19 Colour Centres & Polymorphism

Prof. P. N. Kotru

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19.1 Colour centres

 

In the previous section we have learnt about certain type of point defects and that the presences of either Schottky defects or anti-Schottky defects bring about a change in the density of the crystalline solid. Very often this change is small because the actual number of such defects present is always very small in comparison with the total number of atoms or ions present in the crystal. However, we come across situations which are quite interesting. A crystal of sodium chloride is ordinarily transparent and uncoloured, but it is well known that if this crystal is heated in the presence of sodium vapour the crystal turns yellow. What has brought about this change in sodium chloride crystal? The explanation is that by heating sodium chloride in sodium vapour produces defects by creating vacancies at the sites of chlorine (Cl ─ )ions. Since, a sodium atom is introduced into the structure which means that a sodium ion and an electron are produced; the ion gets adjusted at any of the proper sites for a sodium ion in the lattice whereas the electron is relatively free which wanders around in the lattice. The removal of a negative ion from a particular site means that in the region around that vacant lattice site, there is an excess of positive charge around that site. There is a possibility for the wandering free electron to be attracted to this net positive charge and get trapped in the vacancy created by the chlorine ion which left the crystal. In this way, we have an electron which is moving in a region of positive charge. The situation may be compared with an electron in a hydrogen atom which moves in the field of a positive charge (the proton) and could be, in very approximate way, described as being similar to the Bohr theory of hydrogen atom. According to this theory the electron will only be allowed to move in certain orbits in which does not radiate energy while other orbits are for bidden orbits. This situation is shown in figure 19.1

 

 

Figure 19.1

 

There is a probability for the electron to jump from one orbit to another and in this process it may either absorb or emit a quantum or photon of energy. The frequency ν of the radiation which is emitted or absorbed is given by the relation:

 

E 1  ─  E 2    = h ν    ,

 

Where E1 and E2 are the energies of the electron in the two orbits.

 

It so happens that for the alkali halide crystals the frequency of the radiation is involved in the visible part of the spectrum. Sodium chloride crystals which are heated in excess of sodium vapour become coloured. The vacancy created at the site of Cl ion alongwith the trapped electron is called a colour centre.

 

Ordinarily, a crystal is transparent to the visible range of spectrum. So, a sodium chloride crystal should not absorb visible light and is expected to be perfectly transparent. Crystals which are coloured contain defects known as colour centres. Defects in the lattice which absorb light are called colour centres.

 

Colouring of crystals can be achieved by different ways as described below:

 

(i) By addition of suitable impurities like transition element ions with excited energy levels.

(ii) By adding stoichiometric excess of cations by heating the crystal in the alkali metal vapour followed by

quick cooling. The colour of the crystal would depend on its nature. For example, KCl when heated in excess

of potassium vapour becomes blue; NaCl when heated in excess of sodium vapour becomes yellow and LiF

when heated in excess of Li vapour turns pink.

(iii) Crystals can also be made coloured by exposing them to high energy radiation like X-rays, γ rays or

irradiating them with energetic neutrons or electrons.

 

There are several different kinds of colour centres and the simplest one is called as F centre. F centre is a defect in crystals where an electron is trapped at a negative ion vacancy. Its name has come from the German word “Farbe” which means colour and F centres are also named as “Farbzentren”. The se centres are generally generated by heating a crystal in an excess of alkali vapour or by irradiating the crystal by x-rays. Sodium chloride having F-centres shows main absorption band at about 4650 Å and is known as F-band. Figure 19.2 shows optical absorption plotted against wavelength of some alkali halide crystals. The F-bands associated with the F-centres of some alkali halides are shown in figure 19.2

 

Figure 19.2: Optical absorption Vs. wavelength in case of some Alkali halide crystals

 

 

When point defects in a crystal trap electrons colour centres are formed with the resultant electronic energy levels spaced at optical frequencies. The trapped electron has a ground state energy which is determined by the surroundings of the vacancy. These energy levels lie in the forbidden energy gap and relatively widely spaced levels change to an almost continuous set of levels which fall just below the bottom of the conduction band. Should the crystal be exposed to white light, the energy excites the trapped electron to a higher energy level, it is absorbed in the process and a characteristic absorption peak corresponding to near visible region appears in the absorption spectrum of the crystal with F-centres. The peak does not change when an excess of another metal is introduced in the crystal if the foreign atoms get substituted for the metal atoms of the host crystal. Therefore, making an assumption that the absorption peak is as a result of transitions to excited states close to the conduction band determined by the trapped electron is justified. Energy level diagram for an F-centre as shown in figure 19.3 clearly shows that F absorption band is created due to a transition from the ground state to the first excited state below the conduction band.

