Mechanism of decomposition of manganese(II) oxalate dihydrate and manganese(II) oxalate trihydrate

Article abstract of DOI:10.1016/j.tca.2004.04.001

The thermal behavior and magnetic properties of two manganese(II) oxalates crystallohydrates are studied and compared. MnC₂O₄· 2H₂O crystallizes in monoclinic syngony, while the MnC ₂O₄·3H₂O - in orthorhombic. The difference in the crystal lattice is responsible for the different thermal behavior of the samples. This fact is proved by in situ measurement of the magnetic properties, TGA, DTA, DSC, and X-ray diffraction. The dehydration proceeds in one step with MnC₂O₄·2H₂O (ΔH=86kJ/mol) and in three steps with MnC₂O₄· 3H₂O (ΔH=132kJ/mol). The exchange interaction between the manganese ions in MnC₂O₄·3H₂O is changing from antiferromagnetic to ferromagnetic during the entire dehydration process, while in the case of MnC₂O₄·2H ₂O the same interaction remains almost constantly antiferromagnetic. The two crystallohydrates reveal different initial and final temperatures of dehydration, thus changing the region where the anhydrous oxalate exists. The oxidative decomposition of the anhydrous oxalate leads not only to Mn(III) but also to Mn(IV) and an oxidation process is much weaker when starting from MnC₂O₄·3H₂O. This fact shows that the orthorhombic crystal lattice stabilizes the lower oxidation states of manganese. The annealing of the monoclinic MnC₂O₄·2H ₂O at 450°C leads to its complete transformation into Mn ₃O₄ with hausmanite structure. At the same temperature, the orthorhombic MnC₂O₄·3H₂O gives not only Mn₃O₄ but also a great quantity of cubic Mn ₂O₃.

Full text of DOI:10.1016/j.tca.2004.04.001

Thermochimica Acta 421 (2004) 141–149
Mechanism of decomposition of manganese(II) oxalate dihydrate and
manganese(II) oxalate trihydrate
a,∗
b,1
B. Donkova , D. Mehandjiev
a
Department of Inorganic Chemistry, Faculty of Chemistry, University of Sofia, 1 J. Bourchier Av., Sofia 1164, Bulgaria
b
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Bl. 11, Sofia 1113, Bulgaria
Received 20 October 2003; received in revised form 27 March 2004; accepted 1 April 2004
Available online 24 May 2004
Abstract
The thermal behavior and magnetic properties of two manganese(II) oxalates crystallohydrates are studied and compared. MnC
crystallizes in monoclinic syngony, while the MnC O—in orthorhombic. The difference in the crystal lattice is responsible for the
·3H
different thermal behavior of the samples. This fact is proved by in situ measurement of the magnetic properties, TGA, DTA, DSC, and X-ray
diffraction. The dehydration proceeds in one step with MnC O (ꢀH = 86 kJ/mol) and in three steps with MnC ·3H O (ꢀH =
·2H
32 kJ/mol). The exchange interaction between the manganese ions in MnC O is changing from antiferromagnetic to ferromagnetic
·3H
during the entire dehydration process, while in the case of MnC O the same interaction remains almost constantly antiferromagnetic.
2
O
4
2
·2H O
2
O
4
2
2
O
4
2
2
O
4
2
1
2
O
4
2
2
O
4
·2H
2
The two crystallohydrates reveal different initial and final temperatures of dehydration, thus changing the region where the anhydrous oxalate
exists. The oxidative decomposition of the anhydrous oxalate leads not only to Mn(III) but also to Mn(IV) and an oxidation process is much
weaker when starting from MnC
2
O
4
·3H
2
O. This fact shows that the orthorhombic crystal lattice stabilizes the lower oxidation states of
◦
manganese. The annealing of the monoclinic MnC
structure. At the same temperature, the orthorhombic MnC
2004 Elsevier B.V. All rights reserved.
2
O
4
·2H
2
O at 450 C leads to its complete transformation into Mn
3
O
4
with hausmanite
2
O
4
·3H
2
O gives not only Mn
3
O
4
but also a great quantity of cubic Mn
2 3
O .
©
Keywords: Magnetic properties; Decomposition; Synthesis; Manganese(II) oxalates
1
. Introduction
(space group Pcaa [4]). It is expected that the difference in
the crystal lattice of the compounds and the different way
of bonding of water molecules would affect some proper-
ties, such as a thermal stability, oxidation, and magnetic
behavior.
The investigation of the processes of thermal decomposi-
tion in oxalate systems is related to their wide application as
precursors for the preparation of nano-materials, supercon-
ductors, sorbents and catalysts. For this reason, the detailed
knowledge of the oxidation states of the element under con-
sideration is important.
