Thermal analysis of magnesium tris(maleato) ferrate(III) dodecahydrate physico-chemical studies

Article abstract of DOI:10.1023/A:1011932300499

Thermal analysis of magnesium tris(maleato) ferrate(III) dodecahydrate has been studied from ambient to 700°C in static air atmosphere employing TG, DTG, DTA, XRD, Moessbauer and infrared spectroscopic techniques. The precursor decomposes to iron(II) intermediate species along with magnesium maleate at 248°C. The iron(II) species then undergo oxidative decomposition to give α-Fe₂O₃ at 400°C. At higher temperatures magnesium maleate decomposes directly to magnesium oxide, MgO, which undergoes a solid state reaction with α-Fe₂O₃ to yield magnesium ferrite (MgFe₂O₄) at 600°C, a temperature much lower than for ceramic method. The results have been compared with those of the oxalate precursor.

Full text of DOI:10.1023/A:1011932300499

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 829–835
THERMAL ANALYSIS OF MAGNESIUM
TRIS(MALEATO) FERRATE(III) DODECAHYDRATE
Physico-chemical studies
B. S. Randhawa and K. J. Sweety
Department of Chemistry, Guru Nanak Dev University, Amritsar, 143 005, India
(Received in revised form November 25, 2000)
Abstract
Thermal analysis of magnesium tris(maleato) ferrate(III) dodecahydrate has been studied from am-
bient to 700°C in static air atmosphere employing TG, DTG, DTA, XRD, Mössbauer and infrared
spectroscopic techniques. The precursor decomposes to iron(II) intermediate species along with
magnesium maleate at 248°C. The iron(II) species then undergo oxidative decomposition to give
α-Fe2O3 at 400°C. At higher temperatures magnesium maleate decomposes directly to magnesium
oxide, MgO, which undergoes a solid state reaction with α-Fe2O3 to yield magnesium ferrite
(MgFe2O4) at 600°C, a temperature much lower than for ceramic method. The results have been
compared with those of the oxalate precursor.
Keywords: DTA, DTG, ferrites, magnesium, Mössbauer spectra, TG, XRD
Introduction
Ferrites obtained from the thermal decomposition of various iron(III)carboxylate pre-
cursors have attracted much attention not only as permanent magnets and microwave
devices, but also as a promising material for magnetic recording medium [1]. Ferrites
of pure/manganese doped magnesium have found extensive application in computer
storage systems and switching circuits [2]. Magnesium ferrite has been prepared [3]
by ceramic method which involves milling of the reactants which imparts defects and
strains in the product. Alternatively magnesium ferrite has been obtained from ther-
mal decomposition of the precursor, magnesium tris(oxalato) ferrate(III), which in
turn is synthesised by the liquid-mix technique [4]. This technique does not involve
milling of the reactants and ferrites are obtained at lower temperatures in shorter time
than in the conventional ceramic method. The present investigation of magnesium
tris(maleato) ferrate(III) has been undertaken to get ferrite at minimum possible tem-
perature and compare its thermal stability with respective oxalate complex.
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
Materials and methods
Magnesium tris(maleato) ferrate(III) dodecahydrate was prepared by mixing
stoichiometric quantities of aqueous solutions of ferric maleate, magnesium carbonate
and maleic acid. The reaction mixture was stirred vigorously and concentrated on a water
bath until a brown coloured product is formed after the addition of excess acetone. The
identity of the complex was established by chemical analysis. The content of iron was de-
termined spectrophotometrically using 1,10-phenanthroline method [5]. (Found: Fe
10.91, C 27.08, H 3.01%; calc.: Fe 10.30, C 26.56, H 3.32%).
IR spectra were recorded in KBr matrix on a Pye-Unicam IR spectrophotometer
in the range 4000–200 cm–1. Thermal analysis was performed at USIC, University of
Roorkee, Roorkee on Stanton Redcraft (STA-780) in static air atmosphere at a heat-
ing rate of 10°C min–1. XRD powder patterns were also recorded using Ni filtered
CuKα radiation. The experimental details of Mössbauer investigation were as
reported elsewhere [6]. The isomer shift values are reported w.r.t. pure iron absorber.
Results and discussion
The IR spectrum of magnesium tris(maleato) ferrate(III) dodecahydrate shows a
broad band centred at 3290 cm–1 due to ν(OH) of lattice water and a distinct shoulder
at 2910 cm–1 due to ν(C–H) of the maleate group. The overlapping bands due to
ν(C=C) and νasy(C=O) lie in the range 1570–1650 cm–1. Bands at 1435 and 1230 cm–1
are attributed to νsym(C=O) of the coordinated carboxylate group [7]. Sharp band at
860 cm–1 is assigned to cis (C–H) wagging. Small distinct band at 530 cm–1 indicates
the presence of Fe–O (carboxylate) bonding [8].
Fig. 1 Mössbauer spectrum of magnesium tris(maleato) ferrate(III) dodecahydrate at
25±2°C
Mössbauer spectrum of magnesium tris(maleato) ferrate(III) dodecahydrate at
300 K exhibits a doublet (Fig. 1) with isomer shift and quadrupole splitting values of
0.41 and 0.56 mm s–1 respectively. These parameters are in agreement with those re-
J. Therm. Anal. Cal., 65, 2001

RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
831
Fig. 2 Simultaneous TG–DTG–DTA curves of magnesium tris(maleato) ferrate(III)
dodecahydrate
ported for high spin iron(III) complexes having octahedral geometry [9]. The high
spin nature of the complex has also been confirmed by its µeff value of 5.87 B.M.
Figure 2 shows the simultaneous TG–DTA–DTG curves for magnesium
tris(maleato) ferrate(III) at a heating rate of 10°C min–1. DTA shows three consecutive
endothermic stages with peak temperatures at 97, 140 and 180°C respectively. These
stages are also revealed in TG and DTG curves. The first two stages are associated with
the dehydration which completes at 170°C as indicated by a mass loss of 20% (calc.
loss=19.9%). The third stage is due to the decomposition of anhydrous complex into
Fig. 3 Mössbauer spectrum of magnesium iron(III) maleate heated at 250°C
J. Therm. Anal. Cal., 65, 2001

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RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
Fig. 4 Mössbauer spectrum of magnesium iron(III) maleate calcined at 400°C
magnesium maleate and iron(II) oxalate as shown by a mass loss of 35% (calc.
loss=35%) at 248°C. Mössbauer spectrum of the parent complex calcined isothermally at
250°C for 15 min shows two quadrupole doublets (Fig. 3) one with isomer shift and
quadrupole splitting values of 1.20 and 2.58 mm s–1 respectively indicating the formation
of iron(II) oxalate [10] and the other doublet is due to the parent complex. As heating
continues, TG curve slackens until a mass loss of 46% is reached at 400°C indicating the
oxidative decomposition of iron(II) oxalate into iron(III) oxide along with magnesium
maleate. The corresponding DTG and DTA(exo) peaks both lie at 360°C. The
Mössbauer spectrum of the residue obtained from the parent complex after heating at
400°C for 2 h exhibits a six-line pattern (Fig. 4) with isomer shift and internal magnetic
field values of 0.44 mm s–1 and 505 KOe respectively. These parameters are in close
agreement with those reported [11, 12] for α-Fe2O3.
Table 1 Mössbauer parameters for the thermal analysis products of Mg3[Fe(C2H2C2O4)3]2 12H2O
at 300 K
Temperature of
calcination/°C
δ/
∆
/
HI/
KOe
Cationic(Fe3+)
distribution/%
EQ
Assignment
mm s–1
mm s–1
2.58
0.68
–
250
–
1.20
0.40
0.44
0.38
0.36
0.27
0.47
0.44
0.34
–
–
FeIIC2O4
parent complex
α-Fe2O3
–
–
400
600
–
505
517
493
467
543
527
508
–
0.20
0.01
–
36
25
39
41
27
32
α-Fe2O3
MgFe2O4
–
–
–
0.34
0.12
0.02
α-Fe2O3 (77 K)
MgFe2O4 (77 K)
–
–
–
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RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
833
Further, there is a strong exotherm at 510°C corresponding to a mass loss of
74.5% (calc. loss=74.16%) marking the formation of magnesium oxide from magne-
sium maleate through an intermediate step in which magnesium oxalate is formed as
shown by a shoulder in the respective DTG peak. The liberation of acetylene from
maleate to yield oxalate has been reported during ‘Kolbe electrolysis’ of metal
maleate/fumarate in solution phase [13, 14]. Magnesium maleate formed decomposes
at much lower temperature (410°C) as compared to its pure salt [15] (450°C) due to
the (i) metastable structure of the former and (ii) catalytic effect of α-Fe2O3 present.
Neither TG nor IR spectrum indicates the presence of magnesium carbonate at any
