Synthesis and thermal stability of rare earth compounds REF3, REF3·nH2O and (H3O)RE3F10·nH2O (REu202f=u202fTb ? Lu, Y), obtained from sulphide precursors

Article abstract of DOI:10.1016/j.jfluchem.2017.12.001

The nature of the interaction between sulphides of the rare-earth elements (RE) of the yttrium subgroup RE₂S₃ (Dy ? Lu, Y) with hydrofluoric acid (concentration of 49%) was studied. Powders of RE₂S₃ compounds were obtained by the reaction between RE₂O₃ (Dy ? Lu, Y) and Tb₄O₇ with a stream of H₂S and CS₂ in the 1000–1050 °C temperature range. The reaction between RE₂S₃ and HF was carried out in a glassy carbon crucible in the 21–25 °C temperature range in solution, and the precipitate was washed with H₂O and dried at 90–100 °C. For the hydrates of RE fluorides with decreasing RE radius, the RE content increases the sorption of water for TbF₃·0.5H₂O; DyF₃·0.5H₂O; YF₃·0.5H₂O; HoF₃·0.8H₂O; and ErF₃·0.9H₂O. The powders of REF₃·nH₂O consisted of micro- and nano-sized particles containing the amorphous phase. Water loss occurs in the 50–180 °C temperature range and is accompanied by a natural increase in enthalpy: from 23.6 J/g for TbF₃ · 0.5H₂O to 97.8 J/g for ErF₃·0.9H₂O and from 51.1 J/g for (H₃O)Tm₃F₁₀·1.7H₂O to 64.5 J/g for (H₃O)Lu₃F₁₀·0.9H₂O. Exothermic effects of formation of polycrystalline phase REF₃ were recorded in the 200–300 °C temperature range. The reaction of RE₂S₃ (Tm, Yb, Lu) powders with HF proceeds with the formation of the zeolite type compounds (H₃O)Tm₃F₁₀·1.7H₂O, (H₃O)Yb₃F₁₀·1.0H₂O and (H₃O)Lu₃F₁₀·0.9H₂O. Thermal dissociation occurs with gradual loss of water and HF to form RE fluorides with the β-YF₃ structure as described by

Full text of DOI:10.1016/j.jfluchem.2017.12.001

Accepted Manuscript
Title: Synthesis and thermal stability of rare earth compounds
REF3, REF3·nH2O and (H3O)RE3F10·nH2O (RE = Tb − Lu,
Y), obtained from sulfide precursors
Authors: I.A. Razumkova, O.V. Andreev, A.N. Boyko
PII:
S0022-1139(17)30442-6
DOI:
Reference:
https://doi.org/10.1016/j.jfluchem.2017.12.001
FLUOR 9081
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Received date:
Revised date:
Accepted date:
11-10-2017
30-11-2017
1-12-2017
Please cite this article as: I.A.Razumkova, O.V.Andreev, A.N.Boyko, Synthesis and
thermal stability of rare earth compounds REF3, REF3·nH2O and (H3O)RE3F10·nH2O
(RE=Tb − Lu, Y), obtained from sulfide precursors, Journal of Fluorine
Chemistry https://doi.org/10.1016/j.jfluchem.2017.12.001
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Synthesis and thermal stability of rare earth compounds REF3, REF3·nH2O and
(H3O)RE3F10·nH2O (RE=Tb – Lu, Y), obtained from sulfide precursors
I.A. Razumkova*, O.V. Andreev, A.N. Boyko
Institute of Chemistry (Tyumen State University), 6, Volodarskogo Street, Tyumen, 625003,
Russia
* corresponding author, e-mail: razumkova@list.ru
Graphical abstract
Highlights
 The REF3·nH2O and (H3O)RE3F10· nH2O compounds were synthesized from
RE2S3 and HF.
 The REF3· nH2O compounds have an amorphous phase.
 After heat treatment the REF3·nH2O and (H3O)RE3F10·nH2O compounds moved
into a REF3 compound of crystal structure as β-YF3.
Abstract
The nature of the interaction between sulphides of the rare-earth elements (RE) of the
yttrium subgroup RE2S3 (Dy – Lu, Y) with hydrofluoric acid (concentration of 49%) was studied.
Powders of RE2S3 compounds were obtained by the reaction between RE2O3 (Dy – Lu, Y) and
Tb4O7 with a stream of H2S and CS2 in the 1000–1050 °C temperature range. The reaction between
1

