Thermal decomposition of polycrystalline [Ni(NH3)6](NO3)2

Abstract

The thermal decompositions of polycrystalline samples of [Ni(NH3)6](NO3)2 were studied by thermogravimetric analysis with simultaneous gaseous products of the decomposition identified by a quadruple mass spectrometer. Two measurements were made for samples placed in alumina crucibles, heated from 303 K up to 773 K in the flow (80 cm3 min−1) of Ar 6.0 and He 5.0, at a constant heating rate of 10 K min−1. Thermal decomposition process undergoes two main stages. First, the deamination of [Ni(NH3)6](NO3)2 to [Ni(NH3)2](NO3)2 occurs in four steps, and 4NH3 molecules per formula unit are liberated. Then, decomposition of survivor [Ni(NH3)2](NO3)2 undergoes directly to the final decomposition products: NiO1+x, N2, O2, nitrogen oxides and H2O, without the formation of a stable Ni(NO3)2, because of the autocatalytic effect of the formed NiO1+x. Obtained results were compared both with those published by us earlier, by Farhadi and Roostaei-Zaniyani later and also with the results published by Rejitha et al. quite recently. In contradiction to these last ones, in the first and second cases agreement between the results was obtained.

Full text

Thermal decomposition of polycrystalline [Ni(NH
3
)
6
](NO
3
)
2
Edward Mikuli •Anna Migdał-Mikuli •
Dorota Majda
Received: 14 May 2012 / Accepted: 13 August 2012 / Published online: 28 September 2012
The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract The thermal decompositions of polycrystalline
samples of [Ni(NH
3
)
6
](NO
3
)
2
were studied by thermo-
gravimetric analysis with simultaneous gaseous products of
the decomposition identified by a quadruple mass spec-
trometer. Two measurements were made for samples placed
in alumina crucibles, heated from 303 K up to 773 K in the
flow (80 cm
3
min
-1
) of Ar 6.0 and He 5.0, at a constant
heating rate of 10 K min
-1
. Thermal decomposition process
undergoes two main stages. First, the deamination of
[Ni(NH
3
)
6
](NO
3
)
2
to [Ni(NH
3
)
2
](NO
3
)
2
occurs in four steps,
and 4NH
3
molecules per formula unit are liberated. Then,
decomposition of survivor [Ni(NH
3
)
2
](NO
3
)
2
undergoes
directly to the final decomposition products: NiO
1?x
,N
2
,O
2
,
nitrogen oxides and H
2
O, without the formation of a stable
Ni(NO
3
)
2
, because of the autocatalytic effect of the formed
NiO
1?x
. Obtained results were compared both with those
published by us earlier, by Farhadi and Roostaei-Zaniyani
later and also with the results published by Rejitha et al. quite
recently. In contradiction to these last ones, in the first and
second cases agreement between the results was obtained.
Keywords Hexammine Werner-type complex
[Ni(NH
3
)
6
](NO
3
)
2
Thermal decomposition 
Thermogravimetry Quadruple mass spectrometry 
Infrared and Raman spectroscopy
Introduction
The Werner-type hexammine coordination compounds are
very interesting materials to study for several reasons. One
of these is their rich phase polymorphism and its connec-
tion with the structural and dynamical properties of these
compounds (for example see [1,2]). Another reason is
connected with their application in medicine as compounds
interacting with DNA, and as anticancer drugs [3–5].
The next important reason is that the rich family of
[M(NH
3
)
n
]A
m
complexes represent the large number of
compounds that can potentially be used for reversible,
indirect hydrogen storage. Furthermore, they are found to
exhibit facile ammonia release kinetics [6]. In addition,
recently, these compounds have also been proposed as an
ideal ammonia storage in connection with selective cata-
lytic reduction (SCR) of NO
x
systems [7] in both diesel and
lean-burn gasoline-driven automobiles.
The synthesis, chemical composition, crystal structure,
phase polymorphism, vibrational spectra and other physical
and physicochemical properties of [Ni(NH
3
)
6
](NO
3
)
2
have
been widely described in many papers (for example see
[8–19]). The thermal decomposition of [Ni(NH
3
)
6
](NO
3
)
2
in a flow of Ar was first studied in 2004 by Migdał-Mikuli
et al. [20]. Next, this compound was thermally decomposed
in air atmosphere by Farhadi and Roostaei-Zaniyani [21]in
2011 and quite recently (2012) it was thermally decom-
posed in a flow of He by Rejitha et al. [22]. Because of
some discrepancies in the results obtained by us and by
these last authors, we have decided to repeat thermal
decomposition measurements of this compound, in the
presence of both Ar and He, with the same experimental
conditions in both cases.
Experimental procedure
The identification of the hexamminenickel(II) nitrate(V),
measured by us with formula [Ni(NH
3
)
6
](NO
3
)
2
,is
E. Mikuli (&)A. Migdał-Mikuli D. Majda
Faculty of Chemistry, Jagiellonian University, Ingardena 3,
30-060 Krako
´w, Poland
e-mail: mikuli@chemia.uj.edu.pl
123
J Therm Anal Calorim (2013) 112:1191–1198
DOI 10.1007/s10973-012-2640-8