Figure 19.3: Energy level diagram for an F-centre

 

 

19.2   Colouring of crystals by addition.

 

It has already been explained that the intrinsic concentration of defects in the alkali halide crystals can be upset by getting them in contact with alkali metal. This effect can also be produced in the crystals by getting them in contact with halogen. The latter will be discussed further in the text. In several materials significant effects on ionic and electronic conductivity and optical properties can be produced in this way.

 

Taking the example of KCl, it can be additively coloured by sealing in an evacuated glass tube with potassium metal and then heating in a furnace. Figure 19.4 shows the experimental set-up for additively colouring of KCl crystal. It consists of a double furnace and evacuated glass tube. KCl crystal and potassium metal are placed in a sealed evacuated glass tube. The temperature T1 is around 500⁰C whereas temperature T2 is maintained at some appropriate value as may be required for the desired concentration of defects. In this way an alkali halide crystal such as KCl is heated in an atmosphere of potassium vapour. The temperatures T1 and T2 are maintained for a few hours and then the tube is removed. The crystal is then quenched in an oil bath or preferably on a copper block. It results into colouring of the crystal due to creation of F centres associated with non-stoichiometry.

 

The density of F- centres depends upon the density of potassium ions in the vapour and, therefore, the temperature T2. In fact, it is the temperature T2 which controls the vapour pressure of potassium and hence the concentration of colour centres.

 

Figure 19.6: Spectrum showing the F band and other absorption peaks in an additively coloured KBr
crystal recorded at 10°K

An additively coloured crystal at room temperature when exposed to F band light, some new bands are observed which lie on the long wavelength side of the F-band. Such wavelength bands are a consequence of aggregates of the F-band and can also be produced by holding the coloured crystal at an elevated temperature of say 100⁰C for a few hours or also by exposing the crystal to x-rays. Most prominent of these long wavelength bands are known as M bands. Still higher aggregates of the F centre are called as R and N centres. Figure 19.7 is an example of M, N, R1 and R2 bands when KCl is irradiated in F-band at room temperature. The curves marked II and III are the curves obtained as a result of successive stages of irradiation. The curve marked (I) shows optical density after heating the crystal to 500⁰C and then quenching it to room temperature. M, N and R bands are not formed if the crystal is quenched and then illuminated at low temperature. It means that the additively coloured crystals are stable against the creation of aggregate centres at temperatures of below 200⁰K.

A broad new band known as F/ band is observed when an additively coloured crystal of KBr is illuminated with F light at around 123°K as shown in figure 19.8

Let us know what these different absorption bands observed in alkali halide crystals are. It is summarized as follows:

 

  1. An F-centre is an electron trapped at a negative ion vacancy.
  2. L 1, L 2, L3 are very weak absorption bands and are thought to be higher excited states of the F centre. β band is closely related to f centres. Its formation is attributed to excitons produced by absorption of a photon near about the F-centre.
  3. M-centre is a pair of neighbouring negative ion vacancies in addition to two electrons. So, it is a combination of two F centres.
  4. R-centre is due to three halogen ion vacancies in addition to three electrons. R- and N- centres are higher aggregates of the F-centre.
  5. F/ band is due to single negative ion vacancy in which two electrons are trapped.
  6. α-band is due to exciton absorption in the immediate vicinity of the isolated vacancies.
  7. Heating an additively coloured crystal in hydrogen gas or by growing a crystal with an addition of an alkali hydride results into substitution of negative hydrogen ions (H─ ions) in place of halogen ions. A new absorption band known as U–band in the ultraviolet region at 5.5 eV is shown by KBr.

