Chemistry Reference
In-Depth Information
Rare-earth fluoride, Ln'F
3
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Y
Er
Tm
Yb
Lu
La
CeO
2
Pr
6
O
11
Nd
Sm
Eu
Gd
Tb
4
O
7
Dy
Ho
Y
Er
Tm
Yb
Lu
Figure 14.8 Electrical conductivity of binary RE oxyfluorides Ln
2
Ln'
2
O
3
F
6
measured at 973 K
under oxygen partial pressure of 0.13 Pa.
, more than 1S m
1
;
C
, 0.1 1S m
1
;
, less than
0.1S m
1
. (Reprinted with permission from [15] Copyright (2000) Elsevier Ltd.)
Because the easy oxygen introduction into fluorides changes essentially their conduc-
tivity some of such phases are rather sensitive to the gas atmosphere composition and could
have different applications. For example LaF
3
and PbSnF
4
were used as oxygen and
moisture sensors [17-19].
14.3 Cation Doping of Fluorides
Cation doping of fluorides always leads to a crystal structure distortion. The degree of
distortion depends on the size ratio and properties (in the first place, electronic structure
and electronegativity) of dopant and matrix cations as well as on the replacement type:
isovalent or heterovalent replacement.
14.3.1
Isovalent Replacement in the Cation Sublattice
Isovalent replacement in the cation sublattice has not been studied precisely, probably
because it does not lead to the formation of additional anion defects that increase the
ionic conductivity. Nevertheless the isovalent replacement in the cation sublattice of
fluorite (for example PbF
2
dopedbyCaF
2
or CdF
2
[10])ortysonite(forexampleLaF
3
