Synthesis and thermal decomposition of mixed Gd-Nd oxalates

Article abstract of DOI:10.1007/s10973-007-8503-z

Several (Gd_1-xNd_x)₂[C₂O ₄]₃?nH₂O samples (0≤x≤1) were prepared by a coprecipitation method: the precipitation is quantitative and all the samples are homogeneous in stoichiometry. XRD analyses have shown that a complete solid solution is formed over the whole range of compositions. The dried Gd rich oxalates have initially a low water content which gradually increases with the Nd content. All the oxalates decompose in O₂ around 700°C either into a single mixed oxide or in a mixture of oxides through several steps, which can be ascribed to the loss of water and CO ₂.

Full text of DOI:10.1007/s10973-007-8503-z

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 3, 797–803
SYNTHESIS AND THERMAL DECOMPOSITION OF MIXED
Gd–Nd OXALATES
A. Ubaldini1,2, C. Artini1*, G. A. Costa1, M. M. Carnasciali3 and R. Masini4
1INFM and DCCI, Via Dodecaneso 31, 16146 Genova, Italy
2NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
3INSTM and DCCI, Via Dodecaneso 31, 16146 Genova, Italy
4CNR–IMEM, Via Dodecaneso 33, 16146 Genova, Italy
Several (Gd1–xNdx)2[C2O4]3·nH2O samples (0£x£1) were prepared by a coprecipitation method: the precipitation is quantitative and
all the samples are homogeneous in stoichiometry. XRD analyses have shown that a complete solid solution is formed over the
whole range of compositions. The dried Gd rich oxalates have initially a low water content which gradually increases with the Nd
content. All the oxalates decompose in O2 around 700°C either into a single mixed oxide or in a mixture of oxides through several
steps, which can be ascribed to the loss of water and CO2.
Keywords: oxalates, rare earths, TG/DTA analysis
Introduction
ionic conduction or dielectric properties [20]. They
are crystalline materials and are usually hydrated. The
most known form at room temperature is the monoclinic
decahydrated oxalate (space group P21/c) [20–22], even
if the compound with six water molecules has been
also reported [17, 23].
Rare earth (RE) sesquioxides are interesting materials
for their significant physical and chemical
properties [1]. Actually, they are applied in many
technological fields, such as catalysts for a large
number of organic reactions [2–4], host materials for
powerful lasers [5], improved phosphors [6] or
precursors for superconductors [7]. Particularly, some
superconducting cuprates, where two or three RE are
simultaneously present, exhibit high critical temperature
and current density [8].
Many reports exist on the thermal decomposition
of simple oxalates [12, 15, 19] but the mixed systems
are not well known. This work focuses on the
particularly interesting mixed oxalates of Gd and Nd,
since the two pure oxides, when prepared at
temperatures above 1000°C, have different structures
at room temperature: Nd2O3 is hexagonal (space group
P-3m1) and Gd2O3 is cubic (space group Ia3) [9, 10].
This work precedes a study of the pseudobinary
phase diagram of the system Gd2O3/Nd2O3, where it is
important to establish if the behaviour of the oxides
can be ascribed only to their intrinsic characters rather
than to the possible biphasicity of the starting oxalates.
In order to check the monophasicity of the
system, several mixed (Gd1–xNdx)2[C2O4]3·nH2O (0£x£1)
were prepared using a standardized coprecipitation
method. All the samples were characterised by X-ray
diffraction (XRD), SEM-EDAX and micro-Raman
analyses; their decomposition was analysed by TG
and DTA analysis.
Generally it is difficult to prepare homogeneous
mixed RE oxides or even simply homogeneous mix-
tures of RE oxides following a classical solid state route,
because of their high thermal stability and low diffusion
coefficients [9, 10]. A well known and important tool
for the preparation of mixed oxides is the thermal de-
composition of previously synthesised organic or inor-
ganic salts, like acetates, oxalates, hydroxides or car-
bonates [11–14]. These compounds decompose at rela-
tively low temperature, usually below 700°C, into an
oxide or a mixture of oxides. The decomposition path is
often very complex and undergoes many intermediate
steps [15–19], depending also on the utilized atmosphere.
When it is possible to prepare a salt where the cations
are homogeneously distributed, for example using the
coprecipitation from a solution, the decomposition will
lead to a mixed oxide or, at least, to a homogeneous
mixture of oxides [16].
Experimental
Rare earths oxalates have gained importance due
to their various properties, mainly related to their
All the (Gd1–xNdx)2[C2O4]3·nH2O samples were
prepared by a coprecipitation method, starting from
*
Author for correspondence: cristinaartini@iol.it
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

