correct. An F-center (from the German 'Farbe', meaning color) is an electron trapped in an anionic vacancy. This vacancy is created when an anion leaves its lattice site, typically to balance excess metal ions on the surface or in interstitial positions. - **B. They are responsible for the color of alkali halide crystals.** This is correct. The trapped electron in the anionic vacancy can absorb energy from visible light, get excited to higher energy levels, and then emit light of a complementary color, thus imparting color to the otherwise colorless alkali halide crystals (e.g., NaCl becoming yellow, KCl violet, LiCl pink when heated in metal vapor). - **C. They are formed due to metal excess defects.** This is correct. F-centers are a consequence of metal excess defects, where the crystal has an excess of metal ions. This excess can be due to anionic vacancies (creating F-centers) or extra cations in interstitial sites. - **D. They result in an increase in the density of the crystal.** This is incorrect. F-centers are formed when an anion leaves its site. Even though an electron occupies the vacancy, the mass of the missing anion is significant. When anions leave, the overall mass of the crystal decreases for a given volume, leading to a *decrease* in density, not an increase. Alternatively, if the metal excess is due to interstitial cations, there would be an increase in density, but F-centers specifically relate to anionic vacancies and thus a decrease in density.