Study of the crystallization fields of nickel(II) selenites in the system NiSeO3-SeO2-H2O

Article abstract of DOI:10.1007/s10973-005-7397-x

The solubility of NiSeO₃-SeO₂-H₂O system in the temperature region 298-573 K was studied. The compounds of the three-component system were identified by the Schreinemakers' method. The phase diagram of nickel(II) selenites

Full text of DOI:10.1007/s10973-005-7397-x

Journal of Thermal Analysis and Calorimetry, Vol. 86 (2006) 2, 449–456
STUDY OF THE CRYSTALLIZATION FIELDS OF NICKEL(II)
SELENITES IN THE SYSTEM NiSeO3–SeO2–H2O
L. T. Vlaev*, Svetlana D. Genieva and Velyana G. Georgieva
Department Physical Chemistry, Assen Zlatarov University, 8010 Bourgas, Bulgaria
The solubility of NiSeO3–SeO2–H2O system in the temperature region 298–573 K was studied. The compounds of the three-compo-
nent system were identified by the Schreinemakers’ method. The phase diagram of nickel(II) selenites was drawn and the crystalli-
zation fields for the different phases were determined. Depending on the conditions for hydrothermal synthesis, NiSeO3·2H2O,
a-NiSeO3·1/3H2O, b-NiSeO3·1/3H2O, NiSeO3 and NiSe2O5 were obtained. The different phases were proved and characterized by
chemical, powder X-ray diffraction and thermal analyses as well as IR spectroscopy.
Keywords: hydrothermal synthesis, IR spectroscopy, nickel(II)selenites, powder X-ray diffraction, solubility diagrams, thermal analyses
Introduction
Experimental
Various nickel(II) selenites have been reported in the lit-
erature [1–18]. It is known NiSeO3·4H2O [15],
NiSeO3·2H2O [1–4, 6–10, 12, 17], NiSeO3·H2O [3, 17],
NiSeO3·1/3H2O [13, 14, 17], NiSeO3 [3, 11, 17],
Ni(HSeO3)2·4H2O [16], Ni(HSeO3)2·2H2O [12] and
NiSe2O5·3H2O [5]. Only NiSeO3·2H2O can be found in
the natural form of a mineral – ahlfeldite [19]. Under
common conditions, NiSeO3·2H2O crystallizes from
aqueous solutions [1–4, 6–10, 12, 17]. NiSeO3·2H2O is
isomorphous with other hydrated divalent selenites
MSeO3·2H2O (M=Mg, Mn, Co, Cu, Zn) [6, 18]. Its ther-
mal dehydration, however, does not produce lower
crystallohydrates as separate phases [7, 9, 10]. They can
be obtained by hydrothermal treatment at temperatures
above 373 K [13, 18] or by employing other special
conditions or reagents [2, 18]. Heated to temperatures
within 523–573 K, NiSeO3·2H2O dehydrates to trans-
form into NiSeO3 [7, 10].
The lack of systematic approach in most of the
studies cited does not allow to determine clearly the
types and number of the different solid phases which
are in equilibrium with the liquid phase at various
temperatures, as well as the boundaries of the crystal-
lization fields of the known nickel(II) selenites in the
system NiSeO3–SeO2–H2O. In this respect, useful in-
formation can be obtained from the dissolution iso-
therms of the system at different temperatures and the
values of the parameters characterizing the none vari-
ant (peritonic) points.
