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derivative of a fluorite-type structure), the above fluorides are related to the K
3
UF
7
type (usually characteristic of heavy metals). Heptafluorides are usually obtained
using the methods of chemical interaction at room temperature by sintering in inert
a fluorinating atmosphere and crystallization from KF flux.
Illyushin et al.
[12]
have obtained several millimeters large K
3
ZrF
7
single crys-
tals under hydrothermal conditions at temperatures of 450
500
C and pressures of
100
150 MPa. The initial materials were aqueous zirconium dioxide and potas-
sium fluoride solutions with the concentration 10
zirconium
heptafluoride is a stable fluorine-containing crystalline phase within the range of
temperatures and pressures used in hydrothermal experiments.
Conductivity (
60 wt%. Potassium
σ
)ofK
3
ZrF
7
single and polycrystals was measured in the temper-
ature range 300
were calculated from the impedance
(admittance) frequency plots of an electrochemical Ag/K
3
ZrF
7
/Ag cell. The
conductivity of single crystals is somewhat higher than that of polycrystals, yet no
noticeable anisotropy in conductivity was observed
[12]
. At 300
C, conductivity is
3.9
700 K. The values of
σ
10
2
2
2 ohm
2
1
cm
2
1
3
and 1.17
3
1
for single and polycrystals, respectively
(
Figure 8.6
).
A synthetic Fe
3
1
variety of elpasoite, a rare natural mineral (K
2
NaAlF
6
), was
synthesized in the course of the investigation of the phase formation in the
AF
a
Fe
2
O
3
a
(NH
4
)
2
HPO
4
a
H
2
O system with A
Na, K, Cs
(T
400
C,
5
5
fill
50 wt%)
[13]
. The structural analysis yielded the composition
K
2
NaH
1.5
Fe
0.6
F
6
for the cubic crystals of this system (space group Fm3m). Under
hydrothermal conditions, complex fluorides of the system A
1
1
F
0.7, C
KF
5
30
5
A
2
1
F
2
A
3
1
F
3
(A
1
1
5
Li, Na, K; A
2
1
5
Ca, Sr, Ba, Mg, Zn; A
3
1
5
B, Al, Ga) were also
obtained
[14]
.
Δ
H
α
= 0.30 + 0.01
eV
0
-
-5
Δ
H
α
= 0.51 + 0.03
eV
-
-10
0
2
4
10
3
/
T
(
K
-1
)
Figure 8.6 Conductivity (
σ
) for single and polycrystals
[12]
.