RE2S3 and HF was carried out in a glassy carbon crucible in the 21-25 °C temperature range in
solution, and the precipitate was washed with H2O and dried at 90-100 °C. For the hydrates of RE
fluorides with decreasing RE radius, the RE content increases the sorption of water for
TbF3·0.5H2O; DyF3·0.5H2O; YF3·0.5H2O; HoF3·0.8H2O; and ErF3·0.9H2O. The powders of
REF3·nH2O consisted of micro- and nano-sized particles containing the amorphous phase. Water
loss occurs in the 50-180 °C temperature range and is accompanied by a natural increase in
enthalpy: from 23.6 J/g for TbF3 · 0.5H2O to 97.8 J/g for ErF3 · 0.9H2O and from 51.1 J/g for
(H3O)Tm3F10 ∙ 1.7H2O to 64.5 J/g for (H3O)Lu3F10∙0.9H2O. Exothermic effects of formation of
polycrystalline phase REF3 were recorded in the 200-300 °C temperature range. The reaction of
RE2S3 (Tm, Yb, Lu) powders with HF proceeds with the formation of the zeolite type compounds
(H3O)Tm3F10∙1.7H2O, (H3O)Yb3F10∙1.0H2O and (H3O)Lu3F10∙0.9H2O. Thermal dissociation
occurs with gradual loss of water and HF to form RE fluorides with the β-YF3 structure as
described by
(H3O)RE3F10·(1+n)H2O→~50−170°퐶(H3O)RE3F10→~200−350°퐶HRE3F10~→350−420°퐶3REF3.
Keywords: rare earth fluoride, sulphides of rare earth elements, zeolite type
1. Introduction
The unique electronic structure of rare earth elements (RE) leads to the appearance of
compounds with different optical, electrical [1, 2] and magnetic [3] properties, which have
attracted considerable interest from researchers in recent years. Fluorides of rare earth elements
are widely used in many fields (optical telecommunications, lasers, new optoelectronic devices,
diagnostics and biological markers) [4-7]. In recent decades, a particularly extensive attention was
devoted to rare earth fluorides (REF3) as luminescent materials [8]. Owing to the low energy of
wave oscillations, the decrease in the radiating activity of doping substances in the electronic
excitation state is minimal [9, 10]. As a consequence, a high-luminescence quantum yield can be
achieved [5]. Currently, fluorides of rare earth elements are used or tested in optoelectronics,
optical fibres and amplifiers, lasers, and medicine. In view of their chemical stability, materials
based on REF3 could potentially be used as bioimaging fluorescent probes [5, 8].
Fluorides of rare earth elements La-Nd crystallize in the tysonite (LaF3) structure type (ST)
̅
(trigonal crystal system, space group 푃3푐1). Morphotropics changed with decreasing ionic radius
[11, 12]. For fluorides of Sm-Gd, dimorphism is typically observed with the low-temperature type
β-YF3 (orthorhombic crystal system, space group Pnma) and high-temperature modifications of
tysonite [13, 14]. The REF3 (Tb-Ho) retain the modification characteristic of β-YF3 up to their
melting point, whereas for fluorides of Er-Lu, Y, in addition to the basic orthorhombic
2

