A defect in a crystal that changes the way in which it absorbs light or other electromagnetic radiation. Impurities in the crystal affect the bind structure and allow transitions in different regions of the spectrums. Impurity colour centres are responsible for the characteristic colours of many gemstones. A particular type of colour centre is an F-centre. This is a missing negative ion in an ionic crystal, where the overall charge neutrality occurs by trapping an electron in the vacancy. The electron has energy levels similar to those of a particle in a box. F-centres can be produced by chemical activity or by irradiation.
Subjects: Chemistry — Physics.