 

The energy level scheme for a KCl crystal is shown in figure 19.9

19.3 The V – Centres

 

If an alkali halide crystal is heated in halogen vapour, a stoichiometric excess of halogen ions is introduced in it. We have already known that heating an alkali halide crystal in metal vapour results into a series of colour centres. In the same way, we expect a new series of colour centres as a result of heating the crystal in a halogen vapour. These new centres, known as V-centres, have holes in place of electrons. V-bands can also be produced by exposing alkali halide crystals to intense penetrating x-rays. The absorption bands produced in KBr crystals at 80⁰ K after irradiating them to intense penetrating x-rays are shown in figure 19.10.The familiar F, F/ and α-bands are seen, but in addition there are other centres giving rise to V1, V2 and V3 bands which occur in the ultraviolet region.

Figure 19.10: Absorption bands in KBr crystals at 80⁰ K after irradiating them with x-rays

 

19.4 Colour Centre Models.

 

The models of various colour centres as described in the previous section are shown here through
schematic representation in figure 19.11

Figure 19.11: Model for various colour centres

 

F Centre   – Electron trapped at a negative ion vacancy.

F/ Centre – F centre combined with one electron.

R1 Centre – F centre combined with a vacant anion site or one electron bound to two anion Vacancies.

R2 Centre – Two F centres combined or two electrons associated with two anion vacancies.

M centre – F centre combined with a pair of vacancies of opposite sign or an electron bound to two anion and one cation vacancies.

 

19.5     Model of V- centres

 

The models of various other colour centres as described in the previous section are shown here through schematic representation in figure 19.12.

 

 

V1-Centre  : Antimorph of the F- centre. It is hole trapped at a positive ion vacancy.

 

V2  & V3 Centres : Antimorph of R-centre; V2 is due to two holes trapped by two positive ion vacancies and

 

V3  is due to one hole trapped at a pair of positive ion vacancies.

 

V4- Centre: Antimorph of M-centre

Figure 19.12: Illustration of model for V-centres

 

19.6     Polymorphism

 

In several cases it so happens that the same chemical compound manifests itself in two or more than two different structures and which exhibit divergently different properties and characteristics. It only establishes the fact that physical properties of a crystal do not depend on only upon the atoms or molecules which constitutes it but on how they are arranged in the crystal structure. Crystals manifest structure sensitive properties, be it mechanical, optical, electronic or any other. Polymorphism is a term used for a phenomenon in which a chemical compound crystallizes in more than one form genetically distinct. A chemical compound may crystallize in two different forms, it is then said to be dimorphous and if in three it is called as trimorphous and so on. In general, it is said to be polymorphous and the phenomenon is called polymorphism.

 

There are several examples of such compounds which exhibit dimorphous. Carbon atoms arranged in one particular type of structure makes it diamond whereas when they are arranged in a different structure, it becomes graphite. The two are different structures which display different properties, though both have carbon as constituent atoms. Similarly, calcium carbonate i.e., CaCO3, manifests itself in two forms  such as calcite and as aragonite. Calcite falls under the rhombohedral class of the hexagonal system whereas aragonite appears as orthorhombic crystals. The optical characteristics of calcite and aragonite are entirely different. The specific gravity of calcite is 2.7 which is lower as compared to that of aragonite; the specific gravity of aragonite being 2.9. The divergently different optical and mechanical properties of diamond and graphite are well known. Titanium dioxide (TiO2) is trimorphous. It appears as a tetragonal structure with c= 0.6442 and known as “rutile”. Its second form is tetragonal with c= 1.778 which is known as “octahedrite”.The third form is orthorhombic and is known as “brookite”. The manifestation of titanium dioxide in these three different structures is reflected by their different properties as well.

 

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References.

 

1.Brown, F.C. “The Physics of Solids”, W.A. Benjamin,Inc.,N.Y.,1967.

2. Kittel, C.: “ Introduction to Solid State Physics,”Wiley Eastern Limited(1954)

3.Dana,E.S. “ A text Book of Mineralogy”, revised by W.E.Ford,Asia Publishing House, New Delhi. 1962.

 

Suggested Reading .

  1. Schulman, J.H., Compton ,W.D. “ Colour Centres in Solids”, Pergamon Press, N.Y., 1962.
  2. Fowler, W.B. “ Physics of Colour Centres”, Academic Press, N.Y., 1967.
  3. Markham,J. “ F-centres in Alkali Halides