Problem 18 - Entrance Test

Schottky defects are typically observed in ionic compounds and are characterized by an equal number of missing cations and anions from their lattice sites, maintaining the electrical neutrality of the crystal. The conditions favoring Schottky defects are: - **Small difference in the size of cation and anion**: When cations and anions are of similar size, it is equally probable for both to leave their lattice sites, forming vacancies. If there were a large size difference, the smaller ion would more easily fit into an interstitial void (favoring Frenkel defects). - **High coordination number**: In crystals with a high coordination number, ions are tightly packed, and there are fewer interstitial spaces large enough to accommodate a displaced ion without significant lattice distortion. This makes Frenkel defects less likely, pushing towards Schottky defects. - **High temperature**: Defects are thermodynamic defects, and their formation is an endothermic process. Higher temperatures increase the thermal energy, making it more favorable for ions to leave their lattice sites. - **Absence of impurities**: Impurities introduce their own set of defects (e.g., substitutional defects), which might compete with or influence the formation of Schottky defects, but do not directly 'favor' Schottky defect formation as a primary condition. - **Low temperature and high pressure**: Low temperature would *disfavor* defect formation. High pressure would generally tend to reduce defects by compressing the lattice, making vacancy formation less favorable. Therefore, the condition that favors Schottky defect is a small difference in the size of cation and anion, combined with a high coordination number.