modification (β-YF3), the high-temperature structure with cubic α-YF3 cubic (space group P23)
can be obtained [15,16].
REF3 are obtained by chemical and physics-chemical methods. The most common and
available is the method of precipitation from aqueous solutions, referred to as “soft chemistry” [5,
17, 18]. Methods of obtaining fluorides can be divided into hydrothermal [19-22], micro-emulsion
[23] and microwave [24] thermal decomposition [25] types. Synthesis of nanofluorides of RE is
mainly realised by one of the following methods: pyrolysis of fluorine-containing precursors;
hydrothermal synthesis; micro-emulsion method; solvo-thermal synthesis; and the sol-gel
technique [26-27]. At elevated temperatures, RE fluorides participate in pyrohydrolysis reaction
to form oxy-fluorides (REOF) [29-30]. The preparation of REF3 of the cerium subgroup during
the interaction of hydrofluoric acid with RE2S3 provides a yield of close to 100%, and does not
contain rare-earth oxyfluoride impurities, because the formation of the H2S gas product shifts the
equilibrium towards fluoride formation.
RE2S3 compounds exist in form of 4 basic modifications. RE2S3 compounds with RE=Ce-
Dy have a low-temperature modification of the rhombic system of α-RE2S3 ST α-La2S3, sp.gr.
Pnma and high-temperature cubic modification of γ-RE2S3 ST Th3P4, sp.gr. I43d. The δ-RE2S3
compounds (Re=Y, Ho, Er, Tm) have the monoclinic structure of ST δ-Ho2S3, sp.gr. P21/m. For
ε-Yb2S3 and ε-Lu2S3, the hexagonal structure of ST α-Al2O3, sp.gr. R3c is found. In the literature,
there is only one report on the extent of the RE2S3-xOx solid solutions, mainly for γ-RE2S3: γ-
(Sm2S3 – Sm2S2.94O0.06) [31], γ-(Gd2S3 – Gd2S2.97O0.03) [32].
In the process of hydrothermal synthesis of fluorides of rare-earth elements (Er-Lu, Y), the
interaction of lanthanide oxalates with hydrofluoric acid leads to the formation of compounds δ-
(H3O)RE3F10 ∙ nH2O [33]. A compound with the (H3O+)Y3F10- · nH2O structure was obtained as
the second (secondary) phase by the interaction of a solution of yttrium nitrates and alkaline earth
metal (strontium [34], barium [35]) with hydrofluoric acid (47%). The compound is crystallized
in a cubic structure and consists of octahedral diamond [RE6F32]-14 blocks constructed from square
REF8 anti-prisms [36]. The behaviour of water molecules and the cation-exchange properties are
observed to be of the zeolite type [37]. Ionic radius of the atom RE is an important factor in the
formation of stable fluoride compounds. The connection structure of diamond [38] is formed from
rare earth elements rHo3+ (coordination number (CN) is 8) = 1.015 Å - rLu3+(CN 8) = 0.977 Å, Y:
+
ARE3F10 · nH2O (A+ = K+, Rb+, NH4 , Cs+) [33], (C2N2H10)0.5RE3F10 · nH2O [22],
(C3N2H12)0.5RE3F10 · nH2O [39].
In this paper, the nature of the interaction of sulphides of rare earth elements RE2S3 (Dy -
Lu, Y) with hydrofluoric acid and the reaction products are studied, and their crystal chemical,
physics-chemical characteristics, and thermal stability are considered.
3

2. Experiment
2.1. Materials
We used 99.99 wt.% ultrapure RE2O3 and Tb4O7 (manufacturer Uralredmed, Verkhnyaya
Pyshma, Russia), 99.9 wt.% pure HF and NH4SCN (manufactured by TEK Systems) and distilled
water as our starting material without further purification.
2.2.
Synthesis of RE2S3
Powders of RE sulphides were obtained by reacting powders of Tb4O7 and RE2O3 (Dy-Lu,
Y) oxides with gaseous CS2 and H2S. The maximum mass of one batch does not exceed 7 g. The
rate of supply of sulphiding gases is 20-25 ml / min [40]. The synthesis temperature is 1000-1100
°C, and the duration is 12-18 hours [41]. The experiment was carried out in a quartz reactor, using
argon as a gas carrier, at the feed rate of 10 ml / min.
2.3.
Synthesis of REF3 and (H3O)RE3F10
Sulphide powders were treated with an excess of hydrofluoric acid (49%) in the ratio of 1
to 10. All experiments were carried out in glass graphite crucibles at ambient temperature (21-25
°C). The precipitate that forms in the process is decanted, washed thoroughly with distilled water
and dried at various temperatures in air. The heterogeneous reaction of polycrystalline powders of
RE2S3 with an HF aqueous solution (49 wt%, acid taken in excess) occurs within 10-15 seconds.
Gaseous H2S are isolation shifted for the equilibrium in reaction towards the reaction products.
RE2S3 compounds are completely soluble.
Samples were dried on a sand bath at 90-100 °C for 3 hours. A solid crust of the sample
was formed from the mother liquor during the heat treatment. The end of the sample drying was
fixed by the cracking of its surface. Samples were stored in batches. Work with the samples was
conducted in the air.
2.4.
Characterization
Actual chemical composition of the samples was determined using a Philips SEM JEOL
JSM-6510 LM scanning electron microscope at five different points on the surface of a sample.
The chemical composition of the samples was determined by X-ray spectroscopy (EDS) using a
scanning electron microscope JSM-6510LV, (JEOL, Japan) with the accelerating voltage of 20
kV. The water was monitored by IR spectroscopy FSM1201 (OOO “Monitoring”, St. Petersburg,
Russia).
The phase composition of the obtained samples was studied by X-ray diffraction (DRON
7 diffractometer; CuKα radiation). The processing of the experimental data, including the
refinement of the crystal lattice parameters and the determination of the coherent scattering regions
D, was realized in the software complex "Powder 2" and DifWin. The instrumental expansion of
the diffraction reflection was applied by a survey with a standard substance, for which the line
4