The initial substance used was NiSeO3·2H2O. It was
prepared by precipitating a 0.2 equiv L–1 aqueous solu-
tion of nickel(II) chloride hexahydrate puriss. (Merck)
with a 0.2 equiv L–1 aqueous solution of sodium selenite
pentahydrate puriss. (Fluka) at 298 K. The solutions
with volumes of 1 L were slowly mixed (5 cm3 min–1)
under continuous stirring with blade mixer. The precipi-
tate obtained was ‘aged’ in the initial solution at room
temperature for a week. The crystalline substance
formed was collected on a G4 frit, rinsed vigorously
with deionized distilled water and dried in air at ambient
temperature for another week. The isolated compound
was light-green crystalline powder, which was stable in
air at laboratory temperature. The results from the chem-
ical and powder X-ray analyses showed the compound
had net formula NiSeO3·2H2O. Thus, the NiSeO3·2H2O
obtained was used as initial substance for the study of
the solubility in the NiSeO3–SeO2–H2O system at dif-
ferent temperatures. Teflon-lined steel vessels with vol-
ume 20 cm3 were used for the experiment [20]. In these
vessels, 1 g NiSeO3·2H2O and 15 cm3 aqueous solution
of SeO2 puriss. (Merck) with concentrations varying
from 0 to 70 mass% SeO2 in 5% steps were placed. The
time required for equilibration of the individual samples
was from 2 months (at 298 K) to 20 days (at 573 K). The
temperatures of the hydrothermal synthesis ranged
from 298 to 573 K in 50 K steps. At the end of experi-
ment time, the vessels were cooled and opened and the
precipitate was filtered through porous glass filter. Both
precipitate and filtrate were analysed to determine the
contents of nickel(II) and selenium(IV). Selenium(IV)
was determined iodometrically by Kotarski method
[21]. Nickel(II) was titrated complexometrically using
The aim of the present work is to study the crys-
tallization fields of nickel(II) selenites in the system
NiSeO3–SeO2–H2O in the temperature interval
298–573 K and characterize the phases observed.
*
Author for correspondence: vlaev@btu.bg
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

VLAEV et al.
xylenol orange as an indicator [22]. The Schreine-
maker’s method was used to study the solubility in the
NiSeO3–SeO2–H2O system. The solubility diagram was
drawn according to Gibbs–Roozeboom method [23, 24].
Powder X-ray patterns were taken on a wide angle
X-ray diffractometer with a goniometer URD-6 (Ger-
many), using cells with a diameter of 12 mm, CoKa ra-
diation (l=1.78892 Å) and an iron filter for b-emission.
The lattice parameters were derived from 150–165 ac-
curately measured reflections in the range 3£2q£60°.
The structures were solved by Patterson or direct meth-
ods and refined with the least squares method.
The thermal analyses of the samples was per-
formed on
a
Paulik–Paulik–Erdey apparatus
(MOM, Hungary) by heating to 1073 K at heating rate
10 K min–1 in a flow of nitrogen at a rate of
20 cm3 min–1. The samples (100 mg) were vigorously
ground in agate vibration mortar. The standard used
was a-Al2O3 heated to 1373 K. The curves were reg-
istered with resolutions 1/5 for DTA, 1/15 for DTG
and 1 mg for TG.
The IR absorption spectra were taken on a
spectrophotometer Specord-75 (Carl Zeiss, Jena, Ger-
many) over the region from 400 to 4000 cm–1 (resolu-
tion 1 cm–1). The experiments were carried out at
room temperature using KBr pellets with concentra-
tion of the substance studied 0.3 mass%.
Fig. 1 Solubility isotherms of the system NiSeO3–SeO2–H2O
Results and discussion
at: a – 423 and b – 573 K
formed in the system NiSeO3–SeO2–H2O which was
in equilibrium with the liquid phase (containing
from 0 to 70 mass% SeO2) within the temperature in-
terval 298–573 K: NiSeO3·2H2O (NiO·SeO2·2H2O),
The Schreinemakers’ method was used to study solu-
bility in the NiSeO3–SeO2–H2O system. The solubility
diagrams obtained at 423 and 573 K, constructed by
the Gibbs–Roozeboom method, are presented in Fig. 1.
