Physicochemical study of the compounds in the three-component system Sc2O3-SeO2-H2O AT 100°C

Article abstract of DOI:10.1023/A:1025118532216

The selenites of scandium are used as new materials in producing instrument and semi-conductor engineering. Like tellurites, they can also be applied as materials for making lasers. The process of producing scandium selenides by the reduction of the corresponding selenites has the advantage of taking place at relatively low temperatures and high rate. In this respect, it is necessary to command methods of obtaining high-purity selenites and to obtain data concerning their thermal stability. A physicochemical study of the system Sc₂O₃-SeO₂-H₂O was carried out at 100°C in order to find solutions to these problems. The possible compounds in the system at a given temperature were obtained. The compounds were identified by the Schreinemakers' method and also by chemical and X-ray phase analyses. The mechanism of thermal decomposition was determined by means of a derivatograph.

Full text of DOI:10.1023/A:1025118532216

Journal of Thermal Analysis and Calorimetry, Vol. 73 (2003) 315–321
PHYSICOCHEMICAL STUDY OF THE COMPOUNDS
IN THE THREE-COMPONENT SYSTEM
Sc2O3–SeO2–H2O AT 100°C
G. G. Gospodinov1* and M. G. Stancheva 2
1Prof. Assen Zlatarov University, Bourgas 80 10, Bulgaria
2Technological College, Razgrad 7200, Bulgaria
(Received November 19, 2002; in revised form March 8, 2003)
Abstract
The selenites of scandium are used as new materials in producing instrument and semi-conductor en-
gineering. Like tellurites, they can also be applied as materials for making lasers. The process of
producing scandium selenides by the reduction of the corresponding selenites has the advantage of
taking place at relatively low temperatures and high rate. In this respect, it is necessary to command
methods of obtaining high-purity selenites and to obtain data concerning their thermal stability.
A physicochemical study of the system Sc2O3–SeO2–H2O was carried out at 100°C in order to
find solutions to these problems.
The possible compounds in the system at a given temperature were obtained. The compounds
were identified by the Schreinemakers’ method and also by chemical and X-ray phase analyses. The
mechanism of thermal decomposition was determined by means of a derivatograph.
Keywords: scandium selenites, solubility isotherm, thermal analysis, X-ray pattern
Introduction
Nilson [1] first obtained amorphous scandium selenite by mixing equimolar solutions
of scandium sulphate and sodium selenite. By heating it with selenious acid, scan-
dium hydrogen selenite was obtained. Mayer et al. [2] obtained X-ray amorphous
scandium selenite, calculated its enthalpy of formation and studied its thermal de-
composition. Other authors [3] obtained Sc2(SeO3)3·5H2O by two methods: neutral-
ization of a suspension of scandium hydroxide by a solution of selenious acid and
also by an exchange reaction of a soluble scandium compound with a solution of al-
kaline selenite. The solubility of scandium selenite in a solution of H2SeO3 at 25°C
was studied in reference [4]. By the Schreinemakers’ method it was found that
Sc2(SeO3)3·5H2O and Sc(HSeO3)3 were obtained.
Sc2O(SeO3)2·3H2O was obtained by mixing a solution of H2SeO3 and a suspension
of Sc(OH)3 in equimolar ratios [5]. The same authors suggested that ScH(SeO3)2·H2O
should be obtained by mixing a solution of ScCl3 containing NaHSeO3 in a tenfold ex-
*
Author for correspondence: stanislav@btu.bg
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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GOSPODINOV, STANCHEVA: Sc2O3–SeO2–H2O AT 100°C
cess. Scandium diselenite ScH(SeO3)2·H2O is a white crystalline substance slightly solu-
ble in water.
Other authors [6] obtained anhydrous Sc(HSeO3)3 and determined the parame-
ters of its crystal lattice.
The aim of the present work was to draw the solubility isotherm of the system, to
determine the equilibrium fields of the compounds obtained and their composition,
and also to determine and describe the mechanism of thermal decomposition of these
compounds by derivatograph analysis.
Experimental
Materials
Sc2O3 with purity of 99.9999 mass% and H2SeO3 with purity of 99.999 mass% were
used as initial compounds.
Methods
In order to study the system Sc2O3–SeO2–H2O at 100°C, samples were prepared in quartz
ampules, each containing 2 g of Sc2O3 and varying concentrations of H2SeO3 (from 0 to
85%). The samples were placed in an air thermostat at 100±0.5°C. They were periodi-
cally stirred. In order to determine the time needed to reach chemical equilibrium, kinetic
curves of equilibrium were obtained. For that purpose, more samples with the same
chemical composition were prepared and opened periodically. Equilibrium was consid-
ered to have been reached when the liquid and solid phases no longer changed their com-
position (determined by chemical analysis). Crystallographic equilibrium was reached
when the intensity and interplanar distances of the X-ray patterns of the solid phases did
not change any more. When equilibrium was reached, the ampules were opened, the liq-
uid and solid phases were separated at the experimental temperature and were subjected
to chemical, thermal and X-ray phase analyses.
Scandium was identified as Sc2O3 complexometrically [7], and SeO2 was ana-
lysed iodometrically and gravimetrically [8].The concentration of Sc3+ in the liquid
phase was determined spectrophotometrically on a Spekol-11 apparatus (Carl Zeiss,
Jena, Germany) [9].
X-ray phase analysis was carried out on a URD-6 apparatus (Germany) at Cu anode
for Kα-emission and a nickel filter for β-emission. An OD-102 derivatograph (MOM,
Hungary) was used for thermal analysis by heating the phases from 25 to 1000°C at a rate
5°C min–1 in ceramic crucibles. α-Al2O3 was used as a standard substance.
Results and discussion
The results from studying the system Sc2O3–SeO2–H2O at 100°C are presented in
Fig. 1 and Table 1. Schreinemakers data show that 2 solid phases crystallize in this
system: Sc2(SeO3)3·4H2O and Sc2(SeO3)3·3H2SeO3. The crystallization field of
J. Therm. Anal. Cal., 73, 2003