extensions are due only to the shooting conditions. As a standard, a dispersed (less than 10 μm)
REF3 powder annealed at 800 °C was used.
Thermal analysis is performed by using synchronously thermal analyser STA 449 F3
MOMQ-1000D (NETZSCH, Germany) (crucibles Al2O3, argon, 2 deg/min heating rate, weight of
sample approximately 25 mg). The error in determining the mass loss is established for the TG /
DSC analyses of compounds of precisely known composition: CuSO4 ∙ 5H2O, ZnSO4 ∙ 7H2O and
RE2(SO4)3 ∙ 8H2O. The error value was 0.3%.
3. Result and discussion
3.1.
Characteristics of the precursors RE2S3
The RE2S3 formation reaction proceeds through the dioxysulfide formation stage, in which
the oxide is transformed into the sulphide according to: RE2O3→RE2O2S→RE2S3. The structure
of the RE2OS2 compositions (Sm-Yb, Y) is presented in previous works [42, 43]. The formation
of the RE2OS2 compound as a co-product due to the interaction of oxides of RE with gaseous
carbon disulphide and hydrogen sulphide was not detected. The impurity phase RE2O2S is
identified by X-ray diffraction when its content in the sample is less than 0.25 mol% RE2O2S [44].
According to the XRF data, single-phase compounds of RE2S3 were obtained in three
modifications: γ-cubic structure, type of Th3P4; δ-monoclinic, type of δ-Ho2S3 and ε-rhombohedral
type of α-Al2O3 [45, 46] (Table 1). The parameters of the cell of the connections completely
correspond to the database JCDPS (Table 1) [48]. The sulphur and the rare-earth element contents
in compressed powders were determined by the EDS on the JSM-6510LV. The experimental
sulphur content corresponds to the theoretical value in the 0.2-0.3 wt.% range (Table 1).
The oxygen content in the synthesized samples of RE2S3 can be as high as 0.2-0.3 wt.% O
from the EDS data, which is generally commensurate with the accuracy of the determination
method. Compositions of the synthesized samples of RE2S3 are in the concentration range
RE2.00S3.02 – RE2.00S3.00 – RE2.00S2.98O0.02, according to the EDS data.
Synthesized RE2S3 compounds do not contain impurities of oxygen-containing phases
within the accuracy of XRF.
Chemical reaction of powders of RE2S3 compounds with an HF aqueous solution (49%)
occurred within a few seconds (up to 5-10 seconds) and were accompanied by exothermal effects.
The phase composition of the reaction products of the sulphides of the yttrium subgroup of rare
earth elements with hydrofluoric acid is determined by the reaction conditions and the RE+3 radius
r (CN 8). The relationship between the modification of the initial sulphide and the phase composition
of the reaction products was not established.
3.2.
X-ray analysis
5