Schreinemakers’ data show (Fig. 1a) that two
solid phases crystallize in the system at 423 K –
NiSeO3·2H2O and NiSe2O5. The peritonic point be-
tween NiSeO3·2H2O and NiSe2O5 is at
4.3 mass% NiSeO3, 26.5 mass% SeO2 and
69.2 mass% H2O. At 573 K (Fig. 1b) Schreinemakers’
data show that three solid phases crystallize in the
system – NiSeO3·1/3H2O, NiSeO3 and NiSe2O5. The
NiSeO3·1/3H2O resulting from the hydrothermal syn-
thesis lost its crystallization water at temperature
above 573 K and SeO2 content higher than 22 mass%
to transform into anhydrous NiSeO3. The peritonic
point P1 corresponds to 2.9 mass% NiSeO3,
17.4 mass% SeO2 and 79.7 mass% H2O. At SeO2 con-
centration higher than 45 mass%, the anhydrous
nickel selenite transforms into NiSe2O5. The peritonic
point P2 corresponds to 4.7 mass% NiSeO3,
36.5 mass% SeO2 and 58.8 mass% H2O.
NiSeO3·1/3H2O
(NiO·SeO2·1/3H2O),
NiSeO3
(NiO·SeO2) and NiSe2O5 (NiO·2SeO2). The individ-
ual phases were distinguishable by colour and crystal-
line habitus: NiSeO3·2H2O – light-green crystalline
powder; NiSeO3·1/3H2O – citron-yellow, fine crystal-
line; CoSeO3 – yellow-brown well crystallized and
NiSe2O5 – light yellow, fine powder. The samples ob-
tained were identified by the chemical, X-ray
diffraction and IR spectroscopy analyses.
The interplanar distances and the relative inten-
sities of the peaks are given in Table 1.
According to literary data [5, 18], NiSeO3·2H2O
adopted a structure similar to Cu, Mg, Zn, Mn and Co
selenite dihydrates. The structure of NiSeO3 was
closely related to that of CoSeO3·1/3H2O [4, 14] and
the isotypic compound CoSeO3·1/3H2O [6, 14].
NiSe2O5 was found to be isomorphous with ZnSe2O5,
CoSe2O5 and MnSe2O5 [6, 18]. Table 2 summarizes
the parameters characterizing the crystalline lattices
of the selenites studied.
The results obtained from the experiments car-
ried out showed that the following solid phases
450
J. Therm. Anal. Cal., 86, 2006

CRYSTALLIZATION FIELDS OF NICKEL(II) SELENITES
J. Therm. Anal. Cal., 86, 2006
451

VLAEV et al.
Table 2 Crystallographic data for nickel(II) selenites
Parameter
NiSeO3×2H2O
a-NiSeO3×1/3H2O b-NiSeO3×1/3H2O
NiSeO3
NiSe2O5
colour
space group
a/nm
b/nm
c/nm
a/°
light-green
P21/n
0.63782
0.87734
0.75467
–
citron-yellow
P-1
citron-yellow
P-1
yellow-brown
C2/c
light-yellow
Pnab
0.60754
1.03662
0.67913
–
0.81383
0.84034
0.85724
123.713
90.174
111.823
2
0.80222
0.82133
0.84364
68.654
61.782
66.363
2
1.54915
0.99355
1.48416
–
b/°
81.451
–
111.173
–
–
g/°
–
Z
4
32
4
V/nm3
dR/g cm–3
0.41761
3.524
0.43583
1.429
0.43811
1.422
0.213015
4.630
0.42771
4.605
The comparison of the parameters measured was
in very good accordance with data obtained from other
authors [2, 4, 5, 7, 12, 13, 18]. It should be noted that
two separate isomorphous phases (a and b) were ob-
served for both NiSeO3·1/3H2O and CoSeO3·1/3H2O
and they had different parameters of the crystalline lat-
tice. Their IR spectra, however, were identical.
On the basis of the solubility diagrams of
NiSeO3–SeO2–H2O system at different temperatures
and the compositions at the peritonic points in the
Gibbs–Roozeboom diagrams, the polythermal dia-
gram of the system studied was drawn (Fig. 2).