guaranteed by the results described in many papers mainly
for three reasons. The first reason is that the present
compound measured by us is exactly the same compound
as the one that was measured in our papers [8–12]. The
second reason is based on chemical, spectral and struc-
tural analysis of this compound. The results all of these
analyses confirm its proper composition and structure. The
third reason is that properties of the investigated com-
pound are exactly identical to those which were discov-
ered by other authors [13–19]. Unfortunately, contrary to
us [20], Rejitha et al. [22] did not prove the proper
composition of the compound measured by them. This
could be one of the reasons why our results are somewhat
different to theirs.
A characterization of the decomposition process of the
[Ni(NH
3
)
6
](NO
3
)
2
compound was performed using a
Mettler Toledo TGA/SDTA 851
e
apparatus. Evolved
gaseous products from the decomposition of the com-
pound were identified using a ThermoStar GSD300T
Balzers quadruple mass spectrometer (QMS). The TGA
instrument was calibrated with indium, zinc and alumin-
ium. Its accuracy is equal to 10
-6
g. The mass spec-
trometer was operated in electron impact mode (EI) using
channeltron as a detector. Screening analyses were per-
formed in the selected-ion monitoring (SIM) mode. The
following ions’ characteristics of each molecules, such as
17, 18, 28, 30, 32, 44 and 46, for NH
3
,H
2
O, N
2
, NO, O
2
N
2
O and NO
2
, respectively, were monitored. It is
important to notice that the QMS spectrum of mass 17
can represent not only NH
3
but also the OH
-
fragment of
H
2
O fragmentation.
For the thermogravimetric analysis (TG/DTG), the
samples were placed in alumina crucible. Two measure-
ments were made: one in a flow (80 cm
3
min
-1
) of Ar 6.0
and the second in He 5.0 in the temperature range from
30 C up to 500 C (303–773 K), at a constant heating rate
of 10 K min
-1
. Simultaneously, differential thermal anal-
ysis (SDTA) measurements were carried out.
Fourier-transform far and middle infrared absorption
measurements (FT-FIR, FT-MIR) were performed using a
Bruker VERTEX 70v vacuum spectrometer. The spectra
were collected with a resolution of 2 cm
-1
and with
64 scans per each spectrum. The FT-FIR spectra
(525–100 cm
-1
) were collected for a sample suspended in
Apiezon N grease and placed on a polyethylene (PE) disc.
The FT-MIR spectra (4,000–500 cm
-1
) were recorded for
the sample in KBr pellet.
The Raman spectra (RS) were recorded using a WITec
confocal CRM alpha 300 Raman microscope equipped
with an air-cooled solid-state laser operating at 488 nm and
a CCD detector. 500 scans were registered. The power of
the laser at the sample position was 40 mW and explored
time was 0.5 s.
Result and discussion
In Fig. 1, TG and DTG curves of the thermal decomposi-
tion process of the studied sample obtained in Ar and in He
atmosphere were plotted. They exhibited a very similar
character, which indicates that the decomposition process
is not influenced by a gas used in the experiment. The main
difference can be observed in the temperature of the DTG
peaks. Namely, the decomposition in He generally occurs
in temperatures of about 20 C lower in comparison with
the experiment carried out in Ar atmosphere. It is under-
standable because the thermal conductivity of He is almost
nine times greater than that of Ar. Figure 2presents mass
loss valued on TG curve of the [Ni(NH
3
)
6
](NO
3
)
2
sample
measured in He atmosphere. Figures 3,4and 5present TG
curve and QMS curves of the particular gaseous products
of [Ni(NH
3
)
6
](NO
3
)
2
thermal decomposition in He. The
TG and DTG curves presented in Fig. 2show that the
decomposition of the sample proceeds in three main stages:
I, II and III (each of them is composed from two steps—a
and b). Table 1presents temperature ranges, percentage
mass losses and identified products of the thermal
decomposition of [Ni(NH
3
)
6
](NO
3
)
2
. From the QMS
spectra (see Figs. 3,4), it can be observed that the stages I
and II involve mainly the step-wise (Ia, Ib and IIa, IIb)
freeing of 4NH
3
molecules, whereas the stage III illustrates
the decomposition of [Ni(NH
3
)
2
](NO
3
)
2
to the H
2
O, N
2
,
NO, N
2
O (see Figs. 3,4), oxygen and NO
2
(see Fig. 5) and
solid NiO. The mass losses for particular step of thermal
decomposition was determined from TG and DTG curves
in a manner presented in Fig. 2.
Figure 6presents the profiles of TG, DTG and SDTA
curves of [Ni(NH
3
)
6
](NO
3
)
2
. In the temperature range of
60 to 240 C, the SDTA curve shows four small, broad
100
80
60
40
0
–1
–2
–3
–4
–5
100 200 300 400 500
T/°C
m/%
dm·dT
–1/g °C–1
TG
DTG
[Ni(NH3)6](NO3)2 in Ar
[Ni(NH3)6](NO3)2 in He
Fig. 1 TG and DTG curves of the [Ni(NH
3
)
6
](NO
3
)
2
sample
measured in Ar and in He atmosphere
1192 E. Mikuli et al.
123

