Thermal analysis of some polynuclear coordination compounds as precursors of iron garnets (M3Fe5O12, M=Y3+ or Er3+)

Article abstract of DOI:10.1007/s10973-007-8839-4

The thermal behaviour of four coordination compounds (NH₄) ₆[Y₃Fe₅(C₄O₅H ₄)₆(C₄O₅H₃) ₆]?12H₂O, (NH₄)<

Full text of DOI:10.1007/s10973-007-8839-4

Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 1, 307–312
THERMAL ANALYSIS OF SOME POLYNUCLEAR COORDINATION COM-
POUNDS AS PRECURSORS OF IRON GARNETS (M3Fe5O12, M=Y3+ or Er3+)
Luminita Patron1, Oana Carp1,*, I. Mindru1, G. Marinescu1, J. Hanss2 and A. Reller2
1‘Ilie Murgulescu’ Institute of Physical Chemistry, Spl. Independentei 202, sect.6, 060021 Bucharest, Romania
2Solid State Chemistry, University of Augsburg, Universit¬tsstrasse 1, 86159 Augsburg, Germany
The
thermal
behaviour
of
four
coordination
compounds
(NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O,
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O, (NH4)6[Er3Fe5(C4O5H4)6(C4O5H3)6]·10H2O and (NH4)6[Er3Fe5(C4O6H4)6(C4O6H3)6]·
22H2O has been studied to evaluate their suitability for garnet synthesis. The thermal decomposition and the phase composition of
the resulted decomposition compounds are influenced by the nature of metallic cations (yttrium-iron or erbium-iron) and ligand
anions ( malate or gluconate).
Keywords: coordination compounds, garnets, precursors, thermal analysis
Introduction
performed. The separated fine light brown precipi-
tates were filtered, washed with ethanol and dried.
The compounds were characterized by elemental
chemical analysis: the metal content was determined
by atomic absorption with a SAA1 device and
gravimetric techniques, and the CHN by a Carl Erba
Model 1108 CHNS-O elemental analyzer. Four poly-
nuclear coordination compounds were synthesized:
(NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O,
In the last years, the use of coordination compounds
as sources of mixed oxides has expanded [1–3]. The
decomposition processes of various metal carboxylates
under heating were widely studied. The simplest
compounds like formates and oxalates were scrutinized;
the compounds containing as ligands anions of
polyhydroxycarboxylic, aromatic and unsaturated
carboxylic acids were also investigated [4–12].
In this paper we report the thermal behaviour of
four polynuclear coordination compounds containing
as ligands malate and gluconate anions, potential
precursors of yttrium and erbium garnets (M3Fe5O12,
M=Y3+ or Er3+), emphasizing the influence of the
complexing agent on the thermal behaviour of the
precursor and implicit on mixed oxides properties.
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O,
(NH4)6[Er3Fe5(C4O5H4)6(C4O5H3)6]·10H2O and
(NH4)6[Er3Fe5(C6O7H10)6(C6O7H9)6]·22H2O.
The thermochemical investigations were carried out
under dynamic air atmosphere (15 cm3 min–1), with
sample mass about 40 mg at heating rates of
5–20 K min–1. DSC measurements were performed on
a Netzsch thermobalance STA 409CC/PG/PC type,
while the thermogravimetry/mass spectrometry on a
STA 409 C one. The IR spectra (400–4000 cm–1)
were recorded with a BIO-RAD FTIR 125 type
spectrophotometer in KBr pellets. The crystalline
phases in the calcinated powders were identified by
XRD powder methods using a Philips Xpert X-Ray
diffractometer (CuKa radiation). Diffraction peaks
were fitted assuming Voight-function for the peak
profile. For the determination of the average crystal-
lites size it has been used the Scherrer formula
D=0.91l/(bcosq), where D is the crystallite size, l the
wavelength (CuKa), b the corrected half-width
obtained using a-quartz as reference and q the
diffraction angle. The transmission microscopy inves-
tigations of the obtained oxides were performed on a
Philips CM30 device.
Experimental
As raw materials in the synthesis of precursors,
Fe(NO3)3·9H2O, Y2O3, Er2O3, malic acid and
d-gluconolactone of reagent quality (Merck) were
used. First, the oxides were converted into nitrates by
HNO3 adding. Afterward, from a solution containing
metal nitrates and polyhydrocarboxylic acid, in a ratio
M3+–Fe3+–carboxylic acid=3:5:12 for malic acid and
3:5:24 for d-gluconolactone the polynuclear coordination
compounds were separated by extraction with ethanol. A
complete precipitation required 24 h, time interval in
which repeated adjustings of the pH to 5–6 values by
addition of a 1:1 NH4OH:C2H5OH solution were
*
Author for correspondence: carp@acodarom.ro
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

