Question

What are F-centers in an ionic crystal?

Answer 1

The word F-centre is derived from the Greek word Farbzenter which means colored center. F-centre is a type of metal excess defect, in which an anionic vacancy is filled by unpaired electrons.

Complete answer:
The arrangement of ions bound together through an electrostatic force in a crystal is known as an ionic crystal. Sometimes these specific patterns of crystal are interrupted and have few defects in it. F-center is a type of ionic crystal defect known as a metal excess defect in which the number of cations in the crystal is more than anions.
In F-centre, the crystal is subjected to the heat of ionizing radiation with the alkali metal, which leads to ionization of the crystal where an anion from the crystal is delocalized and moves out of the crystal leaving behind an anion vacancy. This vacant site is then occupied by the unpaired electrons which further are capable of absorbing light from the visible spectrum. This gives color to the F-centres in an ionic crystal.
Example can be a NaCl crystal, when NaCl crystal is heated with alkali metal that is Na, ionization takes place which forms $\text{N}{{\text{a}}^{\text{+}}}$ ions and $\text{C}{{\text{l}}^{\text{-}}}$ ions arranged in the crystal. Due to heating, the $\text{C}{{\text{l}}^{\text{-}}}$ ions from the crystal moves out to bind with Na from the alkali surrounding leading to anion vacancy in the crystal. This crystal is then occupied by an unpaired electron from Na (Na readily gives away electron).
This is known as F-centers in ionic crystals.

Note:
As the electron occupies anion vacancy it leads to a change in color of an ionic crystal. When sodium chloride (NaCl) crystal is heated it shows yellow color, potassium chloride (KCl) crystal shows violet color, and lithium chloride (LiCl) shows pink color on heating.