Two different patterns of interaction of RE sulphides of the yttrium subgroup with an
aqueous solution of hydrofluoric acid (49%) are established by the XRD method.
In the first case, diffraction patterns of samples contain fuzzy reflections of a rhombic
structure of the β-YF3 type and a “halo” at d = 4.135-2.690; 2.157-1.690 Å (Figure 1a). This picture
is observed for RE (Tb - Er, Y). It can be assumed that the product of the reaction is fluoride of RE
(Tb-Er, Y) and an X-ray amorphous phase, based on the XRD analysis. The intensity of the halo in
the RE series naturally increases. Previously, the X-ray amorphous phase was obtained only for
NdF3 · nH2O [30].
In the second case, the diffraction patterns of the samples are identical to the diffraction
patterns of compound (H3O)Yb3F10 ∙ H2O [48] (Figure 1b) and are identified to be in the cubic
̅
system 퐹푑3푚.
Cell
parameters
a((H3O)Tm3F10
∙
1.7H2O)
=
15.380
Å,
a((H3O)Yb3F10 ∙ 1.0H2O) = 15.319 Å, a((H3O)Lu3F10 ∙ 0.9H2O) = 15.263 Å are comparable with
the data of [33, 38, 49]. Impurity reflexes were observed in the XRD data of samples of compounds
(H3O)RE3F10 ∙ nH2O (Tm, Yb). The elements Tm (4f136s2) and Yb (4f146s2) tend to exhibit a valence
in compounds of two. Most likely, the impurity phase peaks are associated with the formation of a
solid solution of the composition (H3O)(RE+3,RE+2)3F10 ∙ nH2O. This aspect has not been studied in
this paper.
The X-ray diffraction patterns of REF3 (Tb-Ho) show a significant expansion of reflections,
which disappears after the thermal treatment. Expansion of diffraction peaks is due to the small
size of the crystallites. According to Scherer’s formula, crystallite sizes for all possible reflections
of fluorides of RE (Tb-Ho, Y) were calculated (Table 2). The X-ray diffraction pattern contains
three series expansions, thereby presenting the likely shape of the particle as an ellipse with a
conventional dimensions D × 2D × 4D, where D ranges from 45 (Tb) and 70 (Ho) nm (Table 2).
The X-ray amorphous phase, the compounds with the composition (H3O)RE3F10 ∙ nH2O,
the hydrates of fluorides of RE REF3 ∙ nH2O at 400 °C and higher are thermally dissociated to
form the fluorides of RE of the β-YF3 structure. Cell parameters of REF3 compounds are consistent
with the JCPDS data (Table 3).
Oxygen-containing impurities REOF, RE2O3 are not found, indicating the absence of RE
hydroxides in the samples of fluorides obtained by the above described procedure.
3.3.
Thermal characterization
Samples of fluorides of RE (Tb-Er, Y) and compounds of zeolite type (H3O)RE3F10 (Tm-
Lu) show an endothermic effect in the 50-175 °C range (Figure 2). This is ascribed to the loss of
adsorbed water. The form of the thermal effect indicates the gradual removal of adsorbed water
from the surface of the solid phase. The enthalpy of the effect in a list of compounds of RE
gradually increases (Table 4). The magnitude of the endothermic effect in the RE series increases,
6

indicating an increase in the strength of the bond between the water molecules and fluoride
compounds.
Thermal experiments unambiguously determine the adsorbed nature of water in the test
samples. The water present in the samples in the form of a surface film (wetness) gives rise to a
separate peak of the non-invariant phase transition H2O(liquid) - H2O(vapor), at 100 °C. The boiling
peaks of water were observed on the undried samples.
The exothermic effect is manifested for RE fluorides in the 200-270 °C range. The
magnitude of the effect increases in the series of Tb-Er compounds (Table 4). Most likely, the
exothermic effect is caused by the transition of the X-ray amorphous phase to the polycrystalline
phase of REF3 and these results are consistent with the XRD results. X-ray analysis showed that
the maximum amount of X-ray amorphous phase was observed for ErF3. After heat treatment at
above 200-250 °C, the X-ray characteristics of the samples changed abruptly with the halo
decreasing, and the reflexes becoming more narrow. The masses of RE fluorides stop decreasing
at the heating temperatures of 410 °C (Tb), 410 °C (Dy), 425 °C (Y), 420 °C (Ho), and 320 °C
(Er). Owing to thermal dissociation, compounds REF3 ST β-YF3 are formed, the crystal chemical
parameters of which are in complete agreement with the JCPDS database data (Table 3).
In the series of fluorides of RE containing adsorbed water, there is a tendency to increase
the content of water molecules per formula unit REF3. The composition of freshly prepared
samples varies from TbF3 ∙ 0.5H2O to ErF3 ∙ 0.9H2O (Table 4). In parallel experiments, the
composition of REF3 ∙ nH2O varies within the range n = ± 0.1, but for all the samples obtained, n
<0.9. When the samples of REF3 ∙ nH2O are held in air, two opposite processes occur, depending
on the water vapour partial pressure: moisture absorption by freshly prepared samples and
weathering of water molecules.
Thermal analysis of samples of compounds (H3O)RE3F10 · nH2O showed two conjugate
effects, the first of which is endothermic and the second is exothermic (Figures 2 b-d, Table 4).
The temperatures of the endothermic effect for samples (H3O)RE3F10 · nH2O (Tm, Yb) practically
coincide with the temperatures at which the sample acquires a mass close to the HRE3F10
composition. It could be assumed that the endothermic effect energy is spent on the expansion of
the zeolite type compound with the remaining dissociation of H2O molecules to HRE3F10
composition. Formation of a polycrystalline structure of REF3 determines the manifestation
exothermic effect. The general scheme of the thermally driven transformation of the
o(H3O)RE3F10 · (1+n)H2O compounds is given by the following:
(H3O)RE3F10·(1+n)H2O→~50−170°퐶(H3O)RE3F10→~200−350°퐶HRE3F10~→350−420°퐶3REF3.
7