The objects of the present study are the crystal hydrates
in the system manganese(II) oxalate–water. It has been
established that two crystal hydrates are formed in this
system—MnC2O4·2H2O and MnC2O4·3H2O [1,2]. Both
crystal hydrates differ in color and structure. The white
While the reports on the thermal decomposition of man-
ganese(II) oxalate trihydrate are quite scarce [5], this pro-
cess is extensively studied in the case of MnC2O4·2H2O in
air atmosphere [6–19]. In addition to DTA and TG, other
methods, such as gas analysis [6–9], isothermal annealing
[6,8–10], solid-phase IR spectroscopy [8,9], and measure-
ment of the electrical properties of the system [10], have
been also applied. The reported results differ in the product
types: MnO [11], mixture of MnO and Mn3O4 [12], Mn2O3
[6,7,13,14], 3MnO2·Mn2O3, transforming immediately into
Mn3O4·2MnO2 [15]. Mn3O4 is obtained either directly
␣
-MnC2O4·2H2O is monoclinic (space group C2/c) [3],
while the pink-colored MnC2O4·3H2O is orthorhombic
[
16,17], or through the intermediate: mixture of MnO and
∗
Corresponding author. Tel.: +359-2-8161215; fax: +359-2-9625438.
II
IV
MnC2O4 [10], Mn O8 (3MnO2·2MnO or Mn2 Mn3 O8)
5
E-mail addresses: nhbd@inorg.chem.uni-sofia.bg (B. Donkova),
[
9] or MnCO3 [8]. The results depend on the environment
metkomeh@svr.igic.bas.bg (D. Mehandjiev).
1
Tel.: +359-2-8703254; fax: +359-2-8705024.
(static, dynamic, self-generated), sample quantity and parti-
0
040-6031/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2004.04.001

1
42
B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
cle size, annealing rate and duration, shape of the TG con-
tainer, etc. [18]. In the present case, the system is extremely
complex due to the existence of a great number of simple
and mixed stoichiometric manganese oxides, the possibil-
ity of formation of non-stoichiometric oxide systems, as
well as the occurrence of secondary gas–gas and gas–solid
processes caused by the evolution of H2O, CO, and CO2.
However, at least one more factor should be added to the
aforementioned ones, namely the effect of the preparation
conditions on the type and structure of the initial specimen.
It has been established that the obtaining of a pure phase
of ␣-MnC2O4·2H2O by spontaneous crystallization strongly
depends on pH [4], on the nucleation mechanism (i.e., the
temperature and concentration), and on the contact time with
the mother liquor (Donkova and Djarova, submitted for pub-
lication). Depending on the synthetic path, the specimens of
the formula MnC2O4·2H2O subjected to thermal analysis
in the literature cited could be pure monoclinic dihydrate,
and washing, the product was dried in a vacuum desiccator
on silica gel.
2.2. Synthesis of manganese oxides—MnOx
For the comparison of the isothermal behavior of the two
crystallohydrates, three samples were taken from each of the
two initial oxalates. Each sample was annealed for 1 h in air
◦
atmosphere at 300, 400, and 450 C. The samples were then
tempered in a desiccator and weighed. Six samples prepared
in this way are denoted as MnO in the text.
x
2.3. Characterization and investigation
The obtained crystallohydrates—␣-MnC2O4·2H2O and
MnC2O4·3H2O, as well as the oxides obtained from
them—MnOx, were investigated by the following methods:
◦
partially dehydrated at 100 C orthorhombic trihydrate, or a
mixture of them.
2.3.1. Thermal analysis
The thermal investigations were performed on a
The aim of the present work is to study the effect of the
starting crystal lattice of the crystallohydrate on the mecha-
nism of decomposition and the type of the products obtained
by investigation and comparison of the processes of thermal
decomposition of ␣-MnC2O4·2H2O and MnC2O4·3H2O,
using in situ measurements of their magnetic properties. This
method allows the direct determination of the oxidation state
Paulik-Erdey MOM OD-102 derivatograph. The DTA and
TG curves were obtained in a static air atmosphere with
◦
sample mass of 0.200 g at a heating rate of 10 C/min in the
◦
2
5–550 C temperature range, using a standard corundum
crucible. The reference sample was pure ␣-Al2O3. DSC
analysis was carried out in a dynamic air atmosphere on
Mettler TA3000 and DSC 20 apparatuses at a heating rate
n+
of the Mn ions and their coordination during the decom-
◦
of 20 C/min.
position process. Additional information was obtained by
means of thermal analysis, as well as by X-ray diffraction.
The only comparison between the dihydrate and the tri-
hydrate, found so far, concerns only their thermal dehydra-
tion and the structure of the anhydrous oxalate [4]. A study
of the thermal decomposition processes by measuring the
magnetic properties of the aforementioned objects has not
been found so far in the literature.
2
.3.2. X-ray diffraction analysis
The XRD analysis was carried out with a D 500 Siemens
powder diffractometer using Cu K␣ radiation in a 2θ diffrac-
tion interval of 10–60 . The stepwise scanning was per-
formed with a 0.02 2θ step and counting time of 2 s per
step. The identification of the phases was realized by means
of the database JCPDS—International Centre for Diffraction
Data.