GOSPODINOV, STANCHEVA: Sc2O3–SeO2–H2O AT 100°C
317
Fig. 1 Solubility isotherm of the system Sc2O3–SeO2–H2O at 100°C
Sc2(SeO3)3·3H2SeO3 takes most of the diagram – from 0.75 to 83.20% of SeO2 in the
liquid phase. The eutonic point between Sc2(SeO3)3·4H2O and Sc2(SeO3)3·3H2SeO3 is
at 0.75 mass% of SeO2 and 2.1·10–3 mass% of Sc2O3.
Table 1 Solubility isotherm of the system Sc2O3–SeO2–H2O at 100°C
Liquid phase/mass%
Solid phase/mass%
No.
Formula composition
Sc2O3·103
SeO2
Sc2O3
SeO2
1
2
2.50
2.80
3.30
3.40
3.68
3.78
4.22
4.60
4.79
5.10
5.56
6.24
6.85
0.40
0.75
24.52
21.60
15.52
14.25
13.50
15.08
13.20
12.22
12.40
14.25
12.12
14.33
9.13
59.20
51.59
73.80
71.10
69.47
73.54
71.75
69.10
72.50
75.56
75.38
77.12
79.90
Sc2(SeO3)3·4H2O
Sc2(SeO3)3·4H2O
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
Sc(HSeO3)3
3
0.75
4
2.53
5
10.68
18.23
33.77
42.11
50.68
61.34
68.34
78.07
83.20
6
7
8
9
10
11
12
13
J. Therm. Anal. Cal., 73, 2003