Samples of the zeolite type compounds dried at 100 °C lost an average of 7.0 wt. % upon
thermal dissociation to 3REF3 (Figures 2 b-d). The maximum mass loss was shown by the
compound containing thulium and reached 9.2 wt.%. For compounds with stoichiometric
composition (H3O)RE3F10 ∙ nH2O, the weight losses were 7.69. 7.51 and 7.45 wt. %, for RE = Tm,
Yb and Lu, respectively. The composition of synthesized fluoride compounds (Table 4) under the
assumption of high content of adsorbed H2O molecules was determined. A trend of decreasing
number of H2O molecules in zeolite type compounds was found with n varying from 1.7 for
RE=Tm 0.9 for Re=Yb. As the drying temperature of the sample increases, the amount of nH2O
molecules in the zeolite type compound decreases.
3.4.
Morphology characterization
As a result of the reaction of RE sulphides with hydrofluoric acid, the particle morphology
varies greatly. The powders for γ-Tb2S3 show a coral-like grained structure, formed by fused
elongated ovals with dimensions of 0.4 × 0.7 μm (Figure 3a). The precipitates of REF3 (Tb-Er, Y)
fluorides have spherical grains with the size of less than 1 micron, combining into agglomerates
of 2-7 μm. It is not possible to distinguish the differences in the grain characteristics of the
crystalline and X-ray amorphous phases or to obtain clear images of nanosized particles (Figure
3b). The characteristics of the sediment surface did not allow us to obtain clear nanoscale images.
A similar morphology for X-ray amorphous phases was also observed for sulphates of RE, for
example, Eu2(SO4)2 [51].
The presence of a “film” of the water phase on the particles was not detected in the samples
of RE fluorides containing adsorbed water (Figure 3b). A water film is present in the raw samples
and appears in the SEM image in separate areas in the form of a vague enveloping sphere, usually
encapsulating several particles and/or agglomerates.
The particles of the (H3O)Tm3F10 ∙ 1.7 H2O compound have the shape of a regular
octahedron with faces of 0.7-1.5 μm size and 4-8 μm particle size (Figure 3c). With a further
decrease in the ionic radius, the particle shape becomes close to a sphere. The compounds
(H3O)Yb3F10 ∙ 1.0 H2O, (H3O)Lu3F10 ∙ 0.9 H2O are present in the form of globules with a diagonal
of 0.5-1.0 μm (Figure 3d). In the spherical particles, the octahedral motif can be traced.
3.5.
IR spectra
The IR spectra of anhydrous REF3 samples contain characteristic absorption bands in the
400-500 cm-1 range caused by the RE-F bond vibration (Figure 4). Samples of RE fluorides
containing adsorbed water and zeolite type (H3O)RE3F10 · nH2O compounds have characteristic
absorptions of bond stretching (3000-3800 cm-1) and deformation (1630, 1440 cm-1) vibrations of
OH groups [52]. As the compounds of composition (H3O)RE3F10 · nH2O are heat treated and the
8