UBALDINI et al.
commercial Nd2O3 and Gd2O3 powders (Aldrich;
Mixed oxalates begin to precipitate immediately
after the addition of oxalic acid because of their high
insolubility in water (for example Ksp for Nd oxalate
is 3·10–27 [25]), leading to the formation of small
particles. SEM images of oxalates with x equal to 1,
0.5 and 0 are shown in Fig. 1a–d respectively.
The average composition of the precipitates is
very close to the nominal one. For instance, EDAX
analysis performed on many 5x5 mm2 randomly se-
lected areas yielded a Nd content of 28.3, 49.6, 80.5%
for the oxalates corresponding to nominal x=0.3, 0.5,
0.8 respectively.
99.99%). They were weighed in order to have a value
of x ranging from 0 to 1 with steps of 0.1. A solution
was prepared from these oxides using a slight excess
of HCl (13% mV–1). In order to ensure a complete
precipitation and to avoid any possible concentration
gradient, the precipitation of a mixed oxalate was
achieved by fast addition of a fresh prepared oxalic
acid solution in large excess. The precipitate was
filtered, washed with deionized water till no trace of
the Cl– anion was detected, and then dried in air at
80°C for 24 h. A little amount of each sample was
dispersed in isopropanol and analysed by SEM-EDAX.
The particle size distribution was determined from
SEM images containing at least 400 particles, using a
public domain plug-in software [24].
No great differences were detected from point to
point, meaning that the precipitation was homogeneous
and quantitative.
The aspect of the precipitates changes with x.
For Nd oxalate the particles are not isolated crystals
but rather aggregates of many thin crystals (some of
these aggregates are shown, at higher magnification,
in Fig. 1d).
The structure of all the samples was investigated
by X-ray powder diffraction using a Philips PW1830
diffractometer (CuKa radiation) in the range 5<2q<90°.
Gd rich samples were also put in direct contact
with a small amount of deionized water for one day
and then dried at room temperature (hereafter referred
as ‘aged samples’).
These aggregates have an irregular shape, formed
by small fused globes. Observed at higher magnification,
these precipitated particles, composed of crystals with
a nearly constant size, have either a cross-shaped or a
‘pop-corn’ like aspect. In any case, it is possible that
the shape and the morphology of the particles might
be modified by the high vacuum exploited during
SEM analysis: the water is released in an abrupt way
from the particles because of the low pressure.
These aggregates are, on the contrary, not very
common for Gd oxalate, particularly the cross-shaped
ones are absent. There are still many aggregates, but
their shape is irregular and the number of isolated
particles is higher.
All samples were further analyzed at room
temperature by Raman spectroscopy using
a
Renishaw System 2000 Raman imaging microscope.
The spectra were the result of 9 accumulations, each
one lasting 10 s, collected using a 633 nm He–Ne
laser; they were recorded on several points, with a
magnification of 50x.
A Netzsch 408 TG/DTA unit was utilized for the
decomposition studies performed on about 100 mg
powdered samples. The thermal schedules of the
measurements consisted of a heating step from room
temperature up to 1100°C at 5°C min–1 in flowing
oxygen (100 cm3 min–1), followed by an isothermal
plateau for one hour and then by cooling down to
room temperature. In order to observe possible
differences in the reaction mechanisms, the thermal
behaviour of (Gd0.4Nd0.6)2[C2O4]3·nH2O was studied
under different atmospheres (stream of O2 and air).
Isothermal annealing experiments were performed at
190°C (6 h).
On average an asymmetric distribution of the
precipitate particle sizes is observed in both cases,
peaked around 35 mm2 for all the compositions. For
x=1, due to the presence of the nearly spherical
aggregates, a second value is observed at 110 mm2.
The situation for x=0.5 seems to be intermediate,
as the cross-shaped particles are practically absent
and the aggregates show a more regular, almost
spherical, shape.
The single crystals forming the aggregates are
smaller and more uniform for high Nd contents. Gen-
erally they have a square or rectangular shape and, for
Nd2[C2O4]3·nH2O, the average crystal size is about
2 mm x 2 mm, for GdNd[C2O4]3·nH2O it is about
4 mm x 4 mm and for Gd2[C2O4]3·nH2O it is about
6 mm x 5 mm.
Results and discussion
Powders synthesis and characterisation
According to our experimental procedure, rare earth
oxalates form following the reaction path:
These results might indicate a different mechanism
of formation for the oxalates of Nd and Gd. When the
oxalic acid solution is added to the RE solution, even
if it is vigorously stirred, the conditions for the
formation of the precipitate are rapidly reached, as the
(1–x)Gd2O3(s)+xNd2O3(s)+6HCl(aq)
®2(Gd1–xNdx)Cl3(aq)+3H2O
®
2(Gd1–xNdx)Cl3(aq)+3H2C2O4+nH2O®
®(Gd1–xNdx)2[C2O4]3·nH2O(s)+6HCl
798
J. Therm. Anal. Cal., 91, 2008

