Frenkel defects and quadropolar deformation
The major defect in silver halides is the Frenkel defect, where silver ions are located interstitially (Agi+) in high concentration with their corresponding negatively charged silver ion vacancies (Agv−). What is unique about AgBr Frenkel pairs is that the interstitial Agi+ are exceptionally mobile, and that its concentration in the layer below the grain surface (called the space charge layer) far exceeds that of the intrinsic bulk.[3][9] The formation energy of the Frenkel pair is low at 1.16 eV, and the migration activation energy is unusually low at 0.05 eV (compare to NaCl: 2.18 eV for the formation of a Schottky pair and 0.75 eV for cationic migration). These low energies result in large defect concentrations, which can reach near 1% near the melting point.[9]