As can be seen from the figure, five different by
area crystallization fields and three nonevariant points
each with three solid phases in equilibrium can be
identified. If the composition of the different selenites
is described by the general formula NiO·nSeO2·mH2O,
then Fig. 2 shows that phases with small m became sta-
ble with the increase of temperature while the phases
with small n were more stable with decreased SeO2
concentration. The boundaries between the crystalliza-
tion fields with different values of m, however, were
not horizontal and those for different n were not verti-
cal. Each boundary had its slope (negative or positive).
Similar pattern was observed for the crystallization
fields of iron(III) selenites [25]. At SeO2 concentra-
tions lower than 30 mass%, the increase of temperature
lead to formation of thermodynamically stable selen-
ites phases with lower values of m: NiSeO3·2H2O®
NiSeO3·1/3H2O®NiSeO3. Besides, the slope of the
boundary between these two fields was negative
(dT/dx<0). It means that, with the increase of SeO2
concentration, the equilibrium shifts in the same direc-
tion in which it shifts with the increase of temperature,
i.e. SeO2 exerts dehydrating effect and the values of m
in the general formula decrease from 2 to 0 (fields I, II
and III) and from 2 to 0 (fields I, V and VI). Similar
considerations can be applied to the values of n at
T=const. and varying SeO2 concentration. In all the
cases studied, the equilibrium shifted to compounds
with lower values of n (from 2 to 1, fields V and I and
fields V and IV) with the decrease of SeO2 concentra-
tion in the solution, i.e. hydrolyzation was facilitated.
The slopes of the curves separating fields 1, II, III
and IV were negative because the increase of tempera-
ture and SeO2 concentration in the solution eased
selenites dehydration. The results obtained clearly
showed that the hydrothermal treatment of
NiSeO3·2H2O does not give NiSeO3·H2O and
NiSe2O5·3H2O in the temperature and concentration
intervals studied. As it has been reported ear-
lier [10, 18], monohydrate was not observed as inter-
mediate product by the dehydration of the dihydrate
and, likewise, NiSeO3·1/3H2O was not observed by the
dehydration of NiSeO3·H2O. According to the same
authors, the reason for this lack of continuity was the
incompatibility of the texture of these crystallo-
hydrates. Figure 3 shows TG, DTA and DTG curves of
thermal dehydration of NiSeO3·2H2O and decomposi-
tion of NiSeO3.
Fig. 2 Crystallization field of selenites in NiSeO3–SeO2–H2O
system
452
J. Therm. Anal. Cal., 86, 2006

CRYSTALLIZATION FIELDS OF NICKEL(II) SELENITES
Fig. 3 TG, DTG and DTA curves of dehydration and decomposition of: a – NiSeO3×2H2O and b – NiSeO3
As can be seen from the Fig. 3a the dehydration
of NiSeO3·2H2O was a one-stage process giving
NiSeO3 (mass loss 16.2 mass%) and the product ob-
tained was found to be X-ray amorphous. Intermedi-
ate products of the dehydration like NiSeO3·H2O and
NiSeO3·1/3H2O were not registered. The highest de-
hydration rate was measured at 563 K. An exo-effect
was observed at 858 K and attributed to the crystalli-
zation of the amorphous phase [10]. Data from both
X-ray and chemical analyses showed that the product
corresponds to the formula NiSeO3. At temperatures
above 873 K it started to decompose by releasing
SeO2 and formation of NiO. The decomposition was
accomplished at about 973 K (mass loss 49.9 mass%)
and the highest decomposition rate was observed
at 923 K. Figure 3b shows TG, DTA and DTG curves
of decomposition of NiSeO3 prepared under
hydrothermal regime of precipitation.
It can be seen that the decomposition proceeded at
maximum rate at temperature of 963 K, which is by 40
K higher than the decomposition temperature of
NiSeO3·2H2O. It can be explained with the greater sta-
bility of the crystal lattice of NiSeO3 obtained by hy-
drothermal synthesis compared to that of the same
product obtained in situ by dehydration of
NiSeO3·2H2O. Based on the method of Coats and Red-
fern [26, 27] for the kinetics of topochemical reactions
under non-isothermal conditions of heating, the values
of the activation energy E, frequency factor A in
¹
Arrhenius equation, changes of entropy DS , Gibbs
¹
¹
free energy DG and enthalpy DH of the processes of
dehydration and decomposition of NiSeO3·2H2O and
decomposition of NiSeO3 were calculated. These val-
ues are presented in Table 3.