◦
◦
2
. Experimental
2.3.3. Magnetic measurements
The specific magnetic susceptibility of the two crys-
2
.1. Synthesis of MnC2O4·2H2O and MnC2O4·3H2O
Samples of MnC2O4·2H2O and MnC2O4·3H2O were ob-
tallohydrate forms, as well as that of the oxides obtained
◦
from them, was studied in the 25–300 C temperature range
(
298–573 K) with a Faraday type magnetic balance. The
tained by spontaneous crystallization. The initial reagents
respective weight losses (WL) were also determined. In the
course of the in situ monitoring of the process of decom-
position, the magnetic measurement was carried out by a
stepwise increase of the temperature. At each step, the tem-
perature was kept constant for 30 min before starting the
measurements.
The effective magnetic moments of the samples MnOx
were calculated on the basis of the Curie–Weiss law, using
the values of the magnetic susceptibilities.
were “high purity” grade MnCl2·4H2O and K2C2O4·H2O.
◦
The preparation conditions were T = 25 C, controlled acid-
ity of the medium (pH = 3.0± 0.1), and continuous stirring
of the reaction system.
The type of crystallohydrate obtained depends on the nu-
cleation mechanism—homogeneous for MnC2O4·2H2O and
heterogeneous for MnC2O4·3H2O. This fact determines the
different concentrations of the initial solutions. The syn-
thesis of MnC2O4·2H2O was carried out for 24 h and the
obtained white precipitate was filtered, washed with bidis-
tilled water, and air dried. The synthesis of the pink-colored
MnC2O4·3H2O took place for 40 min and, after filtration
In the case of in situ measurements, the magnetic moments
were calculated as:
1/2
µ
= 2.828[χ(T − θ)]
eff

B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
143
where µeff is the effective magnetic moment in Bohr’s mag-
neton BM; χ is the molar magnetic susceptibility; T is the
absolute temperature in K; θ is the Weiss constant in K.
While the magnetic moment enables the determination of
the oxidation state of the metal, the Weiss constant depends
on the environment of the given ion and it is used to moni-
tor changes in the character of interaction between the metal
ions [20–23].
3
. Results
3
.1. Thermal analysis
This study was carried out in order to obtain preliminary
information about the thermal behavior of ␣-MnC2O4·2H2O
and MnC2O4·3H2O. The results are represented in Figs. 1
and 2. In the case of the monoclinic MnC2O4·2H2O, the
Fig. 3. DSC curves of MnC2O4·2H2O (1) and MnC2O4·3H2O (2).
◦
dehydration starts at about 130 C and proceeds in one step
molecules. The experimental weight loss of 26.5% is lower
than the theoretical one of 27.8%, this deviation being within
the limits of the method accuracy. However, the slope in
the trend of the TG curve suggests that, more probably,
dehydration has not been fully completed.
◦
peak at 175 C). The experimentally determined weight loss
(
of 20% is in agreement with the calculated value. In the case
of the orthorhombic MnC2O4·3H2O, the dehydration starts
◦
◦
at a lower temperature of ≈80 C and ceases at ≈210 C.
◦
This is a three-step process (peaks at 120, 145, and 190 C),
In order to obtain additional information, the heat of de-
hydration of MnC2O4·2H2O and MnC2O4·3H2O were es-
tablished by measuring the heat capacity. Fig. 3 shows the
DSC curves, which reproduce quite well the trend of the
DTA curves. The range of dehydration of MnC2O4·2H2O is
which proves the different modes of binding of the water
◦
◦
from 121 to about 195 C, with a peak at 160 C. The de-
termined enthalpy of dehydration amounts to 97 kJ/mol, the
respective literature value being 86 kJ/mol [24].
In the case of MnC2O4·3H2O, the range of the step-
◦
wise dehydration is 75–215 C, with peaks at 109, 150, and
2
◦
00 C. These well defined peaks allow the exact determi-
nation of the bond energy for each water molecule. The re-
spective values are ꢀHI = 59 kJ/mol, ꢀHII = 45 kJ/mol,
and ꢀHIII = 16 kJ/mol. For the entire dehydration process
ꢀ
H = 132 kJ/mol; a respective literature value was not
found.
The trends of the DTA curves in Figs. 1 and 2 suggest that
Fig. 1. DTA and TG curves of MnC2O4·2H2O.
a difference exists also in the processes of decomposition and
◦
oxidation, taking place above ≈250 C. For MnC2O4·2H2O,
◦
they result in two exothermal peaks at 330 and 400 C,
◦
while for MnC2O4·3H2O only one exo-peak at 303 C is ob-
served. An obvious difference between the two samples is
◦
the observation of an exo-peak at 480 C in the DTA curve
of MnC2O4·3H2O. The experimentally determined weight
loss in region II of Fig. 1 is 55.5%, the respective theoret-
ical values being 51.4% (MnO2), 55.9% (Mn2O3), 57.38%
(
Mn3O4), and 60.35% (MnO). The analogous region for the
ꢀ
trihydrate (region II in Fig. 2) has an experimental weight
loss of 56.9%, the theoretical ones being 55.85% (MnO2),
59.92% (Mn2O3), 61.27% (Mn3O4), and 63.97% (MnO).