endothermic peaks illustrating the deamination process,
one big, sharp and broad exothermic peak in the temper-
ature range of 240–285 C and another big, sharp and
broad but endothermic peak in the temperature range of
285–315 C.
The exothermic peak can be explained by reduction and
oxidation (redox) processes taking place between the
reductant (NH
3
) and the oxidant (NO
3
-
). The beginning of
the exothermic effect (at ca. 240 C) is correlated with
beginning of N
2
evolution observed as QMS curve
m/z=28 presented in Fig. 4. As can be seen in Fig. 6, the
acceleration of exothermic effect is bounded with accel-
eration of N
2
production and with additional evolution of
N
2
O (curve m/z=44) and with the beginning of NO
(m/z=30) and H
2
O(m/z=18) evolution. All of them
starting above a temperature of 260 C. Next, above
285 C, the endothermic peak on DTA curve is correlated
with the evolution of O
2
(m/z=32) and NO
2
(m/z=46),
which is presented in Fig. 5, besides former evolution of
H
2
O and NO (see Figs. 3,4).
Taking advantage of the results of our previous inves-
tigations [20,23–26] and taking into account the results
obtained by other researchers [27–36], the redox process,
which takes place during thermal decomposition of
[Ni(NH
3
)
6
](NO
3
)
2
, determines that this decomposition may
be presented as the following reactions:
INiðNH3Þ6

ðNO3Þ2!NiðNH3Þ4

ðNO3Þ2þ2NH3;
ð1Þ
II Ni(NH3Þ4

ðNO3Þ2!Ni(NH3Þ2

ðNO3Þ2þ2NH3;
ð2Þ
III Ni(NH3Þ2

ðNO3Þ2!NiO þ1
2O2þN2þN2O
þ3H2O$NiO þ1
2N2þN2OþNO þ3H2O:ð3Þ
In order to prove that [Ni(NH
3
)
2
](NO
3
)
2
is really an
intermediate product of a thermal decomposition of
100
80
90
60
70
40
30
50
100 200 300 400 500
T/°C
m/%
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
dm·dT–1/g °C–1
TG
DTG
[Ni(NH
3
)
6
](NO
3
)
2
in He
Δm =
5.2 %
5.3 %
6.7 %
6.0 %
39 %
12 %
Fig. 2 Mass loss valued on TG curve of the [Ni(NH
3
)
6
](NO
3
)
2
sample measured in He atmosphere
100 200 300 400
T/°C
20
18
16
14
12
10
8
6
m/mg
1400
1200
1000
800
600
400
200
0
Ion current/nA
TG
m/z = 17
m/z = 18
Fig. 3 TG and QMS curves of thermal decomposition of
[Ni(NH
3
)
6
](NO
3
)
2
in a flow of He obtained for NH
3
and H
2
O species
20
18
16
14
12
10
8
6
m/mg
240 260 280 300 320
T/°C
500
400
300
200
100
0
Ion current/nA
TG
m/z = 44
m/z = 30
m/z = 28
Fig. 4 TG and QMS curves of thermal decomposition of
[Ni(NH
3
)
6
](NO
3
)
2
in a flow of He obtained for N
2
, NO and N
2
O
species
20
18
16
14
12
10
8
6
m/mg
240 260 280 300 320
T/°C
Ion current/nA
50
40
30
20
10
0
TG
m/z = 32
m/z = 46
Fig. 5 TG and QMS curves of thermal decomposition of
[Ni(NH
3
)
6
](NO
3
)
2
in a flow of He obtained for O
2
and NO
2
species
Thermal decomposition of polycrystalline [Ni(NH
3
)
6
](NO
3
)
2
1193
123