As can be seen from Table 3, the dehydration of
NiSeO3·2H2O is characterized by twice lower value
Table 3 Kinetic parameters of non-isothermal dehydration and decomposition of NiSeO3×2H2O and NiSeO3
NiSeO3×2H2O
NiSeO3
Parameter
dehydration
decomposition
decomposition
R2
0.9914
2.0
0.9981
1.0
0.9982
1.0
n
Ea/kJ mol–1
A/min–1
93.6
189.2
9.63×1012
48.1
213.2
6×1013
32.2
2.53×108
131.5
89.0
¹
–DS /J K–1 mol–1
¹
DH /kJ mol–1
181.5
225.9
923
205.1
236.2
963
¹
DG /kJ mol–1
161.7
Tp/K
553
¹
¹
¹
The values of DS , DH and DG are calculated at corresponding Tp temperature.
J. Therm. Anal. Cal., 86, 2006
453

VLAEV et al.
In the environment of the octahedral Ni2+ cation,
one of the water molecules is at a distance equal to the
sum of the covalent radii of the metal and oxygen and
at almost equal distances from the oxygen atoms of
the selenite group. This molecule (denoted as H2O–I)
is loaded almost symmetrically, therefore the differ-
ence in the frequencies of its asymmetric nas and sym-
metric na vibrations is almost constant with a value of
100–110 cm–1 [32, 36]. Hence, the band at 3200 cm–1
can be attributed to nas(O–H)(H
and the band
2 O)
at 3126 cm–1 – to ns(O–H)(H O–I) . The second water mol-
2
ecule is at higher distance from Ni2+ ion but at a rela-
tively
small
distance
from
Se=O
Fig. 4 Infrared absorption spectra of nickel(II) selenites at 298
K: 1 – NiSeO3·2H2O, 2 – NiSeO3·1/3H2O, 3 – NiSeO3
and 4 – NiSe2O5
(HOH...OSe<~2.7 Å), forming a strong enough hy-
drogen bond. The bond of the second hydrogen atom
with the oxygen atom in the H2O molecule remains al-
most the same as in a free water molecule. Since in
such cases of asymmetric loading of the (H2O–II)
molecule the difference between the stretching vibra-
tions could be as much as 500–600 cm–1 [36], then the
vibrations along both O–H bonds in this molecule can
be regarded as independent. Therefore, the band with
the highest frequency at 3452 cm–1 can be attributed
to nfree(OH) of the relatively free OH group in (H2O–II)
while that at 2916 cm–1 – nbond(OH) of the OH group
bonded by hydrogen bond in the same water mole-
cule. Consequently, the band at 1516 cm–1 is probably
of the activation energy compared to that of decompo-
sition. Besides, the dehydration is described by ki-
netic equation of second order (n=2) while decompo-
sition – by equation of first order (n=1). In all the
¹
cases studied, however, the change of entropy DS for
the formation of the activated complex was negative,
which means that the complex has higher degree of
arrangement compared to the initial reagent. The
¹
higher absolute values of DS for the dehydration
compared to decomposition showed that the former
process was accompanied by bigger rearrangement of
NiSeO3·2H2O structure. The higher values of the acti-
vation energy of decomposition of NiSeO3 obtained
hydrothermally compared to the decomposition of
NiSeO3·2H2O confirmed the higher thermal stability
of the former product.
The different nickel(II) selenites obtained were
studied by IR spectroscopy and the absorption spectra
are presented in Fig. 4. The bands observed were in-
terpreted according to the works of Simon [28, 29]
and Cody [30, 31] for selenous acid and other authors,
studying IR spectra of nickel(II) selenites [12, 15, 16]
or another selenites [16, 32–34].
