An electron trapped at a negative ion vacancy in an ionic crystal. F-centres are formed by the release of electrons by irridiation with X-rays or by producing stoichiometric excess of anions in the crystal. F^1 -centres consist of F-centres with a further electron trapped in the same vacancy. FA -centres are F-centres modified by one neighbouring cation being different from the cations of the lattice. F-centre aggregates can be formed from arrays of the nearest neighbouring centres. F-centres give rise to broad optical absorbtion bands.