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GOSPODINOV, STANCHEVA: Sc2O3–SeO2–H2O AT 100°C
Thermal curves of Sc2(SeO3)3·4H2O is shown in Fig. 2. Normal scandium sele-
nite is thermostable at 100°C and at that temperature adsorption water is completely
liberated. In the temperature interval 100–280°C (the endothermal peak has a mini-
mum at 195°C) one-stage liberation of 4 moles of crystalline water takes place. The
mass loss during that transformation is 13.63 mass% compared with 14.44 mass%
calculated from the TG curve. The deference of 0.81% is because of the liberation of
adsorption water in the first stage of the process. The exothermal peak at 660°C is due
to crystallization of the X-ray amorphous product. The X-ray pattern of the product
heated at 400–630°C shows that it is amorphous, while the same product heated over
630°C is crystalline. Then, 2 moles of SeO2 are liberated at 2 stages resulting in the
consecutive formation of Sc2O3·2SeO2 (Sc2O(SeO3)2) and an oxoselenite with com-
position (ScO)2SeO3. The latter reaction takes place at 920°C, the temperature at
which the selenite is heated. The mass loss for the formation of this oxoselenite is
54.51 mass% corresponding to 55.0 mass% calculated from the TG curve. After heat-
ing the selenite for 2–3 h at 1100–1200°C, SeO2 is completely liberated and Sc2O3 is
obtained as a pure phase.
Fig. 2 Simultaneous TG and DTA curves of Sc2(SeO3)3·4H2O
Figure 3 shows the thermal curves of Sc2(SeO3)3·3H2SeO3. The selenite is thermo-
stable at 215°C. At 215–335°C, 3 moles of constitutionally bound water are liberated and
it turns into Sc2(SeO3)3·3SeO2. The TG curve shows that mass loss is 6.42 mass% (theo-
retical calculation for this transition is 6.30 mass%). The exothermal peak at 410°C is due
to crystallization of X-ray amorphous scandium hexaselenite. This transformation was
also proved by the X-ray patterns of the products obtained by heating at 380 and 450°C.
Hexaselenite is thermostable at 500°C. Then, SeO2 is liberated at several stages. At
500–580°C, 1 mole of SeO2 is liberated and hexaselenite turns into pentaselenite. The
mass loss is 19.18 mass% (theoretical calculation is 19.24 mass%). Pentaselenite loses 1
mole of SeO2 and at 580–650°C it turns into tetraselenite. The mass loss is 32.02 mass%
J. Therm. Anal. Cal., 73, 2003

Table 2 Chemical analysis of the products of thermal decomposition of scandium selenites
Experimental composition/% Theoretical composition/%
Phase
Standard deviation/S·102
Sc2O3
SeO2
H2O
Sc2O3
SeO2
H2O
Sc2O3
SeO2
H2O
Sc2(SeO3)3·3H2SeO3
Sc2(SeO3)3·3SeO2
Sc2(SeO3)3·2SeO2
Sc2(SeO3)3·SeO2
Sc2(SeO3)3
16.10
17.14
19.91
23.70
29.25
38.32
55.44
99.92
77.63
82.81
80.10
76.30
70.72
61.62
44.57
–
5.57
–
16.08
17.16
19.90
23.71
29.27
38.33
55.42
77.62
82.84
80.10
76.29
70.70
61.67
44.58
–
5.60
–
1.58
3.81
8.51
2.55
3.08
4.95
4.00
3.08
6.08
3.24
2.00
2.00
3.08
4.36
2.64
–
8.03
–
–
–
–
–
–
–
–
–
–
Sc2O3·2SeO2
–
–
–
Sc2O3·SeO2
–
–
–
Sc2O3
–
100
–
–

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GOSPODINOV, STANCHEVA: Sc2O3–SeO2–H2O AT 100°C
Fig. 3 Simultaneous TG and DTA curves of Sc2(SeO3)3·3H2SeO3/Sc(HSeO3)3/
(theoretical calculation is 32.15 mass%). The small endothermal peak at 650–720°C is
due to a mass loss 1 mole of SeO2 and a transition of tetraselenite into normal scandium
selenite. The mass loss is 45.22 mass% (theoretical calculation is 45.14 mass%).
Sc2(SeO3)3 decomposes at several stages, too. At 720–760°C 1 mole of SeO2 is liberated
and oxoselenite Sc2O3·2SeO2 is obtained, which then turns into Sc2O3·SeO2 at
760–845°C. The mass loss is 71.12 mass% (theoretical calculation is 71.04 mass%).
Complete decomposition of selenite and its transformation into Sc2O3 do not take place.
At 940°C, the temperature at which the product is heated, the mass loss is 78.0 mass%
(theoretical calculation is 83.98 mass%).
The mechanism of the thermal decomposition of the two compounds was proved
by gravimetric analysis and also by isolating the intermediate products and subjecting
them to chemical and X-ray phase analyses.
Table 2 presents the data from chemical analysis and the theoretical calculations
for the corresponding compounds. The good agreement between them is a proof of
the correctness of the suggested scheme. X-ray phase analysis also confirms the iden-
tity of the new phases obtained.
Conclusions
The solubility isotherm of the system Sc2O3–SeO2–H2O at 100°C was studied. The com-
pounds in the system are identified by Schreinemakers’ method, and also by chemical,
derivatograph and X-ray phase analyses after isolating them in the pure state.
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