F-center

A type of point defect in crystals where an anion (negative ion) vacancy in the lattice traps one or more electrons, creating a localized site that absorbs specific wavelengths of light, thereby coloring the typically transparent material (e.g. the yellowing of NaCl due to trapped electrons in missing chloride sites); defects that form due to irradiation (e.g. X-rays), heat, or alkali metal vapor treatment and can be removed by annealing.