What are F-centers in crystallography ? How are they created and what consequences do they have on the overall appearance of crystal ?
In crystallography, F-centers, also known as color centers, are lattice defects in which a cation vacancy in the crystal structure is occupied by a free electron. This creates a negatively charged electron trapped in a vacancy, which imparts a color to the crystal due to its ability to absorb and transmit light in the visible range.

F-centers are typically created by exposure of the crystal to ionizing radiation, such as X-rays, gamma rays, or UV light, which causes cation vacancies and free electrons to be generated within the crystal lattice. Alternatively, F-centers can be introduced intentionally through chemical doping of the crystal with impurities, such as metals or metalloids.

The presence of F-centers can significantly affect the overall appearance of the crystal, as they introduce absorption bands in the visible spectrum, giving the crystal a characteristic color. The color of the crystal can range from pale yellow to deep blue or green, depending on the concentration and type of defects present. In addition to their effect on color, F-centers can also impact other physical properties of the crystal, such as its electrical conductivity, thermal stability, and mechanical strength.