What is meant by a non-stoichiometric defect? Ionic solids which have anionic vacancies due to metal excess defect develop colour. Explain with the help of suitable examples.
Non-stoichiometric flaws are those imperfections that do not affect the crystalline substance's stoichiometry. The anionic sites contain electrons, which cause the colour to develop. As a result of absorbing energy from visible light, these electrons get excited. Crystals of NaCl, for example, produce NaCl when they are heated in an environment of sodium vapours. It is during this phase that the Na atoms on the surface lose their electrons and form Na+Na+ ions, while the liberated electrons diffuse inside the crystal to fill the unoccupied anionic sites. They are stimulated when they absorb energy from visible light and give the crystals a yellow colour.
