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.
The temperature dependence of the ionic conductivity σ
dc
(
T
) and thermal stability of BiF
3
crystals with the structure of orthorhombic β-YF
3
(sp. gr.
Pnma
,
a
= 6.5620(1) Å,
b
= 7.0144(1) Å,
c
= ...4.8410(1) Å,
V
/
Z
= 55.71 Å
3
), grown from melt by the vertical directional crystallization technique have been investigated. The electrical characteristics of BiF
3
are obtained from impedance measurements in the temperature range of 360−540 K. The σ
dc
value at
T
= 500 K and the ion transport activation enthalpy Δ
H
a
are found to be 2.5 × 10
−5
S/cm and 0.48 ± 0.05 eV, respectively. The Δ
H
a
value for the crystal studied is smaller by a factor of 1.4 in comparison with the isostructural rare-earth (Tb, Ho, Er, Y) trifluorides, which is due to the high electronic polarizability and large ionic radius of Bi
3+
cations. It was found that BiF
3
crystals are thermally stable at temperatures up to 550−600 K; at higher temperatures degradation was observed due to the sublimation and pyrohydrolysis of this material. The formation of oxofluoride phases
is responsible for the detected conductivity jump in the dependence σ
dc
(
T
) at
T
∼ 600 K.
Fluoride–oxide composites 99NdF
3
⋅1Nd
2
O
3
, 98NdF
3
⋅2Nd
2
O
3
, 97NdF
3
⋅3Nd
2
O
3
, 92NdF
3
⋅5SrF
2
⋅3Nd
2
O
3
, and 91NdF
3
⋅5SrF
2
⋅4Nd
2
O
3
(mol %) have been fabricated in the NdF
3
–Nd
2
O
...3
and NdF
3
–SrF
2
–Nd
2
O
3
systems by the directional crystallization technique. It is found that the target tysonite-type Nd
Sr
x
(O,F)
solid solution is partially decomposed upon cooling with segregation of nonstoichiometric modifications based on the NdO
F
phase. The electrical conductivity of the composites has been analyzed by impedance spectroscopy in the temperature range of 294–522 K. The σ
dc
values at 500 K are 5.2 × 10
–4
, 6.5 × 10
–4
, and 2 × 10
–3
S/cm for 99NdF
3
⋅1Nd
2
O
3
, 97NdF
3
⋅3Nd
2
O
3
, and 92NdF
3
⋅5SrF
2
⋅3Nd
2
O
3
, respectively; after doping of SrF
2
into the composite, the conductivity increases by a factor of 4 due to heterovalent substitutions of cations Sr
2+
for Nd
3+
and formation of anion vacancies. It is shown that cationic substitutions are much more efficient (in comparison with anionic substitutions) for the mechanism of ionic conduction in tysonite-type solid solutions. The σ
dc
value of oxofluoride composite 92NdF
3
⋅5SrF
2
⋅3Nd
2
O
3
is larger than that for fluoride composite 70NdF
3
⋅30SrF
2
by a factor of 50 at room temperature. The composites under study can be applied as medium-temperature solid-state sensors for simultaneous detection of oxygen and fluorine.
In this paper, the anisotropy of anionic conductivity in CeF
3
superionic conductor crystals with tysonite structure (sp.gr.
) is studied for the first time. Temperature (300–600 K) conductivity ...measurements are made along crystallographic axes
a
and
c
of the crystal trigonal unit cell. The maximum conductivity is observed along the
c
axis. The conductivity of CeF
3
superionic crystals is weakly anisotropic, σ
||
c
/σ
||
a
= 2.4, σ
||
c
= 5.6 × 10
–4
S/cm at 500 K. The anionic conductivity anisotropy effect in individual fluorides with tysonite structure due to the special features of their atomic structure is discussed.
We report on the high-resolution Fourier spectroscopy study of KY
3
F
10
:Pr
3+
crystals. The analysis of the transmission and luminescence spectra allowed us to refine and supplement the information ...on the crystal-field levels of the Pr
3+
ion. The value of the hyperfine splitting of the ground state of Pr
3+
in the KY
3
F
10
cubic host is estimated. The observed shape of the spectral lines indicates the presence of defects in the sample under study.
KTb
3
F
10
crystals have been grown from the melt by the vertical directional crystallization. The incongruent character of melting of this compound is confirmed experimentally. Corrections are ...introduced into the phase diagram of the KF–TbF
3
system. It is found that the optimal pre-peritectic composition for the KTb
3
F
10
crystal growth corresponds to a KF content of 27.5 ± 0.5 mol % in melt. The KTb
3
F
10
composition is a part of a nonstoichiometric phase of variable composition. The cubic (sp. gr.
) lattice parameters are limited to the range from 11.679(1) to 11.663(1) Å.
K
R
3
F
10
(
R
= Ho, Er; sp. gr.
) crystals have been grown by the vertical directional crystallization. The incongruent melting of these compounds is experimentally established, and the temperatures ...of the corresponding thermal effects are determined. Narrow regions of homogeneity are found for the studied crystals; these regions are also characteristic of the entire series of K
R
3
F
10
crystals under study. The cubic lattice parameter monotonically decreases along the crystal length and varies in the range of 11.5782(2)–11.5654(5) Å for KHo
3
F
10
and 11.5225(1)–11.5102(4) Å for KEr
3
F
10
. The conditions for growing K
R
3
F
10
crystals of optical quality from melt are optimized.
Composite nanomaterials (1 –
x
)CaF
2
–
x
BaF
2
with 0.4 ≤
х
≤ 0.6 have been prepared by directional melt crystallization in a fluorinating atmosphere. The structural, optical, mechanical, and ...electrical characteristics of the synthesized nanocomposites are studied. Biphasic (1 –
x
)CaF
2
–
x
BaF
2
composites have a lamellar fine microstructure. The lamella thickness decreases with an increase in the BaF
2
content and reaches 30–50 nm for the composition with
x
= 0.5. The composites retain high transparency in the IR range (close to the values for the initial components). The microhardness (
Н
V
≈ 5.0 GPa) and ionic conductivity (σ
dc
= (1–3) × 10
3
S/cm at 823 K) of these composites exceed significantly the corresponding parameters of CaF
2
and BaF
2
crystals. The fluoride nanocomposites under investigation are promising materials for practical applications in photonics and solid-state ionics.
The dependence of the ionic conductivity σ
dc
(
y
) of crystallization products on the concentration is studied by impedance spectroscopy in quasi-binary systems
R
F
3
−PbF
2
(
R
= Pr, Nd) in the ...region of compositions
R
1 −
y
Pb
y
F
3 –
y
(0 ≤
y
≤ 0.22) enriched with the rare-earth component
R
F
3
. The single-phase tysonite (space group
) solid solutions
R
1 −
y
Pb
y
F
3 –
y
are synthesized from the melt by directional crystallization for
y
< 0.1. The maximum conductivity σ
dc
at 296 ± 1 К is found to be 7 × 10
−5
and 3 × 10
−5
S/cm for crystals with the composition Pr
0.96
Pb
0.04
F
2.96
and Nd
0.95
Pb
0.05
F
2.95
, respectively. For 9−10 mol % PbF
2
, the samples become two-phase, the second phase—the fluorite solid solution Pb
1 −
x
R
x
F
2 +
x
—appears, and their ionic conductivity decreases.