PATRON et al.
Results and discussion
a
The isolated coordination compounds are characterized
by the molecular formula
(NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O,
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O,
(NH4)6[Er3Fe5(C4O5H4)6(C4O5H3)6]·10H2O and
(NH4)6[Er3Fe5(C6O7H10)6(C6O7H9)6]·22H2O, where
C4O5H42– , C4O5H33– are malate di- and tri-anions and,
C6O7H21–0 , C6O7H93– , gluconate di- and tri-anions.
In the complexes the metallic ions adopt an octa-
hedral configuration [13, 14]. The anionic ligands are
coordinated by both the carboxylic and hydroxyl
groups [15]. Details about synthesis and physical-chemical
characterization of the coordination compounds are
presented elsewhere [16].
b
Yttrium-iron coordination compounds
thermal decomposition
In the temperature range of 331.9–1160.8 K the malate
compound (NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O
undergoes a stepwise decomposition in five distinct
stages of mass loss. The observed mass loss (70.08%)
is in good agreement with the calculated one (69.85%).
The TG, DSC, DTG and the most representative ion
intensities curves corresponding to m/z=18 (H2O) and
44 (CO2) are depicted in Fig. 1.
Fig. 1 a – TG, DTG, DSC, and b – ion intensities curves
m/z=18 (H2O) and 44 (CO2) of the polynuclear
coordination compound
The first decomposition step (331.9–395.6 K,
max DTG=359.4 K) represents the evolving of six NH3
T
(NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O
(heating rate=5°C min–1)
and two H2O molecules (calcd./found: 6.10/5.64%).
The second step, (395.6–558.1 K, Tmax DTG=450 K),
consists in an overlapping of at least two decom-
position steps: the releasing of the remainder outer
coordination sphere water molecule and the oxidative
degradation of the malate ligand. The formation of an
oxoacetate intermediate corresponding to the mole-
cular formula [Y3Fe5O8(CH3COO)8] may be advan-
ced based on FTIR results (the appearance of
absorption peaks at ~1020 and 1050 cm–1 attributed to
acetate anion [17]) and on the thermogravimetric calculus
(calcd./found: 47.73/46.13%). The development of
acetate intermediates during the degradative oxidation
of malates is already mentioned by literature [18–21].
Rising the temperature, this intermediate decomposes
(558.1–661.9 K, Tmax DTG=620 K) leading to an
amorphous oxocarbonate, namely Y3Fe5(CO3)3O9
(calcd./found: 11.27/12.75%). The last three decom-
position steps (661.9–1160.8 K) represent a stepwise
decomposition of the formed oxocarbonate:
Y3Fe5(CO3)3O9®Y3Fe5(CO3)O11 (661.9–803 K,
Fig. 2 IR spectra of a – (NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·
12H2O and b – (NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O
decomposition intermediates isolated at 500°C
(Fig. 2). The carbonate characteristic absorption peaks
are detected at: ~1550, ~1390 (attributed to the split
anti-symmetrical carbonate stretching) and ~850 cm–1
(attributed to the out of plane bending of carbonate).
T
max DTG=728 K, calcd./found: 3.56/3.39%)®
Y3Fe5(CO3)0.5O11.5 (948.6–1033.8 K, calcd./found:
0.89/0.78%)®Y3Fe5O12 (1054.8–1160.8 K, calcd./found:
0.89/0.83%). The presence of the carbonate anion is
confirmed by FTIR analysis of the reaction intermediate
308
J. Therm. Anal. Cal., 92, 2008

