The
T–x
(temperature–composition) phase state diagrams of 120 systems RF
3
–R'F
3
(pair combinations of 16 trifluorides of rare-earth elements R and R' without ScF
3
) represent the interaction of ...components in terms of phase composition depending on the chemical proximity of RF
3
and R'F
3
, which is defined by the difference Δ
Z
=
Z
(R)
– Z
(R') of the atomic numbers of R and R' (except YF
3
). The lanthanide contraction of R
3+
with increasing
Z
together with temperature causes polymorphism and morphotropy of RF
3
and
The series of RF
3
is divided into structure subgroups (SSs), pair combinations of RF
3
of which give 10 types of systems. A classification of RF
3
–R'F
3
systems by Δ
Z
is proposed, which divides 10 types of systems into four groups of systems (GSs): GS-1 contains 25 systems of RF
3
of identical SSs with Δ
Z
= 2–3; GS-2, 43 systems of RF
3
of neighboring SSs with Δ
Z
= 6
–
7; GS-3, 32 systems of RF
3
separated by one SS with Δ
Z
= 10; and GS-4, 20 systems of RF
3
separated by two SSs with Δ
Z
= 14.
Internal periodicity of phase transitions (fusion and polymorphism) has been revealed in trifluorides of Y, La, and 14 lanthanides. This periodicity is determined by the internal periodicity of ...filling of the 4
f
-electron subshell of rare-earth elements (REEs,
R
), in which cerium
58
Се–
64
Gd and terbium
65
Тb–
71
Lu subsets can be selected. The lanthanide compression of ionic radii
R
3+
(15% towards the smaller one) induces the formation of three structure types: LaF
3
, β-YF
3
, and α-YF
3
(α-UO
3
). The formula volumes
V
form
of REE trifluorides of the LaF
3
and β-YF
3
structure types only partially obey the internal periodicity of lanthanides with an increase in
Z
. The internal periodicity of the fusion and polymorphism of REE trifluorides (without ScF
3
) divides
R
F
3
into four structural subgroups:
A
(
R
= La–Nd),
B
(
R
= Pm–Gd),
C
(
R
= Tb–Ho), and
D
(
R
= Er–Lu, Y), strictly specifying the percentage and elemental composition of each subgroup. The periodicity of the
R
F
3
polymorphism in the products manifests itself in the crystallization of the melts prone to supercooling in the form of a periodic change of single crystals with coarse- or fine-grained blocks of the low-temperature forms.
The combination of 15 RF3 (R – rare earth elements – REEs without ScF3 and YF3) into one system “from LaF3 to LuF3” we call a full quasi-system (QS). It consists of 14 serially linked particular ...systems (PartSs): LaF3–CeF3; CeF3-PrF3; PrF3-NdF3; NdF3-PmF3; PmF3-SmF3; SmF3-EuF3; EuF3-GdF3; GdF3-TbF3; TbF3-DyF3; DyF3-HoF3; HoF3-ErF3; ErF3–TmF3; TmF3-YbF3; YbF3-LuF3. The atomic numbers (Z) of cations from 57 (La) to 71 (Lu) and the temperature (T) are axies of QS. The conditions of including RF3 in PartSs is the neighborhood of R (Ln - lanthanides) positions in Periodic Table of Chemical Elements and |ΔZ| = 1. This securities of maximum chemical proximity (СhProx) of RF3's components. The QS obeys the phase rule, principles of continuity and conformity. Full QS contains all signs of chemical RF3's interactions identified in 34 studied LnF3-Ln’F3 systems. These signs are dependent of ΔZ: perfect and limited isomorphism and two varieties (peritectic and eutectic) of structural true morphotropic transformations (MTs) of phases. Modifications of LnF3 forms the solid solutions (ss) Ln1-xLn'xF3 with average Zav=(1-x)NLn + x(N+1)Ln’ (N from 57 to 71). These ss reveal the lanthanide contraction (LC) in the regions |ΔZav|<1. Full QS is an individual sub-level of chemical classification of LnF3-Ln’F3 systems. It is a tool for studies the chemical interactions LnF3, analyzing fine consequences of LC, creating a special system of Ln3+ ionic radii for LnF3, studies of density anomalies (negative thermal expansion), prediction of phase diagrams unexplored systems RF3-R’F3 and other problems of rare earth trifluorides chemical family.