However, the process is obviously incomplete and passes
through intermediate non-stoichiometric phases, since in the
ꢀ
Fig. 2. DTA and TG curves of MnC2O4·3H2O.
next region III , the TG curve for the trihydrate shows a

1
44
B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
“
stable” decrease in the weight from 56.9 to 58.46%. The
ꢀ
weight loss in region IV is 59.23%.
The products obtained by the thermal decompositions are
not defined from the TG curves, since this is quite question-
able in the case of such complex systems as the oxalates of
the d-elements. The reasons for this are well grounded in
[25]. The data obtained from the thermal analysis show only
the changes occurring in the process of decomposition and
reveal the differences in the two crystallohydrates. As seen
in Fig. 3, the initial temperature of dehydration of the dihy-
drate is the same as that for the separation of the second wa-
ter molecule in the trihydrate, but the final dehydration tem-
perature is different in the latter case. This in turn shifts the
region of existence of the anhydrous oxalate. On the other
hand, the enthalpy of dehydration of MnC2O4·2H2O is com-
pared with the activation energy in [26], and a conclusion is
drawn concerning the composition of the intermediate com-
plex. It should be taken into account that ꢀH of the sec-
ond and third water molecule in the trihydrate (66 kJ/mol)
is close to the reported activation energy value for the dihy-
drate, but it reflects relations of completely different nature.
3
.2. X-ray diffraction analysis
The XRD patterns of MnC2O4·2H2O and MnC2O4·3H2O
Fig. 4. Dash-diagrams of the three MnOx samples, obtained after anneal-
coincide complete those of the reference compounds.
The monoclinic MnC2O4·2H2O was identified accord-
ing to card JCPDS No. 25-0544 and the orthorhombic
MnC2O4·3H2O—according to card JCPDS No. 32-0648.
The X-ray patterns of the obtained after annealing man-
ganese oxides MnOx are represented in Figs. 4 and 5 as dash
diagrams in order to visualize and compare the obtained
phases. The dash diagrams of Mn3O4 (JCPDS No. 24-0734)
and Mn2O3 (JCPDS No. 41-1442) are given as references
in the same figures.
◦
◦
◦
ing of MnC2O4·2H2O (a) at 300 C; (b) at 400 C; (c) at 450 C; (d)
∗
◦
Mn3O4—JCPDS 24-0734. Legend: ( ) Mn3O4; ( ) Mn2O3.
dominantly obtained but the annealing time is not indicated.
◦
After annealing at 300 C for 5 h, the obtaining of slightly
crystalline Mn O8 and small amounts of Mn3O4 is reported
5
in [9]; Mn3O4 is reported to be the main phase after an-
◦
nealing for the same time, but at 600 C. Consequently, in
◦
the temperature range 300–600 C, the type of the oxide ob-
◦
In the case of initial MnC2O4·2H2O, at 300 C the sample
tained depends on both the temperature and the annealing
duration. A detailed analysis of the influence of these fac-
tors on the product types is given in [6]. Depending on the
annealing time, different final products can be obtained at
the same temperature. The results, however, follow the trend
in the thermodynamic stability of the oxides Mn(IV) →
Mn(III) → Mn(III) + Mn(II) [27] and the transitions, es-
tablished with these systems [28].
is still strongly amorphous and the most intensive reflections
in the crystallizing phase are those of Mn3O4. The weight
◦
loss of 54% shows that annealing for 1 h at 300 C is insuffi-
cient for the completion of the oxidative decomposition and
that the process is in region II of the TG curve in Fig. 1. The
presence of MnO2 (WL 51.4%) is possible, together with
◦
Mn2O3 (WL 55.9%) and Mn3O4 (WL 57.4%). At 400 C
(
WL 56.5%), the predominant phase is Mn3O4, although the
From this point of view, the results obtained starting with
the trihydrate (Fig. 5) are more interesting and they con-
firm that the difference between the two crystallohydrates
is not only in the different release of water. When the ini-
◦
strongest line of Mn2O3 emerges, too. At 450 C, there is al-
ready a complete transformation into hausmannite, Mn3O4
(full coincidence with the reference material No. 24-0734,
◦
shown in the same figure), which is confirmed by the exper-
imental weight loss of 56.96% (theoretical value of 57.4%).
In the DTA experiments, such a complete transformation is
tial substance is MnC2O4·3H2O at 300 C, the sample is
less amorphous, which explains the stronger intensity of the
peaks and facilitates their identification. The only crystal-
◦
◦
observed only at 960 C.
lized phase observed is Mn3O4. At 450 C, the difference
The literature data for the isothermal annealing of the di-
hydrate are as follows. Mn3O4, has been obtained after an-
between the phases obtained from the two initial manganese
crystallohydrates is obvious. In this case, two phases are ob-
served in the sample—Mn3O4 and Mn2O3. The experimen-
◦
◦
nealing at 300 C for 1 h in [8], at 350 C, this has been the
only phase and a coincidence with JSPDS No. 24-0734 is
◦
tally determined weight losses are 59.14% (300 C), 59.5%
◦
◦
◦
reported. In [10] at 340 and 400 C, Mn3O4, has been pre-
(400 C), and 60.9% (450 C), the calculated ones being

B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
145
◦
Fig. 6. Temperature dependences (25–300 C) of the specific magnetic
susceptibility of MnC2O4·2H2O (curve 1) and MnOx samples obtained
◦
◦
◦
after annealing at 300 C (curve 2), 400 C (curve 3), and 450 C (curve
4).
