Formation and thermal stability of selenites and hydrogen selenites of samarium

Article abstract of DOI:10.1007/s10973-005-0470-0

The solubility isotherm of the Sm₂O₃-SeO ₂-H₂O system was studied at 100°C. The two compounds obtained in the three-component system were identified by the Schreinemakers' method as well as by chemical, thermoan

Full text of DOI:10.1007/s10973-005-0470-0

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 1057–1063
Short Communication
FORMATION AND THERMAL STABILITY OF
SELENITES AND HYDROGEN SELENITES OF
SAMARIUM
G. G. Gospodinov1* and M. G. Stancheva2
1Prof. Assen Zlatarov University, 8010 Bourgas, Bulgaria
2Technological College, 7200 Razgrad, Bulgaria
(Received Oktober 14, 2003; in revised form January 26, 2004)
Abstract
The solubility isotherm of the Sm2O3–SeO2–H2O system was studied at 100°C. The two compounds
obtained in the three-component system were identified by the Schreinemakers’ method as well as
by chemical, thermoanalytical and X-ray diffraction analyses after their isolation in pure state.
Keywords: phase state, samarium selenites, solubility isotherm, thermal degradation, X-ray
Introduction
The first data concerning the selenites of samarium can be found in the works of
Espil and Cleve [1, 2], who obtained normal and hydrogen compounds with different
composition.
For some time past, a number of samarium salts have been obtained.
Sm2(SeO3)3×5H2O was synthesized by different authors [3, 4]. Other authors [5] ob-
tained Sm2(SeO3)3×H2SeO3 by a reaction between solutions of selenious acid and sa-
marium nitrate. Synthesis of SmH(SeO3)2×2H2O was described in [6]. Another
reference [7] concerns the synthesis of SmH(SeO3)2×2.5H2O whose parameters of the
crystal lattice were determined. Later, Koskenlinna et al. [8] obtained
SmH(SeO3)2×2H2O from aqueous solutions of SeO2 and SmCl3 with a wide range of
*
Author for correspondence: E-mail: ggospodinov@btu.bg
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

1058 GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM
concentrations. The crystal system and the parameters of this product were deter-
mined. Savchenko et al. [9] obtained normal and hydrogen salts with different com-
position and studied their thermal properties. A larger review on inorganic complex
compounds of the rare earths and structures of the rare earths selenites is given
in [10, 11].
The above review leads to the conclusion that the preparative methods used by
the authors do not settle the question about the synthesis of all possible selenites of
samarium.
The data concerning all possible phases in the three-component system
Sm2O3–SeO2–H2O can be obtained by studying and drawing the solubility isotherm,
and this is the aim of the present study.
Experimental
Sm2O3 with the purity of 99.99 mass% (Aldrich, Germany) and SeO2, obtained by the
authors by oxidation of high-purity Se with nitric acid, were used to prepare the sys-
tem. SeO2 was subjected to additional triple sublimation to achieve better purification.
In order to study the Sm2O3–SeO2–H2O at 100°C, 20 samples were prepared
each containing 2 g Sm2O3 and varying concentrations of selenious acid (from 0
to 85 mass%). The samples were sealed in glass ampoules (‘Rasotherm’) and placed
in an air thermostat at 100±0.5°C for two months. To determine the necessary time
for reaching chemical equilibrium, kinetic curves were obtained. For that purpose,
more ampoules with the same composition were prepared and opened periodically.
Equilibrium was reached when chemical analysis showed that the liquid and the solid
phases did not change their composition. It was considered that crystallographic
equilibrium was established when the peak intensities and the interplanar distances in
the X-ray patterns no longer changed. After chemical and X-ray equilibrium was
reached, the liquid and the solid phases were separated at the experimental tempera-
ture and subjected to chemical, thermal and X-ray phase analyses.
Chemical analysis for samarium ions was made by reverse complexometric titration
using xylenol orange as an indicator [12], and SeO23– ions were analyzed iodometrically
and gravimetrically [13]. The concentration of Sm3+ in the liquid phase was determined
spectrophotometrically on a Spekol-11 apparatus (Carl Zeiss Jena, Germany) using
pyrocatecholviolet as an indicator.
X-ray phase analysis was made with a URD-6 apparatus (Carl Zeiss Jena, Ger-
many) at Cu anode and Ka-emission and a nickel filter for b-emission. Thermal analy-
sis was carried out using an OD-102 derivatograph (MOM, Hungary). The operating
conditions of the thermal analysis are the temperature range from 20 to 1000°C, heat-
ing rate 5°C min–1, sample mass 250 mg, thermocouple Pt/PtRh, standard substance
a-Al2O3, in a medium of chemically pure nitrogen using metalloceramic crucibles.
J. Therm. Anal. Cal., 78, 2004

GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM 1059
Fig. 1 Solubility isotherm of the system Sm2O3–SeO3–H2O at 100°C;
1 – Sm2(SeO3)3×4H2O and 2 – SmH(SeO3)2×2H2O
Table 1 Solubility isotherm of the system Sm2O3–SeO2–H2O at 100°C
Liquid phase/mass%
Solid phase/mass%
No.
Formula composition
Sm2O3
SeO2
Sm2O3
SeO2
1
2
2.0×10–3
3.2×10–3
4.8×10–3
5.0×10–3
7.2×10–3
7.3×10–3
1.0×10–2
1.5×10–2
2.7×10–2
2.7×10–2
3.2×10–2
3.9×10–2
4.2×10–2
6.2×10–2
7.4×10–2
8.2×10–2
9.2×10–2
9.6×10–2
0.19
0.62
36.80
37.28
38.10
27.16
39.26
34.56
34.50
46.89
34.87
34.18
36.40
33.10
26.49
25.92
28.82
29.21
28.47
30.42
37.91
47.04
45.80
36.94
49.51
46.47
46.89
49.27
47.25
49.32
49.95
49.53
52.56
54.70
54.90
56.62
57.11
57.05
Sm2(SeO3)3×4H2O
Sm2(SeO3)3×4H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)2×2H2O
SmH(SeO3)3×2H2O
3
0.62
4
4.76
5
10.27
15.88
18.97
25.22
29.13
38.92
44.27
46.04
55.96
65.50
68.85
74.45
78.62
82.54
6
7
8
9
10
11
12
13
14
15
16
17
18
J. Therm. Anal. Cal., 78, 2004

1060 GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM
Results and discussion
The results from studying the Sm2O3–SeO2–H2O are given in Table 1 and Fig. 1.
As figure shows, at 82.54 mass% of liquid SeO2 two compounds are obtained in
the system: Sm2(SeO3)3·4H2O (congruently soluble) and SmH(SeO3)2·2H2O (incon-
gruently soluble). A great part of the diagram consists of the field of crystallization of
SmH(SeO3)2·2H2O. The composition of the eutonic point between Sm2(SeO3)3·4H2O
and SmH(SeO3)2·2H2O is: 0.62 mass% of liquid SeO2 and 4.8×10–3 mass% of Sm2O3.
The presence of two compounds in the three-component system was confirmed
by X-ray data.
Figure 2 shows the derivatogram of Sm2(SeO3)3·4H2O. Heating the compound,
dehydration takes place by stages in the temperature interval 100–310°C, 4 mole of
H2O are liberated, and anhydrous Sm2(SeO3)3 is obtained. This salt is thermally stable,
and crystallization processes start at 360–450°C. This fact can be explained by a con-
siderable amorphisation of the product due to dehydration and its crystallization in the
above temperature interval. An X-ray diagram of a sample of Sm2(SeO3)3, heated
at 340°C, shows that the compound is amorphous, while at 450°C it is already crystal-
Fig. 2 Thermoanalytical curves of Sm2(SeO3)3×4H2O
J. Therm. Anal. Cal., 78, 2004

GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM 1061
Fig. 3 Thermoanalytical curves of SmH(SeO3)2×2H2O
line. Normal selenite is thermally stable at 450°C. At a higher temperature, decomposi-
tion takes place accompanied by liberation of 1 mole of SeO2 and Sm2O3·2SeO2 is ob-
tained. The mass loss is 23.58 mass% (theoretical calculation is 24.28 mass%).
In the temperature interval 665–895°C, Sm2O3·2SeO2 loses one additional mole
of SeO2 and turns into another basic salt – Sm2O2·SeO3. The last stage of decomposi-
tion takes place at 1000°C, and a molecule of SeO2 is liberated. The mass loss
is 50.01 vs. 53.72 mass% calculated theoretically.
Figure 3 presents the thermoanalytical curves of SmH(SeO3)2·2H2O.
The salt is thermally stable at 160°C. In the temperature interval 160–270°C it
loses 8.36 mass% due to the loss of 4 moles of crystallization water vs. 8.18 mass% cal-
culated theoretically. In the temperature interval 270–310°C the hydrogenselenite
loses its constitutional water and turns into Sm2(SeO3)3·SeO2. The mass loss compared
to the initial mass is 10.43 mass% (theoretical calculation is 10.20 mass%). Liberation
of 1 mole of SeO2 from tetraselenite takes place at 390°C. At 390–490°C the salt
loses 36.95 mass% from its mass (theoretical calculation is 35.34 mass%) and turns
into a basic salt Sm2O(SeO3)2. In the temperature interval 580–760°C the salt loses yet
J. Therm. Anal. Cal., 78, 2004

1062 GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM
J. Therm. Anal. Cal., 78, 2004

GOSPODINOV, STANCHEVA: SELENITES AND HYDROGEN SELENITES OF SAMARIUM 1063
another mole of SeO2 and a basic salt with composition Sm2O2×SeO3 is formed. Com-
plete decomposition to Sm2O3 takes place at a temperature higher than 1000°C. The
mass loss at 1050°C is 59.57 vs. 60.48 mass% calculated theoretically.
The decomposition stages for both salts were confirmed by identifying the inter-
mediate decomposition products obtained and also by chemical and X-ray phase
analysis of these products.
The lattice parameters of the unit cells of the selenites obtained from the three-
component system and those obtained by their thermal decomposition were determined
(Table 2) with an exception of Sm2(SeO3)3·4H2O. Our calculation for the parameters of
the unit cells of SmH(SeO3)2·2H2O are in good agreement with those reported in [8].
This paper represents continuation of our investigation on the three-component
systems of the type Ln2O3–SeO2–H2O [14–22].
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