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•The full QS is formed by 14 systems: LaF3–CeF3; CeF3-PrF3; PrF3-NdF3; NdF3-PmF3; PmF3-SmF3; SmF3-EuF3; EuF3-GdF3; GdF3-TbF3; TbF3-DyF3; DyF3-HoF3; HoF3-ErF3; ErF3–TmF3; TmF3-YbF3; YbF3-LuF3 with common axis T and Z.•The full QS have in pairs of components the maximum chemical proximity .|ΔZ| = 1 (the difference in the atomic numbers Z of the cations).•The full QS is the individual sub-level of T-Z RF3-R’F3 systems with .|ΔZ| = 1 having the signs of a T-x system.•The full and short QSs are a models for LnF3-Ln’F3 systems and a tools for analyzing fine structural and chemical properties of compositions with .|Zav|=(1-x)NLn + x(N+1)Ln’ (N from 57 to 71) <1.
Lanthanide contraction of lanthanum and 14 lanthanides (
Ln
), leading to a decrease in
R
3+
ionic radii by ~15% (with respect to the smallest one) with an increase in the element atomic number
Z
, ...is a way to implement a close packing of F
1–
ions in trifluorides of heavy 4
f
elements of the periodic table of elements. The efficiency of lanthanide (
Ln
) and actinide (
An
) contraction in the formation of inorganic fluoride crystals with a minimum unit-cell volume per fluorine ion (
V
F
) is estimated. For
An
F
3
(
An
= Ac–Am), the
V
F
value decreases from ~20 to ~17.4 Å
3
with an increase in
Z
from 89 to 95. The unavailability of heavy
An
makes this value final. The close packing of the fluorine ion in fluorides could not be obtained using the effects of lanthanide and actinide contraction and increase in the cation valence. Fluorides
Ln
F
3
(structure type β-YF
3
) from the end of the series from ErF
3
(15.86 Å
3
) to LuF
3
(15.48 Å
3
) are nearest to the anion close packing. The latter
V
F
value is minimum among all studied
Ln
F
3
compounds and actinide fluorides (both simple and complex) and close to the saturation value. It corresponds to the estimated fluorine ionic radius of 1.246 Å.
The anionic nonstoichiometry in inorganic fluorides is at result of substitution of F
1–
for O
2–
in the anionic sublattice. All families of fluorides exhibit the initial stage of anionic ...nonstoichiometry (ISAN), which was previously studied for trifluorides of rare-earth elements (REEs),
R
F
3
. Partial substitution of F
1–
for O
2–
in
R
F
3
occurs in reactions with H
2
O vapor upon heating (pyrohydrolysis), exchange reactions of
R
F
3
and
R
2
O
3
in melts, hydrothermal solutions, solid phase, and during mechanochemical synthesis. The ISAN is based on the formation of
R
F
3 – 2
x
O
x
oxofluorides, the type and structure of which depend on the crystalline
R
F
3
forms. Congruently melting
tys
-
R
F
3 – 2
x
O
x
compounds are formed based on the tysonite forms
tys
-
R
F
3
(
R
= La–Gd, the LaF
3
type). Berthollide phases ~
tys
-
R
F
3 – 2
x
O
x
, which are isostructural to the above-mentioned phases and melts incongruently above the corresponding
R
F
3
compounds, are formed with
R
= Tb–Ho. The effect of stabilization of
tys
-
R
F
3 – 2
x
O
x
when moving up the temperature scale (+Δ
T
fus
) changes nonmonotonically along the REE series, exhibiting a maximum of ~100°C in the range of Gd–Tb. There are no F
1–
→ O
2–
substitutions in the β-
R
F
3
forms (
R
= Er–Lu, Y) of the β-YF
3
type. The α-
R
F
3 – 2
x
O
x
phases of the α-YF
3
(α-UO
3
) type melt incongruently and decompose at high temperatures. The ISAN products in
R
F
3
may serve as sources of new congruently melting fluorine–oxygen materials.