H2O molecule content decreases, the intensity of the peaks for the OH-group oscillations
decreases. The REF3 · nH2O compounds show characteristic absorption peaks in the 800-1200 cm-
1
range. The phase composition of one of the synthesized samples was determined by IR
spectroscopy as (H3O)Yb3F10 · nH2O + YbF3 · nH2O. The X-ray patterns of the sample show
diffraction maxima of the zeolite type compound and the halo of the X-ray amorphous phase. In
the IR spectrum of the sample, characteristic absorption lines of the zeolite structure as well as
absorption lines of 800-1200 cm-1 REF3 · nH2O are observed. The YbF3 · nH2O phase in the sample
is present in the X-ray amorphous state (Figure 4). Peaks for the vibrations of RE-OH groups
require detailed study, because there is little information on the investigations of the compounds
that are the subject of the article by the IR spectroscopy.
3.6.
The interactions of RE2S3 (Tb-Er) compounds for different ionic sizes varying from
= 1.040 to rEr3+
= 1.004 nm with an aqueous solution of HF (49%) lead to the
Reaction steps
rTb3+
(CN 8)
(CN 8)
formation of REF3 · nH2O with the β-YF3 structure type as described by reaction (1) (Table 4).
RE2S3 + 6HF + nH2O = 2REF3·nH2O↓+ 3H2S ↑ (1)
The precipitates of RE fluorides of (RE=Tb-Er) obtained by reaction (1) with different heat
treatments retain the β-YF3 structure type. It should be concluded that products of reaction (1) are
not crystallohydrates and H2O molecules are not a part of the structure of fluorides but are adsorbed
on the surface of the grains [53]. Water molecules are bound to the solid phase only by van der
Waals forces and the HO-H...F-RE2+ hydrogen bonds.
The REF3 ∙ nH2O composition depends on the following synthesis parameters: acid
concentration, the mass ratio of the sulphide sample and the hydrofluoric acid solution, the
synthesis and drying temperatures, and the storage conditions of the samples. The synthesis
parameters are specified in the methods section.
The reaction of the s Tm2S3, Yb2S3, Lu2S3 (rRE3+(CN 8), Table 4) compound with
hydrofluoric acid proceeds by reaction (2) and leads to the formation of zeolite type
(H3O)RE3F10 · nH2O compounds.
3RE2S3 + 20HF + (2+2n)H2O = 2(H3O)RE3F10·nH2O↓+ 9H2S ↑ (2)
According to [31], the (H3O)RE3F10·H2O composition, the resulting interaction between the
aqueous solution of oxalate rare earth elements with hydrofluoric acid (49%) is formed for rare
earth elements with rRE3+(CN 8) smaller than 1.004 Å (Er -Lu, Y). Attempts to obtain a compound
involving yttrium and erbium ions by reaction (3) were not successful. Reducing the synthesis
temperature to 0 °C and processing temperature to 25 °C did not lead to a change in the results. At
9

different synthesis temperatures, the products of the interaction of Y2S3 and Er2S3 with an aqueous
HF solution are the REF3 ∙ nH2O and the X-ray amorphous phase.
4. Conclusions
The phase compositions of the products of the reactions of the RE2S3 (Tb-Lu, Y)
compounds with an aqueous HF solution (49%) were established. The reactions were carried out
at 21-25 °C, and the precipitate was decanted and dried at 90-100 °C. Rare earth element fluorides
with the compositions TbF3 · 0.5H2O, DyF3 · 0.5H2O, YF3 · 0.5H2O, HoF3 · 0.8H2O, ErF3 · 0.9H2O
were obtained. Upon heating, more than 2/3 of crystalline water passes into the gas phase in the
50-170 °C temperature range. This process is accompanied by an endothermic effect, the enthalpy
of which in the RE series naturally increases. In the 200-270 °C temperature range, the exothermal
effect of the formation of the polycrystalline phase of REF3 is manifested. For the RE =Tm, Yb,
Lu, zeolite type compounds with the δ-KYb3F10 · nH2O type structure were obtained:
(H3O)Tm3F10 · 1.7H2O, a = 15.380 Å; (H3O)Yb3F10 · 1.0 H2O, a = 15.319 Å;
(H3O)Lu3F10 · 0.9 H2O, a = 15.263 Å. The particles of the zeolite type compound have the form
of regular tetrahedra or a spherical shape with dimensions of 0.7 × 1.5 μm. Thermal evolution
profiles of the zeolite phases samples contain endothermic effects at 50-170 °C accompanied by a
transition of water to the gas phase, as well as exothermic effects at 200-360 °C corresponding to
the formation of polycrystalline REF3. The REF3 (ST β-YF3) phases are the final products of
thermal treatments of the REF3 ∙ nH2O and (H3O)RE3F10 ∙ nH2O compounds. IR spectra of RE
fluorides containing adsorbed water and zeolite type compounds contain peaks for bond stretching
(3000-3800 cm-1) and deformation (1630, 1440 cm-1) vibrations of OH groups.
Acknowledgements
The authors express their gratitude to Mrs. Sofia A. Basova for the preparation of this manuscript.
10

