Chemistry Reference
In-Depth Information
14.5.4
Phase Diagrams
Solid solutions usable as solid electrolytes are usually nonequilibrium since they are either
quenched high-temperature phases or single crystals grown up from the melt. Nevertheless
they are rather stable kinetically and show reproduced results for a long time. It is
important to know what can occur after their long usage as electrolytes: whether phase
transformations (for example at temperatures lower than the synthesis temperature) are
possible according to the equilibrium phase diagram or whether crystallization of ordered
phases can occur. Thus the investigation of the corresponding phase diagrams is very
important for the further applications of nonstoichiometric solid solutions. Examples of
some phase diagrams are presented in Figure 14.16 (isothermal section of a ternary phase
diagram) and Figure 14.17 (binary phase diagrams).
BaF
2
- monophase samples
- double-phase samples
- ternary-phase samples
Φ
BiOF +
Φ
R
R+
+
Φ
β
BiOF +
Φ
+
β
I
R+
β
l+
β
+R
II
BiF
3
β
III
1/2(Bi
2
O
3
)
BiOF
Φ
+
β
Figure 14.16 Solid solution in the BiF
3
-Bi
2
O
3
-BaF
2
system at 873 K [34].R: Ba
4
Bi
3
F
17
based
phase, : -BiO
y
F
32y
based phase
,
F
: fluorite-like BaF
2
based solid solution, I, II: tysonite-like
solid solutions, III: ordered tysonite-like phase BiO
y
F
32y
(0.13< y<0.23)
14.6
Ionic Transfer in Fluorite-like Phases
The conductivity
of ionic crystals can be assumed to be due to the mobilities of the
individual ions and electrons:
¼
q
n
mob
m
mob
In this formula q, n
mob
and m
mob
denote respectively, the charge, the concentration and
the mobility of charge carriers. In fluorite-like phases these charge carriers are mainly
fluoride ions.
