what is metal excess defects

The answer is little lengthy but it explains it all.

It arises due to the anion vacancy in a Crystal or extra metal occupying lattice sites or interstitial sites in the crystal. Anion vacancy: when alkali metal halide crystals are heated in the atmosphere of that alkali metal vapours the excess of alkali metal atoms are deposited on the suface of the crystal and then the halide ions diffuse to the surface of the crystal and combine with the alkali metal atom and inturn leave its valance electron in its lattice site, which is then responsible for imparting colours by the crystal. Since the anion leave the Crystal, there is deficiency of anion and excess of cation however the crystal remains electrically neutral. Excess cation occupying interstitial sites: the non- metal atoms leave the sites leaving the electrons behind in the interstitial sites and the metal atoms bind to it remains in the lattice but in the interstitial sites making the crystal rich in cations . However the crystal remains electrically neutral.