Ionic solids, which have anionic vacancies due to metal excess defect develop colour. Explain with the help of a suitable example.

okyee we can understand by the example on NaCl crystal , when NaCl crystal heated in th presense of sodium vapours , the +ve sodium ions are deposited at the surface of lattice due to this , some of the Cl -ve ions come out of the lattice and thereby leaving a vacency, this vacancy is occupied by the electron( which is ejeculated by Na +ION ON SURFACE OF CRYSTAL ) then this position is called Farbe centre or F- centre (electron trapped in anion vacanvy) greater the number of F centre greater will be the intensity of color.