MIXED Gd–Nd OXALATES
Fig. 2 Diffraction patterns of the aged oxalate with x=0, 0.5 and 1
Fig. 1 SEM images of a – Nd oxalate (90x), b – (Nd0.5Gd0.5
)
c=10.04 , b=113.80° for x= 0). As shown in Fig. 3, a
linear increase of all the cell parameters as a function
of x is observed in agreement with the fact that Gd3+ is
smaller than Nd3+. The angle b is almost constant for
both x=0 and x=1, so that a large variation as a func-
tion of the composition can not be expected.
oxalate (90x), c – Gd oxalate (90x) and d – Nd oxalate
at higher magnification (800x)
concentration of H2C2O4 and therefore of C O2– is
2
4
locally high; thus several nuclei appear in small areas
and many crystals quickly grow on them forming the
aggregates. Their size increases and, as they become
too heavy, they start to precipitate.
Hence, we can state that a complete solid solution
forms for all the x values and that the properties and
the structures of the mixed oxides, formed by thermal
decomposition of the oxalates, depend only on the
composition and temperature and not on an eventual
initial polyphasicity of the oxalates.
It seems that, dealing with Nd oxalate, the
nucleation rate is high and likely greater than the
crystal growth rate, so during the whole precipitation
process a large number of nuclei form, leading to a
wide distribution of aggregates sizes. It has been also
reported that the solubility of the RE oxalates slightly
increases with the RE atomic number [26]; thus, the
nucleus critical radius for Nd oxalate could be smaller
than for Gd oxalate. Small crystals can quickly appear
in the solution and in very small areas: for this reason,
they form immediately the aggregates.
Some interesting observations can be drawn
about the oven-dried samples, mainly for the Gd rich
samples. Their XRD patterns, as shown in Fig. 4,
display a different structure: new peaks are detected in
the oven-dried samples. It is important to notice that
the position of the peaks of the oven-dried samples
depend on the composition: there is a systematic shift
towards lower angles as x increases, in agreement
with the different sizes of Gd3+ and Nd3+.
For what concerns Gd oxalate, on the contrary,
the nucleation rate seems to be lower compared to the
previous case and some crystals can thus grow and
reach larger dimensions. Furthermore, the shape is
irregular because they form for random cohesion of
isolated particles or smaller aggregates.
According to the thermal analysis, discussed in
the following section, RE oxalates can easily lose
water and at higher temperature also CO2. Depending
on the synthesis procedure, after the drying stage the
water content can be lower than the equilibrium
value, and this water lack is expected to modify the
crystalline structure of the samples as well.
For the other compositions, the shape and the
size of the aggregates change linearly as a function of
the ratio Nd/Gd, meaning that the nucleation and the
growth rate depend on x.
A drying process at 105°C leads to an almost
complete dehydration, while the same process at 190°C
As shown by the X-ray analysis performed on
the aged samples (Fig. 2), all the oxalates are
crystalline; for clarity, only the diffraction patterns of
the mixed oxalates with x =0, 0.5 and 1 are shown.
In any case the XRD patterns are very similar to
those of other RE2[C2O4]3·10H2O and all the peaks
can be indexed assuming a monoclinic system with
space group P21/c. The reported lattice constants both
for Nd oxalate (a=11.192 , b=9. 612 , c=10.257 ,
b=114.42°) [22] and Gd oxalate (a=11.04 , b=9.
63 , c=10.09 , b=114.10°) [20] are close to those
obtained (a=11.19 , b=9.66 , c=10.27 ,
b=114.25° for x=1 and a=11.02 , b=9.64 ,
Fig. 3 Cell constants of the mixed Gd–Nd oxalates as a
function of x
J. Therm. Anal. Cal., 91, 2008
799