Figure 4 showed that IR spectra of the selenites
studied had some common and some specific bands. For
instance, four absorption bands were registered in the
high frequency region of NiSeO3·2H2O spectrum,
which were attributed to stretching vibrations of O–H
from H2O molecules. In the interval 1500–1700 cm–1
two absorption bands were observed due to the bending
vibrations of H2O molecules. The reason for this is that
the two water molecules in the dihydrate are structurally
unequal according to the following scheme [35]:
due to the bending vibrations das(H
and the band
2 O –I)
at 1628 cm–1 – to das(H
.
2 O–II)
All these considerations were sustained by the
fact that only one absorption band at 1626 cm–1 was
observed for NiSeO3·1/3H2O, representing the bend-
ing vibrations of (H2O–II) molecules, thus confirming
that in this case the water molecules were structurally
equal. Two clearly distinguishable bands at 3413
and 2959 cm–1 were registered in the high frequency
region of the same spectrum. Since the difference be-
tween them was quite big (~400 cm–1), they can be at-
tributed to stretching vibrations of nfree(OH) from
(H2O–II). For NiSeO3, however, no absorption bands
were observed from 1500 to 1300 cm–1 except for the
low intensity broad bands at 1626 and 3446 cm–1
probably due to small quantities of physically ad-
sorbed water on sample surface.
A number of bands were registered in the re-
gion 900–400 cm–1 and their number increased from
NiSeO3·2H2O to NiSeO3, i.e. as dehydration pro-
ceeds, the characteristic bands were more distinctly
measured and the IR spectrum had more details. Four
454
J. Therm. Anal. Cal., 86, 2006

CRYSTALLIZATION FIELDS OF NICKEL(II) SELENITES
Table 4 Infrared absorption spectra of NiSeO3×2H2O and NiSeO3×H2O×D2O
nNiSeO
3 ×2H2
O
NiSeO3×2H2O
n/cm–1
NiSeO3×H2O×D2O
n/cm–1
Band assignment
nNiSeO
3 ×H2O×D2
O
d
500 vs
579 w
700 vs
800 s
493 s
1.014
SeO32–
nNi–O
589 w
716 vs
816 s
0.983
0.978
0.980
0.985
n
as(SeO32–
)
n
n
s(SeO32–
)
841 s
854 w
s(SeO32–
)
924 sh
r
H2
O
1136 w
1200 w
1540 sh
1626 s
1.335
1.357
0.984
1.001
d(OD)(D
d(OD)(D
d(OH)(H
d(OH)(H
2O)
2O)
2O)
2O)
1516 m
1628 s
2213 s
2391 s
2550 s
2924 s
3118 sh
3200 s
3450 vs
1.318
1.338
1.354
1.997
1.003
1.000
1.001
n(OD)(D
n(OD)(D
n(OD)(D
n(OH)(H
n(OH)(H
n(OH)(H
n(OH)(H
2O)
2O)
2O)
2O)
2O)
2O)
2O)
2916 s
3126 sh
3200 s
3452 vs
vs – very strong, s – strong, m – medium, w – weak, b – broad, sh – shoulder, ns – symmetric stretching, nas – antisymmetric
stretching, d – bending and r – rocking
types of characteristic bands were found: nSe–O in
molecules in the crystallohydrate were strongly
bonded and were not exchanged with D2O under
these conditions. According to data from thermal and
X-ray analyses, the heating of NiSeO3·2H2O to 473 K
results in its partial dehydration and amorphization
(spectrum 2). The five consecutive treatments of this
sample with D2O in autoclave at 473 K followed by
drying at 373 K gave substantial changes in the IR
spectrum (spectrum 3). Two new groups of absorp-
tion bands were registered at 1136, 1200 cm–1 and
2213, 2391, 2550 cm–1, respectively. Taking into ac-
count the fact that the coefficient of isotopic shift of
frequencies for D2O is about 1.34 [12, 33], these new
bands should be attributed to deformation and valent
vibrations of D2O molecules in the crystallohydrate.