Fig. 5. Dash-diagrams of the three MnOx samples, obtained after anneal-
◦
◦
◦
ing of MnC2O4·3H2O (a) at 300 C; (b) at 400 C; (c) at 450 C; (d)
∗
◦
Mn2O3—JCPDS 41-1442. Legend: ( ) Mn3O4; ( ) Mn2O3.
5
5.85% (MnO2), 59.9% (Mn2O3), and 61.27% (Mn3O4).
◦
Regardless of close experimental weight loss values the
diffractograms in Fig. 5 show that the residues obtained dif-
fer in composition and prove that the definition of the DTA
product based solely on the weight losses is quite question-
able and the transitions are interrelated.
Fig. 7. Temperature dependences (25–300 C) of the specific magnetic
susceptibility of MnC2O4·3H2O (curve 1) and MnOx samples obtained
◦
◦
◦
after annealing at 300 C (curve 2), 400 C (curve 3), and 450 C (curve
4).
the initial MnC2O4·2H2O and MnC2O4·3H2O and within
◦
2
5–300 C for the oxides. The values of the Weiss constant,
3
3
.3. Magnetic measurements
θ, are given in the same table, as well as the correlation co-
efficients, R. The values R show that the law is valid with a
high degree of reliability. The negative values of the constant
of Weiss indicate that all samples are antiferromagnetic, the
.3.1. Magnetic susceptibility
Figs. 6 and 7 represent the temperature dependences of
the specific magnetic susceptibility (χs) of both initial sam-
ples of manganese(II) oxalates, as well as those of the ob-
tained manganese oxides MnOx in the temperature range of
Table 1
◦
Effective magnetic moments of MnC O ·2H O and MnC O ·3H O (in
2
4
2
2
4
2
2
5–300 C. The decrease in χs with the increase of tempera-
◦
◦
the range of 25–80 C) and of the oxides (in the range of 25–300 C),
prepared by isothermal annealing
ture shows that all the samples studied display paramagnetic
properties.
No. Sample
µeff
θ (K)
R
(
BM)
3
.3.2. Magnetic moments
As it was already pointed out, the calculation of the mag-
1
2
3
4
5
6
7
8
MnC2O4·2H2O
5.92
−13 0.999
−259 0.9972
−351 0.9983
−474 0.9985
−13 0.999
◦
◦
◦
MnOx (from MnC2O4·2H2O) 300 C 4.44
netic moments provides information about the changes in
the oxidation state of the Mn ions during thermal decom-
position. The magnetic moments µeff of the samples studied
were calculated from the magnetic susceptibility using the
Curie–Weiss law. The values obtained for µeff, represented
MnOx (from MnC2O4·2H2O) 400 C 4.75
n+
MnOx (from MnC2O4·2H2O) 450 C 5.07
MnC2O4·3H2O
5.92
◦
◦
◦
MnOx (from MnC2O4·3H2O) 300 C 4.75
−299 0.9939
−266 0.9981
−387 0.9975
MnOx (from MnC2O4·3H2O) 400 C 4.79
MnOx (from MnC2O4·3H2O) 450 C 5.02
◦
in Table 1, were calculated within the range of 25–80 C for

1
46
B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
Fig. 8. Temperature dependence of the magnetic moments (curve 1) and the weight during in situ magnetic measurements (curve 2) of (A) MnC2O4·2H2O
and (B) MnC2O4·3H2O.
◦
temperature of Neel being lower than 25 C. The lower ab-
solute θ values in the initial oxalates indicate weaker anti-
ferromagnetic interaction in these samples. The higher |θ|
values in the oxides are an indication of stronger antiferro-
magnetic exchange interaction.
ture dependences of the magnetic moments of the two crys-
tallohydrates MnC2O4·2H2O and MnC2O4·3H2O are also
different (curves 1 in Fig. 8A and B).
The dehydration of MnC2O4·2H2O proceeds in region a
in Fig. 8A. Since the oxidation state of manganese(II) is be-
ing preserved during the entire dehydration process, it fol-
lows that the magnetic moment should not change. Indeed,
µeff remains constant in this temperature range, which is an
indication that there is only Mn(II) in the sample.
3
.4. Changes in the magnetic moments of MnC2O4·2H2O
and MnC2O4·3H2O upon dehydration and decomposition
The magnetic moments of the initial oxalates at a given
temperature are shown as curves 1 in Fig. 8A and B. The
results are compared with the weight losses of the samples
of MnC2O4·2H2O and MnC2O4·3H2O versus temperature
In the case of MnC2O4·3H2O, dehydration occurs with
a stepwise decrease of the weight (region a in Fig. 8B). If
one assumes that θ has a constant value or equals zero, then
the calculated magnetic moment µeff shows an unexpected
increase in its value upon dehydration of the sample. How-
ever, there is no noticeable change in the oxidation state,
in the coordination state, or in the field strength, since this
would lead to a decrease not to an increase in the magnetic
moment of Mn(II). Therefore, one should suppose that θ is
changing upon dehydration of this sample, while the oxida-
tion state is being preserved, i.e., µeff remains constant.
