Types of Colour Centres

TYPES OF COLOUR CENTRES

There are different types of colour centres viz., F-centre, F '-centre, R₁-centre, R₂-centre, M-centre, V-centre etc.

F CENTRES

F-centre: Trapping of an electron at a negative ion vacancy is called F-centre.

Explanation: When a NaCl crystal is heated in an atmosphere of sodium vapour, then the NaCl crystal will absorb the sodium atoms from the sodium vapour. Each of this excess sodium atoms absorbed from the sodium vapour may then ionise into a positive sodium ion and an electron.

i.e., Na→ Na⁺ + e^-

Thus a number of Na⁺ ions are created and hence an equal number of negative ion vacancies will also be created to keep the crystal neutral as a whole. Since the negative ion vacancies represent a net positive charge within the crystal, they attract the electrons (produced due to ionisation) and captures it.

This trapped electron will be shared by all + the positive (Na⁺) ions nearby the vacant site as shown in Fig. 4.4.

Properties of F-Centre

(i) Due to excess metals added to the crystal, equal number of positive ions and negative ion vacancies are created. Therefore, the density of the crystal will decrease (due to creation of vacancies). i.e., coloured crystals are less denser than uncoloured crystals.

(ii) Width of F-band (Energy levels of electrons below the conduction band energy levels) increases with the increase in temperature.

(iii) F-absorbtion band is independent of the metal ions added to it.

(iv) In STOICHIOMETRIC crystals the bands are formed due to irradiation of ultraviolet radiations, X-rays etc.

F '-centre: An F centre with two electrons trapped in it is called F '-centre.

Explanation: When the temperature of the F-centre is decreased, then the F-centre will be destroyed and the electron from one F-centre will be captured by another F-centre to form a new colour centre called F ' centre (Fig. 4.5)

R1, R2 AND M CENTRES

When the crystals containing F-centres are irradiated with light, under a suitable temperature range, coagulation will occur. Due to coagulation, various colour centres viz., R1, R2 and M centres are produced as shown in Fig. 4.6.

R1 Centre: It is the colour centre formed due to the combination of an F-centre and a negative vacant site as shown in Fig. 4.6.

R2 Centre: It is the colour centre formed due to the combination of two F centres as shown in Fig. 4.6.

M Centre: It is the colour centre formed due to the combination of an F-centre with positive and negative vacant sites as shown in Fig. 4.6.

V-CENTRES

V-centre: Trapping of a hole at a positive ion vacancy is called V-centre.

Explanation: When alkali halides such as KBr, is heated in Br2 vapour, then the excess of bromine atoms produce positive holes. These holes are trapped by the positive ion vacancies to produce different colour centres viz., V-centre, V₁-centre, V₂-centre etc as shown in Fig. 4.7.

V1 centre: The combination of two V-centres give rise to a colour centre called V₁ centre as shown in Fig. 4.7.

V2 centre: The combination of a V-centre and a positive vacant site is called V₂ centre and is as shown in Fig. 4.7.