Table 4 shows the absorption frequencies of the bands
observed and their designations.
SeO23– anion, rH , nCo–O and dO–Se–O. These absorp-
2 O
tion bands were observed in the IR spectra of the
other selenites; they were analyzed by other
authors [32, 36, 37].
The spectrum of NiSe2O5 (Fig. 4, spectrum 4)
showed two low intensity and wide absorption bands
at 3428 and 1620 cm–1. They were due to the stretching
and bending vibrations, respectively, of small amounts
of physically adsorbed water on sample surface. Tak-
ing into account the composition and specific features
of the thermal decomposition of the diselenites,
NiSe2O5 can be regarded as a product of attach-
ment – co-ordination bonding of a molecule SeO2 to
NiSeO3. Therefore, the IR spectrum showed both ab-
sorption bands for the almost free SeO2 [38, 39] and
ions [34–39]. The large number of bands present in the
interval 850–400 cm–1 are usually connected with the
vibration structure of the diselenite anion
[O2Se–O–SeO2]. The spectrum is considered to sum-
marize the vibrations of (SeO2)-groups and (Se–O–Se)
bridges [34–39]. According to the authors who have
studied various selenites, the bands in the interval
Beside absorption bands characteristic for D2O,
spectrum 3 shows also bands characteristic for H2O. It
900–860 cm–1 are due to nas(SeO ) ; 840–780 cm–1 – to
2
ns(SeO
;
600–560
530–2475 cm–1 – ns(Se–O–Se); 410–330 cm–1 – d
those from 300 to 250 cm–1 – to dSe–O–Se
cm–1
–
nas(Se–O–Se)
;
)
and
(SeO2
)
.
The IR spectra of the samples obtained after
treatment with D2O confirmed the different thermal
stability of the crystallization water (H2O–I) and
(H2O–II) in NiSeO3·2H2O (Fig. 5).
The treatment of NiSeO3·2H2O with D2O
for 10 days followed by drying at 373 K did not lead
to changes in the IR spectrum. It means that the H2O
Fig. 5 Infrared absorption spectra of initial 1 – NiSeO3×2H2O,
partially dehydrated and amorphized at 2 – 473 K and
after deuterated with D2O at 3 – 473 K
J. Therm. Anal. Cal., 86, 2006
455

VLAEV et al.
10 V. N. Makatun, V. V. Pechkovskii, R. Ya. Melnikova and
means that, even at temperature of 473 K, not all H2O
molecules can be exchanged for D2O molecules. The
following two reasons were considered to explain the
observation. The first one lies in the insufficient size
of sample surface area leading to difficulties in the ex-
change for deuterium. The second one is the high acti-
vation energy of the exchange which implies strong
bonds in the water molecules in NiSeO3·2H2O. This
was confirmed by the fact that full dehydration of
NiSeO3·2H2O was achieved as high as 623 K, accord-
ing to the data from the thermal analysis. It may be
concluded, therefore, that both water molecules in the
crystallohydrate are structurally and energetically un-
equal. Thus, it is impossible to obtain lower crystallo-
hydrates (NiSeO3·H2O and NiSeO·1/3H2O) by step-
wise dehydration of NiSeO3·2H2O. They can be ob-
tained only under conditions of hydrothermal synthe-
sis within certain intervals of temperature and con-
centration of aqueous solutions of SeO2.
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Conclusions
It may be concluded, that the absorption bands observed
in the IR spectra of the different selenites studied, to-
gether with the results from the powder X-ray diffrac-
tion and chemical analyses irrefutably prove the exis-
tence of different crystalline forms of nickel(II) selenites
and provide enough data to determine the corresponding
crystallization fields of stability in the solubility diagram
of the system NiSeO3–SeO2–H2O.
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Received: October 5, 2005
Accepted: December 19, 2005
OnlineFirst: May 23, 2006
DOI: 10.1007/s10973-005-7397-x
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J. Therm. Anal. Cal., 86, 2006