THERMAL ANALYSIS OF SOME POLYNUCLEAR COORDINATION COMPOUNDS
Fig. 3 a – TG, DTG, DSC, and b – ion intensities curves
m/z=18 (H2O) and 44 (CO2) of the polynuclear
coordination compound
Fig. 4 DSC curves registered for
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O
(heating rate=5°C min–1)
a – (NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O and
b – (NH4)6[Er3Fe5(C6O7H10)6(C6O7H9)6]·22H2O
compounds (heating rate 20°C min–1)
For the gluconate coordination compound
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O six decom-
position mass losses are evidenced in the temperature
range of 316.3–1050.2 K (Fig. 3). The experimental
and the theoretical mass losses assuming Y3Fe5O12 form-
ation are close (calcd./found: 76.34/76.39%). The TG,
DSC, DTG and m/z=18 (H2O) and 44 (CO2) curves
are presented in Fig. 3.
The first step (316.3–417 K, Tmax DTG=342.8 K)
represents the endothermic release of six ammonia
and two water molecules (calcd./found: 4.42/4.31%).
The next four decomposition stages (417–480.8 K,
results: (i) the stoichiometric results (mass loss
calcd./found.=70.51/70.67%) (ii) the presence in the
FTIR spectrum of the intermediate isolated at 773 K
of an absorption peak at 2350 cm–1, characteristic to
the trapped CO2 [22] and the absence of the CO32–
typical absorption bands. The formed decomposition
intermediate decomposes in the temperature range
1020.3–1050.2 K (Tmax DTG=1030.5 K) with the eli-
mination of one CO2 molecule, stoichiometry sus-
tained from both thermogravimetric and mass spect-
rometry calculation (calcd./found.=1.41/1.38%). This
decomposition process is associated with an overall
exothermic process, which suggests that the endo-
thermic elimination of CO2 is accompanied or has as
effect structural rearrangements such as crystal-
lization. The exothermic effect associated to the
structural rearrangements prevails in comparison with
the endothermic release of CO2. Performing experi-
ments with high heating rates (20 K min–1), it was
possible to discriminate the existence of the two
processes attributed to CO2 elimination and structural
rearrangements. The obtained DSC curve (Fig. 4a)
evidenced two thermal effects the first at 1050.9 K
and the second as a shoulder at ~1091 K. The TEM
investigations (Fig. 5) of the final decomposition
T
max DTG=355.7 K; 480.8–507 K, Tmax DTG=493.6 K;
507–627.4 K, Tmax DTG=562.9 K; 627.4–735.8 K,
max DTG=672.4 K) associated with exothermic effects
T
represent the oxidative degradation of the ligand.
Their thermal stoichiometry can not be established
due to the partially overlapping processes. The
corresponding mass losses are: 23.96, 8.57, 29.74 and
8.4% and the main evolved gaseous products are CO2
and H2O. The formed reaction intermediate may be
formulated as an oxide (amorphous as resulted from
X-ray investigations) containing entrapped CO2, with
the molecular formula Y3Fe5O12(CO2). The above
formulation is sustained by the following experimental
J. Therm. Anal. Cal., 92, 2008
309

