Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
The shade develops due to the presence of electrons withinside the anionic sites. These electrons take in power from the seen a part of radiation and get excited. For example, while crystals of NaCl are heated in an environment of sodium vapours, the sodium atoms get deposited at the floor of the crystal and the chloride ions from the crystal diffuse to the floor to shape NaCl with the deposited Na atoms. During this process, the Na atoms at the floor lose electrons to shape Na ions and thereleased electrons diffuse into the crystal to occupy the vacant anionic sites. These electrons get excited by soaking up power from the seen mild and impart yellow color to the crystals.