Environmental Engineering Reference
In-Depth Information
Furthermore, due to a predominance of Frenkel-type disorder, pure AgBr is
always in equilibrium with some cation vacancies and cation interstitials. For
example:
AgBr s Ag
i
V
′
Ag
Br
x
Br
(6.15)
and also
[
V
′
Ag
][Ag
i
]
K
F
(6.16)
where
K
F
is the equilibrium constant for ionized Frenkel defect formation at a
constant temperature, and [
V
′
Ag
] and [Ag
i
] represent the concentration of ionized
cation vacancies and cation interstitials per m
3
of AgBr.
When AgBr is doped with CdBr
2
, the following defect formation reactions
are expected:
(a) CdBr
2
s Cd
•
Ag
V
′
Ag
2Br
Br
(6.17)
or
CdBr
2
Ag
i
s Cd
•
Ag
Ag
x
Ag
2Br
x
Br
(6.18)
and
1
2
Br
2
(v)
(b) CdBr
2
h
•
s Cd
•
Ag
Br
Br
(6.19)
According to Eq. (6.14), an increase in [
V
′
Ag
] must decrease
p
, i.e., with in-
crease of
σ
h
•
should decrease. As AgBr is predominantly an ionic lattice,
positive hole transport through AgBr is the rate-limiting step for subsequent
growth of the halide film. Thus, the rate of bromination decreases for Cd-doped
Ag compared with that for undoped Ag. This model has been verified for Ag-
halogen systems [7,8], and the kinetic data as reported by Hauffe [5] are presented
in Fig. 6.3.
As regards doping effect, one has to remember that the effect will be more
pronounced only when the intrinsic defect concentration of the reaction product
is small. If the inherent defect concentration of a compound is already quite high,
very little influence is expected, e.g., doping effect of the FeO growth rate on
Fe will invariably be less pronounced than those for NiO or Cu
2
O on the corre-
sponding metals. This is because of the fact that the inherent defect concentration
in FeO is more than 10 at.% while those in NiO or Cu
2
O are less than 0.1 at.%
at 973 K while exposed to the same oxygen pressure.
σ
i
,
