Environmental Engineering Reference
In-Depth Information
type of disorder involving cations in a compound (MX) is shown in Fig. 5.9b.
The most common examples of a system exhibiting such a disorder on metal
sublattice include AgBr and AgCl.
As a rough rule, Schottky disorder is energetically favored in stoichiometric
compounds where the cations and anions are of comparable sizes, whereas Fren-
kel disorder predominates in systems where the sizes of the cations and anions
are appreciably different.
To achieve nonstoichiometry and maintain charge balance in an ionic crystal,
secondary defects also must exist. Whether the defects will remain free and ran-
domly distributed or in association will depend on the concentration of the defects
and temperature. Usually at low concentration and high temperature, the defects
will be in free form or unassociated. At low temperatures with high concentra-
tions of point imperfections, the defects are expected to be in associated form
leading to formation of defect complexes or clusters.
5.4.4 Defects in Nonstoichiometric Compounds
Nonstoichiometric compounds are of two main types: (1) oxidant-deficient or
metal excess with respect to stoichiometric composition and (2) metal-deficient
or oxidant excess with respect to stoichiometric composition. The electrical neu-
trality of nonstoichiometric compounds is conserved through the formation of
point defects and complementary electronic defects. The subsequent discussion
relates to compounds formed in metal-oxygen systems.
Oxygen-Deficient Oxides (
n
-Type Semiconductors)
This could be due to presence of vacant sites in the oxygen sublattices as shown
in Fig. 5.10a. Typical oxides in this group ZrO
2
, SiO
2
, TiO
2
,Ta
2
O
5
,Nb
2
O
5
,
U
3
O
8
,V
2
O
5
, etc.
It could also be due to metal excess at interstitial sites as shown in Fig. 5.10b.
Examples of this group are ZnO, CdO, BeO, Al
2
O
3
, etc.
It may be noted that the formation of both oxygen vacancies and interstitial
cations leads to the formation of complementary quasi-free electrons. In oxygen-
deficient oxides, therefore, the electronic conductivity is due to the transport of
electrons in the conduction band. Such oxides are termed n-type semiconductors
(negative charge carrier).
Metal-Deficient Oxides (p-Type Semiconductors)
When a nonstoichiometric oxide is metal deficient, the predominant defects may
be either metal vacancies or interstitial oxygen atoms.
Figure 5.11 shows a defect structure involving predominantly metal vacancies
(M
1
δ
O). The formation of charged metal ion vacancies leads to the formation
of complementary electronic defects (positive holes in the valence band). In such