[Ni(NH
3
)
6
](NO
3
)
2
, we have repeated thermal decompo-
sition of hexammine complex and stopped this process at
240 C, similar to what we did in our previous work [20],
but this time in He atmosphere. The heated substance
changed its colour from bright violet to dark green. After
cooling this substance, we performed its chemical and
infrared plus Raman spectroscopy analyses, which
indicated that both the composition and coordination
character of this compound is as such expressed by the
Table 1 Comparison of temperature ranges, percentage mass losses and identified products of the thermal decomposition of [Ni(NH
3
)
6
](NO
3
)
2
obtained in four different investigations
Stage
numbers
Temperature ranges (C) Mass loss (%) Products of thermal decomposition
This study [20][21][22] This study [20][21][22] Calculated This study [20][21][22]
Ar He Ar Air He Ar He Ar Air He Ar He Ar Air He
Ia 25–125 25–105 27–202 0–167 78–116 4.4 5.2 12.0
aa
5.98 NH
3
2NH
3
2NH
3
NH
3
Ib 126–180 106–160 116–164 6.0 6.0
a
5.98 NH
3
NH
3
IIa 181–234 161–214 203–234 168–207 164–300 5.2 5.3 6.3
aa
5.98 NH
3
NH
3
2NH
3
4NH
3
?NO
3•
?NO
2
IIb 235–264 215–239 235–272 7.0 6.7 6.0 5.98 NH
3
NH
3
IIIa 265–285 240–285 273– 208–250 37.0 39.0 49.5
a
49.87 N
2
?N
2
ON
2
?N
2
ON
2
?N
2
O
IIIb 286–303 286–320 13.0 12.0 ?NO ?3H
2
O?NO ?3H
2
O?NO ?3H
2
O
Temperature ranges (C) Mass after decomposition (%) Products of thermal decomposition
This study [20][21][22] This study [20][21][22] Calculated This study [20][21][22]
Ar He Ar Air He Ar He Ar Air He Ar He Ar Air He
Solid 303–500 320–500 –477 251–600 27.4 25.8 26.2
aa
26.21 NiO NiO NiO NiO
a
This value was not presented in the paper
20
15
10
5
–0.2
–0.4
–0.6
100 150 200 250 300 350
T/°C
Exo
2
1
0
–1
ΔT/°C
TG
DTG
DTA
dm·dT–1/g °C–1
m/mg
Fig. 6 TG, DTG and DTA curves obtained for thermal decomposi-
tion of [Ni(NH
3
)
6
](NO
3
)
2
in a flow of He
4000 3000 2000 1000 0
Wavenumber/cm–1
RS
FT–IR
Raman intensity/a.u. Absorbance/a.u.
Fig. 7 Comparison of FT-FIR and RS room temperature spectra of
[Ni(NH
3
)
6
](NO
3
)
2
and [Ni(NH
3
)
2
](NO
3
)
2
1194 E. Mikuli et al.
123

formula: [Ni(NH
3
)
2
](NO
3
)
2
. Figure 7and Table 2prove the
above statement. Figure 7presents the infrared (FT-IR) and
Raman light scattering (RS) spectra of diamminenickel(II)
nitrate(V). Table 2contains the frequencies in cm
-1
of the
vibrational modes associated with the bands present in these
spectra of [Ni(NH
3
)
2
](NO
3
)
2
, compared with those obtained
from the spectra of [Ni(NH
3
)
6
](NO
3
)
2
.
On subsequent heating of the intermediate product
[Ni(NH
3
)
2
](NO
3
)
2
, we assume that its thermal decompo-
sition may be presented at least as several different reac-
tions, for example:
Ni(NH3Þ2