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13

Figure 1. XRD patterns of the samples, obtained at 100 °C
a): 1- TbF3 ∙ 0.5H2O; 2- DyF3 ∙ 0.5H2O; 3- HoF3 ∙ 0.8H2O; 4- YF3 ∙ 0.5H2O; 5- ErF3 ∙ 0.9H2O;
b): 1- (H3O)Tm3F10 ∙ 1.7H2O; 2- (H3O)Yb3F10 ∙ 1.0H2O; 3- (H3O)Lu3F10 ∙ 0.9H2O; 4-LuF3,
obtained at 800 °C
14

a
b
c
d
Figure 2. TG, DSK curves for samples dried at 100 °C:
(a) ErF3 ∙ 0.5H2O, (b) (H3O)Tm3F10 ∙ 1.7H2O, (c) (H3O)Yb3F10 ∙ 1.0H2O,
(d) (H3O)Lu3F10 ∙ 0.9H2O.
15

a
b
c
d
Figure 3. SEM images of particles of powders of phase compositions: (a) Tb2S3, (b)
TbF3 ∙ 0.5H2O, (c) (H3O)Tm3F10 ∙ 1.7H2O, (d) (H3O)Lu3F10 ∙ 0.9H2O.
16

Figure 4. IR spectrum of fluoride compounds
17

Table 1. Crystalline chemical characteristics and the content of sulphide sulphur in the powders
of RE2S3 compounds.
RE2S3
Sp.gr. № JCPS Cell parameters, Å
EDS (wt.%)
a
b
-
-
c
-
-
α/β°
-
-
RE(exp.) S(exp.) S(theor)
γ-Tb2S3
I43d
26-1424 8.349
26-0594 8.298
76.6
77.3
23.4
22.7
35.0
23.23
22.84
35.10
γ-Dy2S3 I43d
δ-Y2S3
δ-Ho2S3 P21/m 73-0466 17.476 4.002 10.149 98.18 77.3
δ-Er2S3 P21/m 81-0926 17.427 3.979 10.097 98.67 77.5
δ-Tm2S3 P21/m 44-1157 17.369 3.956 10.044 98.65 78.0
P21/m 79-2250 17.486 4.079 10.027 97.86 65.0
22.7
22.58
22.5
22.0
21.6
21.7
22.33
22.16
21.75
21.56
ε-Yb2S3 R3c
ε-Lu2S3 R3c
82-1182 7.208
85-0744 7.221
-
-
-
-
55.82 78.4
55.81 78.3
18

Table 2. Calculation of the size of the area of coherent scattering.
REF3
D, nm
1 0 1
43
43
43
0 2 0
188
169
184
262
1 1 1
49
45
57
77
2 1 0
79
87
102
118
1 2 1
93
90
92
109
TbF3
DyF3
YF3
HoF3
62
19

Table 3. Cell parameters of REF3 (ST β-YF3), obtained at an annealing temperature of 400 °C.
REF3
№ JCPDS
Cell parameters, Å
a
b
c
TbF3
DyF3
YF3
HoF3
ErF3
TmF3
YbF3
LuF3
84-0179
32-0352
74-0911
84-0178
86-0710
32-1352
74-2178
32-0612
6.496
4.472
6.379
6.402
6.344
6.278
6.213
6.149
6.932
6.919
6.874
6.875
6.835
6.814
6.782
6.762
4.377
4.383
4.409
4.371
4.378
4.411
4.430
4.476
20

Table 4. Phase composition of the starting compounds, the reaction (RE2S3+HF) products, thermal
properties of fluoride of rare earth compounds.
Composition of Thermal effects in the sample, heated to 500 °C
rRE3+, samples
[50]Å reaction, dried at
100 °C
of
Thermal effects, °C
RE
Enthalpy, J/g
ΔH1 ΔH2 ΔH3
T1
T2
T3
type
A*
n
1.040
1.027
1.019
1.015
0.5
0.5
0.5
0.8
50-175
50-180
50-150
40-140
-
-
-
-
200-220 23.6
210-240 27.0
270-305 45.0
215-255 50.0
-
-
-
-
-0.8
-1.5
-12.0 5.86
-50.2 5.79
-64.3 6.46
3.76
4.00
Tb
Dy
Y
Ho
1.004
0.994
0.985
0.977
0.9
1.7
1.0
0.9
55-160
-
230-270 97.8
-
Er
65-155 170-225 225-270 51.1
70-175 280-360 360-420 55.2
60-175 270-350 365-425 64.5
23.4 -18.8 8.01
18.3 -38.0 5.77
6.0
Tm
Yb
Lu
B**
-57.5 6.07
A* REF3·nH2O
B** (H3O)RE3F10·nH2O
21