**2.1.4 Association of point defects with charges**

Anionic vacancies VO •• (or cationic vacancies ''' V ) can be associated with electrons (or holes) Al to form F centers (or V centers). In fact, upon trapping one electron (or two electrons), VO •• becomes a F+ center (or a F center). Anionic vacancies can also be associated with a substitutional divalent cation ' M to form F Al cation center (such as FMg or FCa). The F, F+ and Fcation act as a donor centers. The energy levels of F and F+ are respectively estimated around 3 and 3.8 eV below the edge of the conduction band (Kröger, 1984).
