How does a vacancy change the electrical and optical properties of a crystal?

In crystallography, a vacancy—the absence of an atom or molecule from its normal lattice position—significantly alters a crystal's electrical and optical properties.1

Electrical Properties

In semiconductors, the movement of electrons can create an electron vacancy within a covalent bond. This vacancy, known as a hole, acts as a mobile, positively charged particle. When an electric field is applied, these holes move through the crystal in a direction opposite to electrons, contributing to electrical conductivity. By intentionally introducing specific impurities—a process called doping—scientists can create p-type semiconductors where holes serve as the primary charge carriers.23

Optical Properties

Vacancies can also function as colour centres, which influence how a solid interacts with light. For example, an F-centre occurs when a vacancy in an ionic solid traps an electron. This trapped electron can absorb specific wavelengths of light, such as ultraviolet or visible radiation, thereby lending a characteristic colour to the crystal that it would not otherwise possess.14