142
3 – Transport in ionic solids
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Figure 58 – Conductivity of NiO as a function of oxygen
partial pressure in logarithmic coordinates at 1 000 °C.
We observe a linear conductivity as a function oxygen partial pressure.
Two domains with different slopes are apparent. The linear parts of each
domain have slopes of 0.16 and 0.23 for lower and higher oxygen partial
pressure, respectively.
2. At high oxygen partial pressure, nickel oxide is a p-type semiconductor. It
has nickel vacancies and holes in the cationic sublattice and a full anionic
sublattice. The reaction with oxygen leads to the formation of atomic and
electronic defects
2
1
O 2 m O
#
O + V ′′ Ni + 2h
•
The equilibrium constant K g1 for this reaction is given by
] [ ]
K
P
h
gl
O
Ni
2
2
1 2
=
:
#
[Vll
At high oxygen partial pressures, the electroneutrality equation reduces to
[h
•
] = 2[V ′′ Ni ]
from which we have
[ ]
[ ] (
)
K
P
h
and h
K
P
2
2
gl
O
gl
O
3
2
2
1 2
1 3
1 6
=
=
:
:
#
#