Thermal analysis of the selenites of the ternary system Nd 2O3-SeO2-H2O at 100°C

Article abstract of DOI:10.1023/A:1025890716155

The solubility isotherm of the system Nd₂O₃-SeO ₂-H₂O at 100°C was studied and drawn. All possible selenites of neodymium were obtained and characterized. Thermal decomposition of all phases in the system was st

Full text of DOI:10.1023/A:1025890716155

Journal of Thermal Analysis and Calorimetry, Vol. 73 (2003) 835–841
THERMAL ANALYSIS OF THE SELENITES OF THE
TERNARY SYSTEM Nd2O3–SeO2–H2O AT 100°C
G. G. Gospodinov 1* and M. G. Stancheva 2
1Prof. Assen Zlatarov University, Bourgas 8010, Bulgaria
2Technological College, Razgrad 7200, Bulgaria
(Received September 10, 2002; in revised form January 10, 2003)
Abstract
The solubility isotherm of the system Nd2O3–SeO2–H2O at 100°C was studied and drawn. All possi-
ble selenites of neodymium were obtained and characterized. Thermal decomposition of all phases
in the system was studied and its mechanism was described.
Keywords: Nd2O3–SeO2–H2O, selenites of neodymium, ternary system
Introduction
The selenites of neodymium and other rare-earths have been attracting attention recently
since their reduction in the corresponding gas medium is one of the ways of obtaining
selenides. They possess valuable technical properties. The data concerning the conditions
of obtaining neodymium selenites, their composition and properties are scarce.
The results of a study of the interaction between solutions of NdCl3 with Na2SeO3
and H2SeO3 as well as the solubility of the system Nd2O3–SeO2–H2O at 25°C were re-
ported in [1]. A normal salt Nd2(SeO3)3 and two acid salts, Nd2(SeO3)3 Η2SeO3 3Η2O and
Nd(HSeO3)3 Η2O, were obtained.
A procedure for obtaining anhydrous selenites of neodymium by mixing a
slightly acidic solution of NdCl3 and a solution of Na2SeO3 used in equimolar ratios
was suggested in [2].
An acid salt with composition Nd(HSeO3)3 2.5Η2O was described in [3]. Crys-
talline acid selenite was synthesized by mixing solutions of NdCl3 (containing NH3)
and H2SeO3.
In [4], Nd(HSeO3)(SeO3) 2Η2O was synthesized and its X-ray parameters were
determined.
Experimental
In order to study the solubility isotherm of the system Nd2O3–SeO2–H2O at 100°C,
20 samples were prepared by adding 2 g of neodymium selenite to 50 g of H2SeO3 with
*
Author for correspondence: E-mail: ggospodinov@btu.bg
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