Fig. 9 represents the temperature dependences of the con-
stant θ for the two crystallohydrates calculated at µeff =
(
curves 2 in Fig. 8A and B) during in situ magnetic mea-
surements. The regions denoted on the TG curves basically
correspond to the following processes: (a) dehydration of
MnC2O4·2H2O and MnC2O4·3H2O, respectively; (b) region
of existence of anhydrous oxalates; (c) decomposition of the
oxalates.
4
. Discussion
The value of the manganese magnetic moment depends
on its oxidation state, its coordination state, and the strength
of the crystal field [20–22]. In the case of the oxalate ions,
it should be expected that the crystal field is of an interme-
diate strength and the values of µeff for the various oxida-
tion states are 5.92 BM for Mn(II), 4.70 BM for Mn(III), and
3
.44 BM for Mn(IV) [21]. The value of the magnetic mo-
ment, calculated for the initial oxalates (Table 1, samples 1
and 5) fully coincides with the theoretically expected value
of 5.92 BM, which means that manganese is in the (+2) ox-
idation state.
The TG curves of the initial oxalates (curves 2 in Fig. 8A
and B) demonstrate both the different mode of binding of
the water in crystal lattice and the temperature differences
upon decomposition. An interesting fact is that the tempera-
Fig. 9. Temperature dependence of the Weiss constant, θ, for
MnC2O4·2H2O (curve 1) and MnC2O4·3H2O (curve 2) calculated at
µeff = 5.92 BM.

B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
147
5
.92, corresponding to the existence of Mn(II)-ions. The
ment with literature data, see Section 3.2), the substantially
lower µeff value is an indication of the presence of a great
amount of Mn(IV) and Mn(III) species in an amorphous
state. For this reason, as reported in [9], a slightly crystalline
2MnO·3MnO2 together with traces of an Mn3O4-like phase
is obtained only after annealing for 5 h of MnC2O4·2H2O
since the time period is necessary for the reorganization and
crystallization of the Mn(II) and Mn(IV) species. The ob-
taining of 3MnO2·Mn2O3, transforming immediately into
Mn3O4·2MnO2, is reported in [15]. In [10], by studying the
change in the electrical properties of MnC2O4·2H2O, it is re-
ported that the conductivity characteristic of Mn3O4 cannot
constant of Weiss depends on the number of neighbors, with
which the respective ions are interacting, and also on the
sign of the exchange integral [23]. While in the case of
MnC2O4·2H2O (curve 1), the negative value of θ is pre-
served almost within the entire temperature range, a change
in the sign is observed for MnC2O4·3H2O (curve 2) from
the very beginning of the dehydration process. This obser-
vation shows that upon dehydration of MnC2O4·3H2O the
indirect exchange between the Mn(II) ions changes from an-
tiferromagnetic into a ferromagnetic one. At the end of the
process, θ acquires again a negative value, which is an evi-
dence of antiferromagnetic interaction.
◦
be achieved at about 300 C, regardless of the fact that XRD
The regions denoted by b in Fig. 8A and B represent the
temperature ranges of existence of the anhydrous oxalates,
and they are different for the two crystallohydrates. When
the initial material is monoclinic ␣-MnC2O4·2H2O, the an-
hydrous oxalate is stable at lower temperatures. The absence
of considerable changes in the TG curves for both samples
shows that there are no transformations, occurring within
this temperature range. This fact allows us to assume that the
oxidation state, respectively the magnetic moment µeff, is
preserved almost constant (curves 1, Fig. 8A and B), while
θ is varying due to the change in the nature of the exchange
interaction.
data confirm its formation. In our opinion, the presence of
higher oxidation states as well is the most probable expla-
nation of the increase in electroconductivity at T > 226 C
◦
observed in [10].
The comparison of the samples obtained at 300 C and
◦
the other oxides in Table 1 shows that µeff is raising with the
increase in annealing temperature. It is seen, however, that
this increase in the magnetic moment is much stronger in
the specimens obtained from MnC2O4·2H2O. The obtained
µ
eff
values are in agreement with the phases established in
the XRD analysis.
Based on the values of the magnetic moments of the ox-
ides obtained after annealing of the two crystallohydrates
(Table 1), one can determine the approximate content of
Mn(IV) in the samples (Table 2).