PATRON et al.
a
b
Fig. 5 TEM image of the product resulted from
(NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O decomposition
products reveal the existence of spherical voids (with
diameters up to 50 nm) characteristic for entrapped
gases evolving.
Er-Fe coordination compounds thermal decomposition
The thermal decomposition of the coordination compound
(NH4)[Er3Fe5(C4O5H4)6(C4O5H3)6]·10H2O occurs in the
temperature range of 325.8–1002.5 K. The registered
experimental loss is 63.76% in comparison with the
theoretical one 63.24% assuming Er3Fe5O12 form-
ation as the final product. The TG, DTG, DSC and
m/z=18 (H2O) and 44 (CO2) curves are depicted in
Fig. 6.
Fig. 6 a – TG, DTG, DSC, and b – ion intensities curves
m/z=18 (H2O) and 44 (CO2) of the polynuclear
coordination compound
(NH4)6[Er3Fe5(C4O5H4)6(C4O5H3)6]·10H2O
(heating rate=5°C min–1)
leads to the formation of a final product with formula
Er3Fe5O12 (calcd./found: 73.02/73.16%). The TG,
DSC, DTG and m/z=18 (H2O) and 44 (CO2) curves
are presented in Fig. 7.
The decomposition starts (325.8–417 K,
max DTG=367 K) with the endothermic evolving of
T
6NH3 and 2H2O molecules (calcd./found: 5.28/4.50%).
The second process occurs at 417–562.8 K,
(Tmax DTG=478 K). Besides the evolving of the remainder
water molecules, the decomposition of the organic
ligand occurs. As in the case of yttrium-iron malate
coordination compound, the formation of an oxoacetate
decomposition intermediate corresponding to the
Er3Fe5O6(CH3COO)12 (calcd./found: 34.84/34.48%)
may be advanced. The same absorption peaks
characteristic to acetate anion are presented in the
FTIR spectrum of the above mentioned intermediate.
Further, the intermediate decomposes in the temperature
range of 562.8–652.4 K, leading to the formation of
the oxocarbonate Er3Fe5(CO3)4O8 (calcd./found:
16.47/17.56%). Further, a two-stepped decomposition
of Er3Fe5(CO3)4O8 to Er3Fe5O12 via Er3Fe5(CO3)2O10
The evolving of the 6NH3 and H2O molecules
(calcd./found: 5.82/5.81% represents the first
decomposition step (330–392.3 K). The next decom-
position stage (392.3–742.4 K) associated with
exothermic effects, represents besides the elimination
of the remainder water molecules (the peak from
187°C on m/z=18 curve) the oxidative degradation of
the ligand. Three peaks were identified on both DTG
and DSC curves (Tmax DTG=484.6, 552.9 and ~597 K,
Tmax DSC=484.5, 558.8 and 602.5 K). The shape of both
curves are characteristic for overlapping processes.
The decomposition intermediate isolated at 773 K
corresponds to the molecular formulation Er3Fe5O12(CO2)
(calcd./found: 65.99/65.28%), being amorphous from
crystallographic point of view. Similar to the intermediate
obtained from yttrium-iron gluconate compound, the
FTIR spectrum of Er3Fe5O12(CO2) intermediate
presents an absorption peak at 2350 cm–1, characteristic
to the trapped CO2. The last decomposition step
(1012.5–1042.5 K) corresponds to the evolving of the
remainder CO2 molecules and the formation of
Er3Fe5O12 garnet (calcd./found: 1.21/1.12%). As in
the case of analogous yttrium compound, two processes
occurs at 652.4–1002.5 K, (652.4–802.4 K, Tmax DTG
=
737.4 K; 952.4–1002.5 K, Tmax DTG=987 K;
calcd./found: 3.32/3.62%; calcd./found: 3.32/3.18%).
The single gaseous product detected during the two
last processes is CO2.
Thermal decomposition of the gluconate
compound, (NH4)6[Er3Fe5(C6O7H10)6(C6O7H9)6]·
22H2O which occurs in the range of 330–1113 K
310
J. Therm. Anal. Cal., 92, 2008

THERMAL ANALYSIS OF SOME POLYNUCLEAR COORDINATION COMPOUNDS
Fig. 8 X-ray diffraction pattern of the Y3Fe5O12 obtained from
gluconate compound at 800°C (1 h calcination time)
Fig. 7 a – TG, DTG, DSC, and b – ion intensities curves
m/z=18 (H2O) and 44 (CO2) of the polynuclear
coordination compound
(NH4)6[Er3Fe5(C6O7H10)6(C6O7H9)6]·22H2O
(heating rate=5°C min–1)
were identified on the DSC curve the Fig. 4b, corres-
ponding to CO2 and structural rearrangements.
Mixed oxide characterization
The X-ray analysis of the oxides obtained by the
decomposition of the four coordination compounds
reveals the following:
• The thermal decompositions of both malate compounds
generate at 973–1073 K a mixture of simple (Y2O3,
Er2O3, Fe3O4, a- and g-Fe2O3) and mixed (YFeO3,
ErFeO3 and Er3Fe5O12) oxides. While in the case of
the product derived from yttrium-iron malate
compound Y3Fe5O12 crystallizes only in small
amounts, at temperatures higher than 1173 K, in the
decomposition products obtained from erbium-iron
malate compound, the garnet represents the majority
phase. The mean crystallite sizes of yttrium and
erbium garnets obtained at 1173 K are 335
respectively, 455 .
• The thermal decompositions of yttrium-iron and
erbium-iron gluconate compounds lead to the
obtaining of pure garnets (Y3Fe5O12 and Er3Fe5O12)
at 1073 K (Fig. 8). The mean crystallite sizes of
yttrium and erbium garnets obtained at 1073 K are
430 and 300 , respectively.
Fig. 9 FTIR spectra of the end products obtained by
decomposition of
a – (NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O and
b – (NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O coordina-
tion compounds (heating rate 5°C min–1)
J. Therm. Anal. Cal., 92, 2008
311

PATRON et al.
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DOI: 10.1007/s10973-007-8839-4
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