Colour centres are point defects or defect clusters in crystal lattices that cause the material to change color. They occur when electrons or holes become trapped at defect sites. Common examples are the F-centre in alkali halides, which forms when an electron is trapped at a halide ion vacancy, and the H-centre and V-centre in alkali halides, which involve trapped holes. Defect clusters can also form through the interaction of multiple point defects, such as pairs or groups of F-centres. The defects cause color changes by absorbing visible light and exciting trapped electrons or holes to higher energy states.

1. “Colour Centers In
Alkali Halide Crystals”
BY
RONY REGI
ROLL NO : 16
MSc. CHEMISTRY
SNC KOLLAM

2. COLOUR CENTRES
 In crystals due to charge compensationa set of defect is
created which change the colour of the crystal. These defects
which change the colour of the crystal is known as colour
centres.
 These are point defects or point defect clusters associated
with trapped electrons or hole normally in transparent
materials.
 The centres cause the solid to become coloured when the
electronic ground state of the defect is excited to higher
energy states by the adsorptionof visiblelight.
 Colour centres can occur naturally in compounds(metallic
oxides) because when heated to higher temperature the ions
become excited and are displaced from their normal
crystallographicpositions, leaving behind some electrons in
vacant spaces.

3. THE F-CENTRE

4. METHOD OF PREPARATION
 F-centres can be prepared by heating an alkali
halide in vapour of an alkali metal. NaCl heated in
Na vapour becomes slightly non-stochiometric due
to the uptake of Na to give , which has a
greenish yellow colour.
 Another means of producing F- centres is by
irradiation. Using one of the normal methods of
recording an X-ray diffraction pattern. Here
powdered NaCl turns into greenish yellow colour.
After bombardment with X-rays. The cause of the
colour is again trapped electrons, but in this case
they cannot arise from a non stochiometric excess
of Na. They arises from ionization of some C𝑙−
ions.

5. TYPES OF COLOUR CENTRES
Many other colour centres have been characterized in alkali halide
crystals : two of these , the H-centre and V-centre.
 V-centres are unusual point defects as they involve no missing or extra
atoms. A hole is introduced into a pristine lattice, and the charge induces
the formation of a bond between two nearest neighbour halide ions,
producing an open-shell dihalide species.
 In contrast to the V-centre, the H-centre is where hole-capture and dimer
formation involves excess halide (for example, an interstitial) interacting
with a lattice site.

6.  In the V-centre, two lattice iodides form a dimer, while in the
H-centre the dimer is formed from a lattice iodide and
interstitial iodide.
 It has been shown that H-centres can be thermally converted
into V-centres, and the two defects have similar but
distinguishable optical and electronic signatures.

7.  The F’-centre, which is two electrons trapped
on an anion vacancy;
 The 𝐹𝐴 centre, which is an F-centre, one of
whose six cationic neighbours is a foreign
monovalent cation, e.g. K+ in NaCl;
 The M-centre, which is a pair of nearest
neighbour F-centres;
 The R-centre, which is three nearest neighbour
F-centres located on a (111) plane;
 Ionised or charged cluster centres, such as 𝑀+
𝑅+
and 𝑅−
.
Other defect centres which have been
identified in the alkali halides include:

8. DEFECT CLUSTERS
 As defects are studied in more detail, using high-resolution electron
microscopy and other techniques, it is clear that the apparently
simple point defects such as vacancies and interstitials are often
more complex; instead, larger defect clusters tend to form.
 The presence of vacancies in both metals and ionic crystals causes
a relaxation of the structure in the immediate environment of the
vacancy. In metals, the atoms surrounding the vacancy relax
inwards by a few percent, i.e. the vacancy becomes smaller,
whereas in ionic crystals the reverse occurs and as a result of an
imbalance in electrostatic forces the atoms relax outwards.
 Vacancies in ionic crystals are charged and, therefore, vacancies of
opposite charge can attract each other to form clusters. The
smallest cluster is either an anion vacancy/cation vacancy pair or
an aliovalent impurity (e.g.Cd2+)/cation vacancy pair. These pairs
are dipolar, although overall electrically neutral, and so can attract
other pairs to form larger clusters.

9. WUSITE :
 One of the most studied and best understood defect systems is
wustite, Fe1−xO: 0 ≤ x ≤ 0.1. Stoichiometric FeO has the rock salt
structure with Fe2+ ions on octahedral sites. Density measurements
show that nonstoichiometric Fe1−xO contains a deficiency of iron
rather than an excess of oxygen, relative to stoichiometric FeO.
 Using point defect considerations one would anticipate that non-
stoichiometric Fe1−xO would have the structural formula
Fe2+1−3xFe3+ 2x VxO in which Fe2+, Fe3+ and cation vacancies
were distributed at random over the octahedral sites in the ccp oxide
ion array.
 The defect structure is different, however, since neutron and X-ray
diffraction studies have shown that Fe3+ ions are in tetrahedral sites
and that defect clusters appear to form. This form of cluster
formation is called Koch Cluster.

10. REFERENCES:
 Solid State Chemistry and its Applications
Second Edition ANTHONY R. WEST
 H-Centre and V-Centre Defects in Hybrid
Halide Perovskites published by Lucy Whalley,
Rachel Crespo Otero, and Aron Walsh
 Internet