The oxidative decomposition of the anhydrous oxalates
proceeds in the regions c in Fig. 8A and B. For the two sam-
ples, however, the initial temperatures of this process are dif-
ferent. In the case of MnC2O4·2H2O, the respective value is
Table 2
◦
◦
T ≥ 205 C, while for MnC2O4·3H2O T is ≥243.5 C. The
Mn(IV) content in the MnOx samples obtained after annealing
values, obtained for µ (curves 1 in Fig. 8A and B), are
eff
Annealing
temperature ( C)
Manganese(IV) (%) in the MnOx
an indication that the mechanism of this oxidation depends
on the initial crystallohydrate form. In order to preserve the
value of 5.92 BM for the magnetic moment (characteristic
◦
From MnC2O4·2H2O
From MnC2O4·3H2O
3
400
450
00
87
50
9
50
45
16
of Mn2 ) at these two temperatures, the absolute value of
the constant of Weiss should be very high |θ| ≈ 610, which
is impossible. This is an evidence of the occurrence of ox-
idation processes. However, it should be noted that: (i) the
decrease in the magnetic moments is much stronger than
expected, if one assumes that the oxidation proceeds only
+
The differences in the starting temperatures of decompo-
sition of the obtained anhydrous oxalates and the different
rates of decrease in their magnetic moments are associated
with the crystalline structure of the initial oxalates; it affects
the different ways of dehydration and different texture, and
probably the structure of the anhydrous product.
to the formation of Mn3O4 (for the pure hausmanite µeff
.14 BM) and (ii) the magnetic moments of the two oxalates
decrease to a different extent.
=
5
In the in situ investigations, the oxidation process with the
Dollimore ascertained that upon dehydration and during
decomposition, or just before decomposition, 3d-oxalates
undergo a strong increase in their surface area. This in-
crease and the changes in the physical texture depend on
numerous factors, such as shape and size of the initial par-
ticles, the compactness and the adhesion between parti-
cles, the ratio between the densities of the oxalate and of
the solid product, as well as on whether the gases formed
are removed as quickly as possible [17,18]. The investi-
gated crystallohydrates differ in shape. Scanning electron
microscopic observations were carried out in the present
work. It was established that the monoclinic MnC O ·2H O
initial MnC2O4·2H2O leads essentially to Mn(IV), as indi-
cated by the obtained µ , which is closer to the theoretical
eff
value for Mn(IV) than to that for Mn(III). The respective
µ
in the case of initial MnC2O4·3H2O indicates that at
eff
the same temperature the oxidation process is less intensive.
These results are confirmed also by the values of µ
eff
◦
for the MnOx obtained after isothermal annealing at 300 C
Table 1, samples 2 and 6). Again, the calculated µeff for
(
both samples are considerably lower than 5.14 BM and the
value for sample 2 is lower than that for sample 6. Irre-
spective of the XRD data, suggesting that the crystal phase
emerging in both samples is Mn3O4 (which is in agree-
2
4
2
represents “rose-like” agglomerates with irregular arrange-

1
48
B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
ment of their rounded “leaflets,” while the orthorhombic
MnC2O4·3H2O is in the shape of long, smooth needles. The
difference in the dispersities of the two crystallohydrates is
obviously due to their mechanisms of nucleation, the lat-
ter being homogeneous for MnC2O4·2H2O and heteroge-
neous for MnC2O4·3H2O (Donkova and Djarova, submit-
ted for publication). These facts could explained the dif-
ferent change of the surface area of the two products. An
optical microscopic comparison of the product obtained af-
ter dehydration of the initial dihydrate and trihydrate is
made in [4]. It shows that a powder is obtained in the
first case, while in the second case the shape of the ini-
tial crystals is preserved. The our scanning electron micro-
scopic observations established that the “leaflets” of the di-
hydrate breaking down during dehydration due to crack for-
mation caused by the water release. The situation with the
trihydrate is completely different. After dehydration, quite
straight grooves are observed on the surface of the initial
long smooth needles. Breakdown of these needles was not
established.
to manganese(III) but also to manganese(IV), which is
further pass to lower oxidation states. The oxidation pro-
cesses proceeds to a different extent depending on whether
the initial material is MnC2O4·2H2O or MnC2O4·3H2O.
This fact shows that the orthorhombic crystal lattice of
the trihydrate stabilizes the lower oxidation states of
manganese.
The annealing of the monoclinic ␣-MnC2O4·2H2O at
◦
450 C leads to its complete transformation into tetrago-
nal Mn3O4. At the same temperature, the orthorhombic
MnC2O4·3H2O gives not only Mn3O4, but also a large quan-
tity of cubic Mn2O3.
Both crystallohydrates are paramagnetic. The exchange
interaction between the manganese ions in MnC2O4·3H2O
is changing from antiferromagnetic to ferromagnetic dur-
ing the entire dehydration process, while in the case of
MnC2O4·2H2O the same interaction remains almost con-
stantly antiferromagnetic.
The enthalpy of dehydration of MnC2O4·3H2O is deter-
mined to be 132 kJ/mol, the respective values for the first,
second, and third molecule being 59, 45, and 16 kJ/mol.
Therefore, using monoclinic MnC2O4·2H2O as the start-
ing material, a considerably greater change in the surface
and structure is established, which leads to amorphization of
the sample, and which is not the case with the orthorhom-
bic MnC2O4·3H2O. This determines the differences in the
thermal and chemical stability of the obtained products and
affects the kinetics of the thermal decomposition. The more
developed surface possesses a greater reactivity, it is oxi-
dized faster and to a greater extent, which explains the lower
decomposition temperature of the product obtained from the
dihydrate and the stronger decrease of its magnetic moment.
