Color Center

In crystallography, we have seen that there are different kinds of point defects that cause disturbance in the crystal structure. The major point defects were either a Schottky defect or any anti-Schottky defect and the presence of these two causes the change in density of the crystalline solids. These defects will ultimately result in the change in the composition of the crystal and some of these defects will change the colour of the crystal and such defects are known as the Color Center or Color Centers. Basically, the Color Center is the lattice defect that absorbs visible light. 

In this article, we will understand what are Color Centers, types of Color Centers such as the f centre defect, etc…

F-Centre Defect

We know that according to the definition of Color Center it is a kind of defect due to which the composition of the crystal changes in such a way that the original colour of the crystal. To be precise, the Color Center is the defect in the regular spacing of atoms within a solid that absorbs visible light of a particular wavelength thus leading a characteristic colour to the solid. Each Color Center is always associated with the absence of an atom from the place it would normally occupy in the crystal and the radiation of an excited state with such an empty place or a vacancy. 

The Color Centers can generally be found in alkali halides, for example, KCl, NaCl, etc… Thus the Color Centers occur mainly in the Schottky defects and metal excess defects. On heating alkali halides in their vapours, we will notice the change in colour, generally, these solids are colourless in the visible range of the spectrum. 

Now, What is the F-Centre Defect?

• The f centre defect is an abbreviation of Fermenter, a Greek word Fabrezenter refers to the combination of two words Fabre means colour and the zenter means centre together we call it as the Color Center. 

• The f centre defect is one of the most important and widely studied point defects in crystallography. The f centre defect is a kind of metal excess defect, where the number of cations will be more than the number of anions. 

• The f centre defect is defined as it is a type of crystallographic defect in which an anion vacancy in the crystal lattice is occupied by another pair of electrons. 

• Thus, the electrons trapped in anion vacancies are known as the f centre defects because that will impact the composition of the crystal and finally change its colour.  Crystals like NaCl, KCl, LiCl execute the F centre defect when they are heated with their respective vapour lamps.

NaCl Colour

The NaCl (Sodium Chloride) crystal is naturally colourless or transparent, but when it is exposed and heated in the presence of a sodium vapour lamp the crystal turns yellow. This happens because when the sodium chloride is heated in the presence of sodium vapour results in defects that causes vacancies at the site of Cl -ions in the crystal lattice and hence an electron gets trapped in the empty lattice site. Thus a sodium ion and an electron pair are produced. Thus the NaCl colour on heating changes to yellow colour is an evident result of F Color Center. 

When a NaCl crystal is heated in the presence of sodium vapour, we notice that the chlorine ion gets in contact with sodium vapour and produces sodium chloride by losing an electron. 

⇒ Na_vapour + Cl^- ➝ NaCl^- (crystal) + e^- (free electron)

Now, since one of the chloride ions is combined with the sodium vapour, thus in NaCl crystal it causes an empty site and this vacant place will trap the free electron. Now on heating this crystal structure, the free electron will get excited and it will radiate the same amount of energy that is absorbed to excite. Thus, it will radiate or emit yellow radiation thus Nacl colour turns yellow on heating. 

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Removal of an ion from a particular site means that in the region around that vacant site we have an excess number of positive charges around that region. We know that electrons will be wandering around the lattice site, due to the creation of the vacant site nearby excess positive charges, there is a possibility that a wandering electron may get trapped in the vacant site which further will cause a change in the composition of the crystal structure. Thus the colour of NaCl or NaCl colour due to F centre defect is usually yellow.

Different alkali halides will have different colours, depending on their composition and absorption of wavelength from the visible spectrum, for example, lithium chloride turns pink, potassium chloride turns violet colour on heating. 

Did You Know?

• Frequently, Color Centers with a sufficiently high density result in substantial absorption of light at optical wavelengths where there would normally be no possible absorption since the photon energy is almost below the bandgap energy. 

• The effect of the defects can be described as introducing additional defect levels in the energy level diagram, which lie between the conduction and balance band so that lower-energy photons are allowed to participate in absorption processes. 

• As a result of the additional absorption, some naturally completely transparent and colourless crystals exhibit pronounced colours due to some density of Color Centers.

• It is also possible that these crystal defects allow the emission of light at absolutely new wavelengths, like spontaneous emission and also stimulated emission. The latter is exploited in Color Center lasers.