836
GOSPODINOV, STANCHEVA: Nd2O3–SeO2–H2O
different concentrations. The samples were thermostated at 100°C by continuous stirring
for 24 h. The samples were then sealed in glass ampules which were placed in air thermo-
stat at 100+0.5°C. The ampules were periodically shaken. In order to determine the time
needed to reach equilibrium, the kinetic curves of the equilibrium were obtained. Further
samples with the same chemical composition were prepared. Every 10 days, the samples
with the same composition were opened periodically and the solid and the liquid phases
were subjected to chemical analysis. It was shown that after 30 days the composition of
the solid and the liquid phases no longer changed, in other words, chemical equilibrium
was reached. Crystallographic equilibrium (proved by studying the changes in the peaks
lines on the X-ray patterns) was reached for 3 months. After equilibrium was reached, the
samples were taken out of the ampules at the experimental temperature and were filtered
through a G4 filter. Both phases were subjected to chemical analysis, and results were
used to draw the solubility isotherm of the system by the Gibbs-Rosebom method. The
compounds obtained were identified by Schreinemakers’ method, chemical analysis and
X-ray phase analysis.
The Schreinemakers’ method involves graphic measurement of the percentage
of the metal oxide and selenium dioxide at the intersection point of the conodes con-
necting the equilibrium liquid and solid phases in a particular crystallization field.
The intersection point gives the percentage of the metal oxide, SeO2 and H2O in the
ideally dried solid phase. Before chemical analysis, the solid phase was washed with
alcohol and ether in a ratio 1:1. Then it was dried at a room temperature for 4 or 5 h
and subjected to chemical analysis.
X-ray analyses of 5 samples of the solid phase from each crystallization field of the
metal selenite showed that the products from a given crystallization field are identical.
Chemical analysis was carried out by reverse complexometric titration using xy-
lenol orange as an indicator [5], and SeO23– ions were analyzed iodometrically and
gravimetrically [6]. The concentration of Nd3+ in the liquid phase was determined
spectrophotometrically on a Spekol-11 apparatus (Germany) using pyrocatechol-
violet as an indicator.
X-ray phase analysis was carried out on a URD-6 apparatus (Germany) at Cu
anode for Kα-radiation and a nickel filter for β-emission. An OD-102 derivatograph
(MOM, Hungary) was used for thermal analyses. The operating conditions were as
follows: sample mass 250 mg, placed in metalloceramic crucible; temperature range
up to 1200°C; heating rate 10°C min–1; standard substance α-Al2O3; chemically pure
nitrogen as a medium.
Results and discussion
The results from studying the system Nd2O3–SeO2–H2O at 100°C are presented in
Table 1, and the solubility isotherm is shown in Fig. 1.
In this system four selenites crystallize: Nd2(SeO3)3 4H2O, NdH(SeO3)2 2H2O ,
Nd2(SeO3)3 3H2SeO3 and Nd2(Se2O5)3 H2SeO3 2H2O.
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Table 1 Solubility isotherm of the system Nd2O3–SeO2–H2O at 100°C
Liquid phase/mass%
Solid phase/mass%
Formula composition
No.
Nd2O3
SeO2
Nd2O3
SeO2
1
2
5.3 10–3
5.2 10–3
7.5 10–3
8.1 10–3
8.4 10–3
8.7 10–3
8.6 10–3
9.0 10–3
1.0 10–2
1.2 10–2
2.2 10–2
2.4 10–2
2.5 10–2
2.4 10–2
2.6 10–2
3.5 10–2
3.0 10–2
2.7 10–2
2.4 10–2
2.4 10–2
2.6 10–2
2.9 10–2
0.08
0.87
42.80
38.89
36.88
29.50
30.19
32.90
36.10
36.52
32.12
26.30
30.85
26.03
32.78
27.50
29.65
26.09
23.17
26.62
20.62
24.48
23.02
25.04
39.02
37.25
47.38
40.41
42.10
45.38
49.12
49.76
48.22
47.57
50.08
50.63
50.31
61.60
58.90
62.79
63.05
65.90
65.51
67.00
68.10
68.54
Nd2(SeO3)3 4H2O
Nd2(SeO3)2 4H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
NdH(SeO3)2 2H2O
Nd(HSeO3)3
3
0.87
4
5.22
5
7.31
6
15.20
24.30
29.51
39.16
42.27
47.87
49.08
50.63
50.63
52.12
56.65
60.17
60.17
63.55
69.15
73.20
76.86
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Nd(HSeO3)3
Nd(HSeO3)3
Nd(HSeO3)3
Nd2(Se2O5)3 H2SeO3 2H2O
Nd2(Se2O5)3 H2SeO3 2H2O
Nd2(Se2O5)3 H2SeO3 2H2O
Nd2(Se2O5)3 H2SeO3 2H2O
Nd2(Se2O5)3 H2SeO3 2H2O
The eutonic points have coordinates as follows:
The eutonic point between Nd2(SeO3)3 4H2O and NdH(SeO3)2 2H2O is at
5.5 10–3 mass% of Nd2O3 and 0.87 mass% of SeO2. The second eutonic point be-
tween NdH(SeO3)2 2H2O and Nd2(SeO3)3 3H2SeO3 is at 7.8 10–2 mass% of Nd2O3
and 50.63 mass% of SeO2. The eutonic point between Nd2(SeO3)3 3H2SeO3 and
Nd2(Se2O5)3 H2SeO3 2H2O is at 9.3 10–2 mass% of Nd2O3 and 60.17 mass% of SeO2.
The DTA and TG curves of Nd2(SeO3)3 4H2O in Fig. 2 show that decomposition
takes place in stages.
Dehydration of the crystal hydrate starts at 100°C with a minimum at 130°C and
takes place in two stages. Anhydrous Nd2(SeO3)3 is thermally stable at 810°C. At
600–670°C there is an endothermal peak which is not connected with a mass change
in Nd2(SeO3)3. The exothermal peak at T=690°C is not associated with a change in
the mass of the anhydrous selenite either, but is connected with a change in the size of
J. Therm. Anal. Cal., 73, 2003