According to literature data [7,13,29], in air atmosphere an
oxidation process to Mn(III) takes place just before decom-
position and calalyzes the later. After decomposition the
References
[1] M. Gorgeu, Compt. Rend. 47 (1858) 929.
[2] Otto Hauser, F. Wirth, J. Pract. Chem. 79 (2) (1909) 358.
[3] R. Deyrieux, C. Berro, A. Peneloux, Bull. Soc. Chim. Fr. 1 (1973)
25.
[4] A. Huizing, H.A.M. van Hal, W. Kwestroo, C. Lahgereis, P.C. van
Loosdregt, Mater. Res. Bull. 12 (1977) 605.
[5] K.B. Zaborenko, S.-C. Kung, L.L. Melikhov, V.A. Portyanoi, Ra-
diokhimiya 6 (6) (1964) 749.
[6] D. Dollimore, J. Dollimore, J. Little, J. Chem. Soc. (A) (1969) 2946.
[7] M.E. Brown, D. Dollimore, A.K. Galway, J. Chem. Soc., Faraday
Trans. 1 (70) (1974) 1316.
[8] A.K. Nohman, H.M. Ismail, G.A. Hussein, J. Anal. Appl. Pyrolysis
m+
evolved CO reduces Mn
present Mn
to lower oxidation states and the
m+
n+
and Mn catalyzes the oxidation of CO to
34 (1995) 265.
CO2 [29]. In the present work was established that the oxi-
dation proceeds not only to Mn(III) but also to Mn(IV). For
the above reasons, all redox processes are much more inten-
sive when starting with MnC2O4·2H2O. Taking into account
the necessity of crystallization of the obtained in this case
amorphous phase, one can explain the more complex shape
of the respective DTA curve.
The corresponding interval for MnC2O4·3H2O is much
narrower, proving comparatively limited oxy-reduction pro-
cesses. In this case (1) the initial particles are considerable
bigger, (2) the anhydrous oxalate has a higher degree of crys-
tallization, and (3) the smaller change of the surface leads to
a more weakly oxidized product and respectively to a slower
decomposition process and a lower degree of reduction.
[9] M.I. Zaki, A.K. Nohman, Ch. Kappenstein, T.M. Wahdan, J. Mater.
Chem. 5 (7) (1995) 1081.
[10] A.K. Nikumbh, A.E. Athare, S.K. Pardeshi, Thermochim. Acta 326
(1999) 187.
[11] C. Duval, Inorganic Thermogravimetric Analysis, Elsevier, Amster-
dam, 1953.
[12] J. Robin, Bull. Soc. Chim. Fr. (1953) 1078.
[13] E.D. Macklein, J. Inorg. Nucl. Chem. 30 (1968) 2689.
[14] Gy. Baksy, A.J. Hegedus, Thermochim. Acta 10 (1974) 399.
[15] M.U. Jacob, D.D. Perlmutter, Thermochim. Acta 49 (1981) 207.
[16] D. Dollimore, D. Griffiths, D. Nicholson, J. Chem. Soc. 166 (1963)
2617.
[
[
[
[
17] D. Dollimore, D. Nickolson, J. Chem. Soc. 165 (1962) 960.
18] D. Dollimore, Thermochim. Acta 117 (1987) 331.
19] D. Dollimore, J. Term. Anal. 11 (1977) 185.
20] F.E. Mabbs, D.J. Machin, Magnetism and Transition Metal Com-
plexes, Chapman and Hall, London, 1973, p. 153.
[21] D. Mehandjiev, S. Angelov, Magnetochemistry of Solid State, Nauka
I Izkustvo, Sofia, 1979, p. 116.
5
. Conclusions
[
22] R. Boca, Theoretical Foundations of Molecular Magnetism, Else-
vier, Amsterdam, Lousanne, New York, Oxford, Shannon, Singapore,
Tokyo, 1999, p. 504.
On the basis of the obtained magnetic data, it is proved
that the oxidation of the anhydrous product leads not only
[23] J.H. Van Vleck, J. Phys. 12 (1951) 262.

B. Donkova, D. Mehandjiev / Thermochimica Acta 421 (2004) 141–149
149
[
[
[
24] K. Nagase, K. Sato, N. Tanaka, Bull. Chem. Soc. Jpn. 48 (2) (1975)
[27] D. Mehandjiev, R. Proiniva-Mehandjieva, Compt. Rend. Acad. Bulg.
Sci. 33 (8) (1980) 1077.
[28] P. Pascal, Nouveau Traitè de Chimie Minerale, vol. XVI, 1960, pp.
748, 785.
4
39.
25] M. Maciejewski, E. Inger-Stocka, W.-D. Emmerich, A. Baiker, J.
Therm. Anal. Cal. 60 (3) (2000) 735.
26] D. Dollimore, N. Guindy, Thermochim. Acta 58 (1982) 191.
[29] D. Dollimore, D.L. Griffiths, J. Therm. Anal. 2 (1970) 229.