Search for chlorine-conducting solid electrolytes with a high ionic conductivity σ
293K
at room temperature (293 K) is based on the analysis of temperature measurements of the electrical conductivity ...σ(
T
) of simple and complex compounds, solid solutions, composites, and glasses. A comparison of the thermal stability of chlorides is based on the analysis of the dependences σ(
T
) in cyclic heating–cooling measurements and the physicochemical data. Maximum σ
293K
values were found for the solid solutions
(3 × 10
−5
S/cm) and
(1 × 10
−4
S/cm). Promising directions of search for chlorine-ion solid electrolytes for chemical current sources and chlorine sensors operating at room temperature are the method of crystallochemical heterovalent substitutions and implementation of the vacancy mechanism of electrical conductivity in crystal structures of the cotunnite (PbCl
2
) and fluorite (CaF
2
) types. The parameters of the ionic conductivity of cotunnite and fluorite crystals of chlorides and fluorides are analyzed.
The anionic electrical conductivity σ
dc
(
T
) of the superionic LaF
3
conductor single crystal has been studied by impedance spectroscopy in a wide (312–1073 K) temperature range. Electrical ...measurements in a protective nitrogen atmosphere have been carried out. It is shown that the σ
dc
values change from 1.5 × 10
–5
to 7 × 10
–2
S/cm (by a factor of ~5 × 10
3
) in the investigated temperature range. A high-temperature anomaly of the σ
dc
(
T
) dependence has been found at
T
0
≈ 970 K, which is apparently indicative of the onset of a diffuse (Faraday) phase transition in the LaF
3
tysonite structure.
Correlation between the parameters of the dispersion relations of refractive indices and coordination numbers of cations in the structure is found for langasite-family La
3
Ga
5
SiO
14
optical ...crystals (C-a
3
Ga
2
Ge
4
O
14
structure type, sp. gr.
P
321).
The temperature dependence of static electrical conductivity σ
dc
(
T
) of LiNbO
3
single crystal (sp. gr.
R
3
c
), oriented along the crystallographic
c
axis, has been studied using impedance ...spectroscopy. The impedance was measured under temperature effect in vacuum in a heating–cooling cycle. The σ
dc
(
T
) dependence with activation energy
E
a
= 1.22 eV, caused by migration of lithium vacancies
, was obtained upon heating. Upon cooling, the σ
dc
(
T
) curve can be divided into three regions with activation energies of 0.66, 0.915, and 0.285 eV. After the measurements in vacuum, color centers containing oxygen vacancies
and electrons were formed in the crystal bulk. A thermal effect in vacuum on a LiNbO
3
crystal changes the mechanism of electrical conductivity from ionic to mixed ionic–electronic. At 673 K, the ionic conductivity upon cooling is 3.2 × 10
–6
S/cm, which exceeds the σ
dc
value upon heating by a factor of about 4. At 500 K, the electronic conductivity upon cooling is 4.3 × 10
–7
S/cm, which exceeds the σ
dc
value upon heating by a factor of about 515 (extrapolation).
The dynamic viscosity coefficients of fluoride ionic melts in the SrF
2
–RF
3
(R = lanthanide (La, Gd, Tb, Yb, or Lu)) binary systems were calculated from diffusion data. A strong dependence of the ...melt viscosity is observed at low lanthanide concentrations. The Sr
1 −
x
R
x
F
2 +
x
melts have their viscosity decreasing as the RF
3
concentration increases, while increasing in response to decreasing R
3+
ionic radius.
The effect of strong stoichiometry violations in the structural type of fluorite (CaF
2
) on the microhardness of Ba
0.9
R
0.1
F
2.1
(R = rare-earth elements La–Lu) crystals is studied. The ...mechanical hardening of heterovalent solid solutions Ba
0.9
R
0.1
F
2.1
as compared to the BaF
2
matrix is due to the formation of nanosized clusters of point defects Ba
8
R
6
F
69
substituting structural fragments Ba
14
F
64
(block-type isomorphism). The microhardness of nanostructured Ba
0.9
R
0.1
F
2.1
crystals is found to increase by 73% as the cluster size decreases from Ba
8
La
6
F
69
to Ba
8
Lu
6
F
69
.
A crystallophysical model of ion transport is proposed based on the electrical and structural data for Ba
1 –
x
La
x
F
2 +
x
and Ca
1 –
x
Y
x
F
2 +
x
superionic conductors (sp. gr.
), in ...which charge carriers are mobile interstitial
ions formed as a result of heterovalent substitutions of M
8
R
6
F
69
structural clusters (R = La or Y) for M
14
F
64
fluorite fragments (M = Ca or Ba). Single crystals of Ca
1 –
x
Y
x
F
2 +
x
(0.02 ≤
x
≤ 0.16) and Ba
1 –
x
La
x
F
2 +
x
(
x
= 0.31) solid solutions are prepared using directional solidification. Mobilities of ion carriers in Ba
0.69
La
0.31
F
2.31
, Ca
0.84
Y
0.16
F
2.16
, Pb
0.67
Cd
0.33
F
2
, and Pb
0.9
Sc
0.1
F
2.1
isostructural superionic conductors are compared. Ba
1 –
x
La
x
F
2 +
x
and Ca
1 –
x
Y
x
F
2 +
x
crystals with improved conductometric and mechanical characteristics are promising for replacement of conventional electrolyte CaF
2
in galvanic cells for thermodynamic studies of chemical compounds.
Unit cell parameters
a
cal
(
x
) and X-ray densities ρ
X
(
x
) are calculated in terms of a model that accounts for ionic bonding, additivity of properties, and dimensional factor upon aliovalent ...substitutions for 45 new nonstoichiometric phases Sm
1 −
x
R
x
F
2 +
x
, Eu
1 −
x
R
x
F
2 +
x
and Yb
1 −
x
R
x
F
2 +
x
(R = La−Lu, Y) based on SmF
2
, EuF
2
, and YbF
2
fluorite matrices (the fluorite CaF
2
type). A comparison of the calculated unit cell parameters
a
cal
(
x
) and the experimental
a
(
x
) values available in the literature for these phases shows satisfactory match. Concentration-dependent relationships
a
cal
(
x
) and ρ
X
(
x
) can be used for monitoring the composition of melt-grown Sm
1 −
x
R
x
F
2 +
x
, Eu
1 −
x
R
x
F
2 +
x
, and Yb
1 −
x
R
x
F
2 +
x
crystals in studies of their defect structures and fundamental properties.
Concentration dependences of the lattice parameter
a
=
f
(
x
) and density ρ =
f
(
x
) have been studied for single crystals of the ternary solid solution Sr
0.8
La
0.2 –
x
Lu
x
F
2.2
(fluorite type, ...CaF
2
; 0 ≤
x
≤ 0.2,
x
is the mole fraction of LuF
3
) grown from the melt by the Bridgman method. The experimental dependences
a
(
x
) and ρ(
x
) obey the additivity law. Densitometric data support the scheme of heterovalent isomorphism in the Sr
0.8
La
0.2 –
x
Lu
x
F
2.2
solid solution: Sr
2+
→ (1 − 5
x
)La
3+
+ 5
x
Lu
3+
+
. The excess of positive charge in the cationic sublattice of crystals is compensated by the formation of interstitial F
i
–
ions. These regularities make it possible to calculate the lattice parameters and density of a large number of new functional fluoride materials—fluorite solid solutions
in ternary systems MF
2
–R'F
3
–R"F
3
(M = Ca, Sr, Ba; R', R" = La–Lu, Y).