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GOSPODINOV, STANCHEVA: Nd2O3–SeO2–H2O
Fig. 1 Solubility isotherm of the system Nd2O3–SeO2–H2O at 100°C
Fig. 2 DTA and TG curves of Nd2(SeO3)3 4H2O
the peaks in the X-ray pattern. The X-ray data show a change in the crystal lattice of
the selenite indicating that a new polymorphous compound results at that tempera-
ture. It is stable at 810°C. The endothermal peak at 885°C is associated with the de-
composition of the high temperature modification of Nd2(SeO3)3 and the formation of
oxiselenite by the following scheme:
Nd2(SeO3)3 = Nd2O3 2SeO2+SeO2
At 975–1000°C another mole of SeO2 is liberated and Nd2O3 SeO2 is formed.
Pure Nd2O3 is obtained at a temperature higher than 1100–1200°C. These transfor-
mations were also proved by chemical and X-ray phase analyses.
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GOSPODINOV, STANCHEVA: Nd2O3–SeO2–H2O
839
Figure 3 shows the DTA and TG curves of NdH(SeO3)2 2H2O. At 130°C the
crystal hydrate loses 4 moles of crystallization water and turns into anhydrous hydro-
gen selenite Nd2(SeO3)3 Η2SeO3. Mass loss is 8.17 mass% (theoretical calculation is
8.27 mass%). The constitutional water of hydrogen selenite is liberated at 280°C to
form tetraselenite Nd2(SeO3)3 SeO2 (or Nd2Se4O11). Mass loss is 11.15 mass% (theo-
retical calculation is 10.84 mass%). Tetraselenite is stable up to 280°C but above this
temperature 1 mole of SeO2 is liberated and normal Nd2(SeO3)3 is formed. At 438°C,
pyrolysis of the normal salt obtained results in Nd2O3 2SeO2. Further decomposition
of Nd2O3 2SeO2 takes place at 698°C yielding a stable base salt Nd2O3 SeO2 or
(NdO)2SeO3. This salt is stable up to 1100°C. At 1100–1200°C it loses its last mole of
SeO2 and turns into Nd2O3. The scheme of thermal decomposition was also proved by
chemical analyses of NdH(SeO3)2 2H2O and the phases obtained by modelling the
conditions of thermal decomposition as well as by X-ray phase analysis comparing
the X-ray data of the phases.
The second acid salt Nd2(SeO3)3 3H2SeO3 decomposes within the acceptable
thermal analysis error by a scheme described in detail in [1]. The temperatures after
liberation of crystallization water differ from the temperatures reported by Savchenco
et al. [1] by no more than 100°C.
Figure 4 shows the DTA and TG curves of the acid salt Nd2(Se2O5)3 H2SeO3 2H2O.
This salt is stable at 120°C. First, 2 moles of crystallization water are liberated and anhy-
drous salt is formed at 200°C. Mass loss is 4.50 mass% (theoretical calculation is
Fig. 3 DTA and TG curves of NdH(SeO3)2 2H2O
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GOSPODINOV, STANCHEVA: Nd2O3–SeO2–H2O
Fig. 4 DTA and TG curves of Nd2(Se2O5)3 H2SeO3 2H2O
3.08 mass %). This difference may be explained by the fact that the acid salt obtained has
a very well developed surface and because of that it contains adsorbed water. At
200–260°C the acid salt liberates the constitutional water and turns into heptaselenite
with composition Nd2(Se2O5)3 SeO2. At 300–360°C the mass loss corresponds to the lib-
eration of 1 mole of SeO2 resulting in hexaselenite with composition Nd2(Se2O5)3. At
400°C the next mole of SeO2 is liberated and pentaselenite with composition
Nd2(SeO3)3 2SeO2 is obtained. The mass loss is 22.20 mass% (theoretical calculation is
23.63 mass%). At 420–500°C more SeO2 is liberated and the mass loss is 43.00 mass%.
This transformation corresponds to the formation of normal salt Nd2(SeO3)3. At
800–840°C 1 mole of SeO2 is liberated and oxiselenite with composition Nd2O3 2SeO2 is
formed. In the temperature interval 880–920°C another mole of SeO2 is liberated and the
other oxiselenite with composition Nd2O3 SeO2 is obtained. The sample was heated up to
1020°C but complete liberation of selenium dioxide did not take place. Mass loss is
70.70 mass% (71.17 mass% is the mass loss for the complete liberation of SeO2 and the
formation of a pure phase Nd2O3).
For the three compounds, all intermediate phases from the thermolysis were
subjected to chemical and X-ray phase analyses after they were isolated in the pure
state, and the data obtained confirmed the identity of the separate phases.
Conclusions
The solubility isotherm of the system Nb2O3–SeO2–H2O at 100°C was studied. The
equilibrium phases of the system were identified by the Scheinemakers’ method and
by chemical and X-ray analysis after they were isolated in the pure state. All com-
pounds of this system were subjected to derivatograph analysis and mechanism of
their decomposition was described.
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GOSPODINOV, STANCHEVA: Nd2O3–SeO2–H2O
841
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J. Therm. Anal. Cal., 73, 2003