ðNO3Þ2$Ni(NO3Þ2þ2NH3!NiO þ1
2O2
þN2þN2Oþ3H2O$NiO þ1
2N2þN2OþNO
þ3H2O;ð4Þ
and some of them are reversible. We suppose that, at the
beginning of [Ni(NH
3
)
2
](NO
3
)
2
decomposition, the solid
nickel nitrate(V) and gaseous ammonia were created,
but immediately Ni(NO
3
)
2
decomposed according to the
following several possible reactions:
Table 2 The list of band positions of the infrared (IR) and Raman (RS) spectra of [Ni(NH
3
)
6
](NO
3
)
2
—(6) and [Ni(NH
3
)
2
](NO
3
)
2
—(2) at room
temperature (vw very weak, wweak, sh shoulder, mmedium, sstrong, vs very strong, br broad, sp sharp)
IR RS Assignments
62NO
3-
62NO
3-
3,560 m,br 3,390 m 3,629 s m
as
(NH)F
2g
m
as
(NH)F
1u
3,346 s 3,355 s,sh m
as
(NH)F
1u
3,295 s 3,368 s m
s
(NH)A
1g
3,242 sh 3,150 s,br m
s
(NH)F
1u
3,195 w 3,292 vs m
s
(NH)E
g
3,016 w,sh 2d
as
(HNH)F
1u
3,160 s,br 2d
as
(HNH)F
2g
1,764 w 1,764 w,sp 2m
2
(NO
3-
)A
2
00
1,662 m 2m
4
(NO
3-
)E’
1,630 m,br 1,627 m d
as
(HNH)F
2g
1,616 m 1,627 m,br d
as
(HNH)F
1u
1,383 vs 1,386 vs 1,370 1,350 w,br 1,416 s,br 1,370 m
3
(NO
3-
)E’
1,350 w,br 1,304 m d
s
(HNH)A
1g
1,192 s 1,193 w,sp 1,204 w,br d
s
(HNH)F
1u
1,180 w,br d
s
(HNH)E
g
1,045 vw 1,000 w,br 1,050 vs 1,051 vs 1,049 m
1
(NO
3-
)A
1
0
888 w,sh 991 w m
2
(NO
3-
)A
2
00
826 m 824 s,sp 830 818 vw m
2
(NO
3-
)A
2
00
719 705 s 721 s 719 m
4
(NO
3-
)E’
677 m 670 s,br q
r
(NH
3
)F
1u
374 vw 460 m,br q
r
(NH
3
)F
2u
472 m q
r
(NH
3
)F
2g
370 s 384 m m
s
(NiN)A
1g
327 vw 378 s,sp m
as
(NiN)F
1u
260 sh m
as
(NiN)E
g
250 s d
as
(NNiN)F
2g
223 vw,br 212 s d
as
(NNiN)F
1u
197 sh m
L
(lattice)A
1g
143 s,sh m
L
(lattice)F
1u
90 w,sh 92 vs m
L
(lattice)E
g
Thermal decomposition of polycrystalline [Ni(NH
3
)
6
](NO
3
)
2
1195
123