The relationship between the polymorphism, isomorphism, and morphotropy has been traced for the homologous series of 16 trifluorides of rare-earth elements (REEs) (without ScF
3
) and
R
1 –
x
R
F
3
...phases (
R
is an REE) based on the
T
–
x
diagrams of
R
F
3
–
R
'F
3
systems. The polymorphism is determined by the REE atomic numbers
Z
and
Т
values, which change the cation/anion radius ratio
r
+
/
r
–
. Four structural
R
F
3
subgroups are selected according to the polymorphism and types of LaF
3
, β-YF
3
, and α-YF
3
(α-UO
3
) structures:
A
(
R
= La–Nd),
B
(
R
= Pm–Gd),
C
(
R
= Tb–Ho), and
D
(
R
= Er–Lu, Y). Combinations of
R
F
3
form 10 types of
R
F
3
–
R
’F
3
systems. Isomorphism (both perfect and limited) manifests itself in the homogeneity range of
R
1 –
x
R
F
3
phases. It affects the
R
1 –
x
R
F
3
structure via the
r
+
/
r
–
ratio by preparing (jointly with
Т
) their morphotropic transformations (MTs). The morphotropy of
R
1 –
x
R
F
3
is regulated by the parameters
Т
and
х
via the ratio
r
+
/
r
–
and is implemented by means of phase reactions of melt with
R
1 –
x
R
F
3
and
R
1 –
y
R
F
3
of different structures at peritectic (MT-I) and (or) eutectic (MT-II) temperatures. Morphotropic transformations of
R
1 –
x
R
F
3
structures occur at the boundary of GdF
3
–TbF
3
(
Z
= 64.43–64.51; МT-I; 1186 ± 10°С) and HoF
3
–ErF
3
(
Z
= 67.67–67.36; МT-II; 1120 ± 10°С). A definition of true morphotropy for systems in
T
–
x
coordinates is given.
Anionic nonstoichiometry in fluorides (replacement of F
1–
with O
2–
) deteriorates the quality of optical materials. Two reviews have been devoted to the initial stage of anionic nonstoichiometry: ...fluorides
M
F
2
(
M
= Ca, Sr, Ba) are considered in this paper, and fluorides
R
F
3
(
R
are 16 rare-earth elements) will be considered in the next one. These 19 fluorides play an important role in the fluoride materials science, comprising more than 70% of 27
M
F
m
that are used to design two-component fluoride crystalline materials. The initial stage of anionic nonstoichiometry in
M
F
2
is the only one in which oxofluorides
M
F
2 – 2
x
O
x
with low oxygen content are formed. Partial replacement of F
1–
with O
2–
in the fluorite structure is accompanied by thermal stabilization of the structure type when moving up the temperature scale with a maximum in the melting curves of the oxofluoride phase, which decomposes upon cooling. No other intermediate phases containing fluorine and oxygen were found in the
M
F
2
–
M
O systems studied.
This paper reviews a wide range of questions in astrophysics and cosmology that can be answered by astronomical observations in the far-IR to millimeter wavelength range and which include the ...formation and evolution of stars and planets, galaxies, and the interstellar medium, the study of black holes, and the development of the cosmological model. These questions are considered in relation to the Millimetron Space Observatory (Spectrum-M project), which is equipped with a aperture cooled telescope and can operate both as a single-dish telescope and as part of a space-ground very long baseline interferometer.
► We obtain reproducible results in ceramics synthesized by different methods. ► We report an incommensurate modulated structure for Bi0.7La0.3FeO3 ceramics. ► We report the piezoelectric coefficient ...for this composition. ► The obtain structure is related with Raman spectra and ferroelectric properties.
Single phase Bi0.7La0.3FeO3 ceramic samples were successfully synthesized by sol–gel combustion and co-precipitation methods, performing a final sintering at 820–870°C from 10 up to 180min. Rietveld refinements of the XRD data detected small satellite peaks that were successfully indexed by an incommensurated modulated structure model. Lanthanum doping improves magnetic response, reduces the leakage current and dielectric losses. The piezoelectric coefficient was reported for the first time in the Bi0.7La0.3FeO3 composition.
The intrinsic fluorine-ion conductivity σ
lat
of BaF
2
(CaF
2
fluorite type) and LaF
3
(tysonite type) crystals is studied by the impedance spectroscopy method. These compounds represent two major ...structural types taken as the basis to form the best nonstoichiometric fluorine-conducting solid electrolytes. The conductivity σ
lat
caused by thermally activated defects is manifested in the field of high temperatures, where conductometric measurements are complicated by pyrohydrolysis. The experiments carried out in inert atmosphere with application of the impedance method have for the first time produced the reliable values of σ
lat
of fluoride crystals in conditions of suppression of pyrohydrolysis (BaF
2
) or partial pyrohydrolysis (LaF
3
). Values of the σ
lat
at 773 K for BaF
2
and LaF
3
crystals grown from melt by the Bridgman method using the vacuum technology are 2.2 × 10
–5
and 8.5 × 10
–3
S/cm differing by a factor of ~400. The tysonite structural type has been proved feasible for making high-conductivity solid fluoride electrolytes based on the analysis of energy characteristics of formation and migration of anionic defects.
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