stage and thus the decomposition of mixed maleate resembles that of magnesium ox-
alate/malonate which also directly yield oxide [16, 17]. As heating continued, solid
state reaction between the oxides of magnesium and iron occurred as revealed by the
exotherm in DTA, resulting in magnesium ferrite, MgFe2O4. The presence of this
product has been affirmed by Mössbauer spectrum of the final residue of analyses
(Fig. 5). Mössbauer spectrum displays three different oxide phases, the parameters of
which (recorded at 77 and 300 K) are listed in Table 1. The parameters of the outer
sextet correspond to α-Fe2O3 while those of the inner ones [18] are assigned to
MgFe2O4. The cationic distribution of this ferroimagnetic spinel in octahedral and tet-
rahedral sites have been found to be 25 and 39% respectively.
XRD data of the final thermal analysis product (listed in Table 2) also confirms
the formation of ferrite.
Table 2 Room temperature XRD powder pattern of the thermal analysis residue of magnesium
tris(maleato) ferrate(III) dodecahydrate at 600°C
2θ
d/Å
Relative intensity
Assignment
a
MgFe2O4
18.20
30.17
33.28
35.60
37.00
40.80
43.04
49.54
54.00
57.00
4.870
2.960
2.692
2.520
2.420
2.210
2.101
1.840
1.697
1.613
23
26
MgFe2O4
b
α-Fe2O3
39
63
MgFe2O4
MgFe2O4, MgOc
25
18
α-Fe2O3
100
22
MgFe2O4
α-Fe2O3
20
MgO, MgFe2O4
MgFe2O4
25
ASTM card No.: a 17–464; b 13–534; c 4–0829
In the light of the above results, the following thermal decomposition scheme is
proposed for magnesium tris(maleato) ferrate(III) dodecahydrate:
115°C
Mg3[Fe(C2H2C2O4)3]2 12H2O
→ Mg3[Fe(C2H2C2O4)3]2 8H2O+4H2O(g)
170°C
Mg3[Fe(C2H2C2O4)3]2 8H2O
→ Mg3[Fe(C2H2C2O4)3]2+8H2O(g)
J. Therm. Anal. Cal., 65, 2001

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RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
248°C
Mg3[Fe(C2H2C2O4)3]2
→ 3MgC2H2C2O4+2FeIIC2O4+3C2H2(g)+2CO2(g)
400°C
2FeIIC2O4
→ α-Fe2O3+3CO(g)+CO2(g)
470–530°C
3MgC2H2C2O4
→ 3MgO+gaseous products
530−600°C
α-Fe2O3+3MgO
→ MgFe2O4+2MgO
Thus the maleate complex, i.e. seven-membered chelate decomposes at a lower
temperature (170°C) than the respective five-membered chelate i.e. oxalate complex
[4] (200°C). It can be explained on the basis of high temperature acid-base theory –
the weaker the anion base is the higher the temperature of decomposition [19] is; oxa-
late ion is a weaker anion base than maleate ion. Irving et al. [20] have also studied
the influence of ring size upon the stability of metal chelates and have generalized
that the stability of chelate complexes decreases with increasing ring size. Decrease
in stability with increasing chain length of the ligand has been explained, to a large
extent, on the basis of chelate/entropy factor.
Fig. 5 Mössbauer spectrum of the final thermal analysis product of magnesium
iron(III) maleate at 300 K
An attractive feature of the precursor technique undertaken is the preparation of
ferrite (end product) at a temperature much lower than that of conventional ceramic
method (>1000°C). It is even lower than the temperature for ferrite formation
(700°C) from the thermolysis of respective oxalate precursor [4]. These studies are
important because of the extensive application of ferrites as device materials in re-
cording and microwave appliances.
J. Therm. Anal. Cal., 65, 2001

RANDHAWA, SWEETY: MAGNESIUM TRIS(MALEATO) FERRATE(III)
835
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