Ni(NO3Þ2!NiO þ2NO2þ1
2O2$NiO þ2NO þ3
2O2
$NiO þNO þNO2þO2:ð5Þ
Thermal decomposition of Ni(NO
3
)
2
results in the
formation of non-stoichiometric NiO
1?x
, which can
catalyse many redox reactions. Wojciechowski and
Małecki [29] indicated that the concentration of Ni
3?
achieves 1 % of Ni
2?
concentration. The system of Ni
3?
/
Ni
2?
with anionic vacancies in the oxide lattice can work
as electron and oxygen transmitters between reductive NH
3
molecules and oxidative nitrate ions or nitrogen oxides.
The rate of acceleration during final [Ni(NH
3
)
2
](NO
3
)
2
decomposition may be caused by an autocatalytic effect of
NiO
1?n
(a mixture of (1 -n)NiOnNiO
2
, where n{0,1}).
The products of reactions (4) and (5) can react with each
other, for example:
2NO þ3
2O2þ2NH3!1
2O2þN2þN2Oþ3H2Oð6Þ
NO þNO2þO2þ2NH3!þ1
2N2þN2OþNO
þ3H2O:ð7Þ
It is also a well-known fact that NO, NO
2
and O
2
are in equilibrium with each other, and above 150 C, the
main components are NO and O
2
(and NO
2
is a
minor component) and decomposes completely at higher
temperatures. It is also sure that in the presence of a
catalytically active nickel oxide, NH
3
can reduce the NO
2
,
O
2
or NO, with the formation of N
2
OorN
2
, and the N
2
O
can also react with NH
3
.
From the point of view of the probability of the above-
presented different reactions which take place during the
thermal decomposition of [Ni(NH
3
)
6
](NO
3
)
2
, and taking
into account the QMS analysis results, it is quite reasonable
to accept that the final products of this process is as such
presented by Eqs. (1–3), and specified in Table 1, where the
temperatures, percentage mass losses and the products of the
[Ni(NH
3
)
6
](NO
3
)
2
thermal decomposition in a flow of Ar
and He at the particular stages of this decomposition process
are presented . This table also presents the comparison of the
results obtained in this study with the results obtained by us
earlier [20] for a sample in corundum crucible (in Ar atmo-
sphere) with results of Farhadi and Roostaei-Zaniyani [21]
obtained in air atmosphere and with the results of Rejitha
et al. [22] obtained in He atmosphere. All measurements
were performed at a constant heating rate of 10 C min
-1
.
Unfortunately, Rejitha et al. did not state what kind of sample
container was used and, additionally, and more importantly,
these authors did not present the values of the mass loss at
particular stages of the decomposition process. As can be
seen from this comparison, there are only some small, rela-
tively insignificant differences between our presentation and
previous result and between the results obtained by Farhadi
and Roostaei-Zaniyani [21]. However, some quite distinct
difference can be observed between the above mentioned
and Rejitha et al.’s [22] results. These differences concern
our IIIa ?IIIb and their II and III stages of the decomposi-
tion. Namely, in contrary to [22], we propose
that the simple release of NH
3
molecules stopped on
[Ni(NH
3
)
2
](NO
3
)
2
, not on [Ni(NH
3
)
4
](NO
3
)
2
. Moreover, the
second difference relies on a composition of the final gaseo us
products of the decomposition. In our opinion, the compo-
sition of 4NH
3
?NO
3•
?NO
2
, proposed by Rejitha et al.
[22], is barely little probable. First, the authors did not
present MS identification of NO
3•
radical. Second, this
radical can react with NO
2
giving the products which were
presented among others by us in Eq. (2).
To summarise, our interpretation of the [Ni(NH
3
)
6
]
(NO
3
)
2
thermal decomposition process is best supported by
experimental results rather than the interpretation proposed
by Rejitha et al. [22] and, moreover, it is consistent with
the results of the structural [2,10,12], vibrational [13,14]
and reorientational [1,9,11,16,18,19] dynamics inves-
tigations of this compound.
Conclusions
The thermal decomposition process of [Ni(NH
3
)
6
](NO
3
)
2
undergoes two main stages. First, simple deamination of
[Ni(NH
3
)
6
](NO
3
)
2
to [Ni(NH
3
)
2
](NO
3
)
2
takes place on a
step-by-step basis, and 4NH
3
molecules per formula unit
are liberated in two stages (I and II) in four steps (Ia ?Ib
and IIa ?IIb). Then, decomposition of survivor
[Ni(NH
3
)
2
](NO
3
)
2
undergoes directly to the final decom-
position products: NiO
1?x
,N
2
,O
2
, nitrogen oxides and
H
2
O, without the formation of a stable Ni(NO
3
)
2
, because
of the autocatalytic effect of the formed NiO
1?x
. Obtained
results were compared both with those published by us
earlier and also with the new results published by Rejitha
et al. and agreement between our former and present result
was obtained. In both cases, 4NH
3
molecules per formula
unit are liberated and the final products of decomposition
are the same (N
2
,O
2
, NO, N
2
O, NO
2
,H
2
O and NiO).
However, some disagreement was ascertained when com-
paring our results with those of Rejitha et al. [22], in which
only 2NH
3
molecules are directly liberated in two steps
(stages I and II). Next, in stage III, [Ni(NH
3
)
4
](NO
3
)
2
decomposes into 4NH
3
?NO
3•
?NO
2
?NiO. On the
other hand, these authors finally concluded that the final
products of the [Ni(NH
3
)
6
](NO
3
)
2
pyrolysis as N, N
2
,O
2
,
NO, N
2
O, H
2
O and NiO. So, these products are (but with
the exception of N and NO
2
) nearly the same as those
which we had observed.
1196 E. Mikuli et al.
123

Acknowledgements The FT-IR investigations were carried out
with the equipment purchased thanks to the financial support of
the European Regional Development Fund in the framework of
the Polish Innovation Economy Operational Program (Contract
No. POIG.02.01.00-12-023/08).
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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