UBALDINI et al.
initial system is completely converted into the oxide.
Since it has been observed that lower heating rates
result in a decrease of the transformation temperatures,
we can conclude all the transformation temperatures
depend on the heating rate. Furthermore, after a
prolonged annealing (48 h) at 600°C, the oxalate was
completely converted into the mixed oxide,
suggesting that the equilibrium temperature can be
close to this value, as confirmed for all the other rare
earths oxalates [28].
The structure and the composition of the formed
oxides depend on the decomposition temperature and
on the ratio between Nd and Gd. For example, at
1200°C Gd2O3 is cubic, Nd2O3 is hexagonal and
NdGdO3, similarly to LaGdO3 [29], is monoclinic.
Moreover, solid solutions and biphasic regions exist,
depending on the temperature. Detailed studies about
the structure of the mixed oxides as a function of the
temperature are in progress.
Fig. 4 Diffraction patterns of three Gd rich oxalates after the
drying stage, compared to an aged sample
causes also the amorphization of the sample and the
loss of some CO2 molecules. A similar behaviour has
been reported also for other oxalates. For instance,
Hwu et al. [27] reported that BaTiO(C2O4)2·4H2O is
crystalline and that the precipitate becomes amorphous as
the drying temperature exceeds 100°C.
In the case of the decahydrated samples, the total
mass loss is equal to the theoretical mass variation cor-
It has been observed that the hydration water can
be fully recovered through an ‘aging’ process in a moist
environment. This result highlights that the oxalates
modify their structure and they can slowly reach the
equilibrium content getting water from the atmosphere;
this process is faster for Nd rich oxalates.
responding
to
the
conversion
of
(Gd1–xNdx)2[C2O4]3·10H2O into (Gd1–xNdx)2O3. In Ta-
ble 1 the calculated and the experimental mass losses
between 30 and 750°C for each value of x are shown:
the percentage theoretical mass loss increases linearly as
a function of x, as Nd is lighter than Gd, meaning that
the decomposition is complete and effective.
Thermal decomposition
For x=0.8, a slight loss of weakly bound water
can be already observed from room temperature up to
about 85°C. The water loss takes place in more steps:
seven water molecules are lost at first and three more
molecules afterwards. Two endothermic peaks are
associated to these mass losses: the former with T
onset at about 120°C, maximum at about 190°C and a
shoulder at 160°C, the latter with T onset at about
235°C and a maximum at 270°C. Moreover, the
A study of the thermal decomposition of rare earths mixed
oxalates into their oxides is complex and uncommon in
literature; only recently Ubaldini et al. [16] studied
the thermal stability of the mixed Ce/Gd oxides.
The
decomposition
route
reported,
of
as
(Gd0.2Nd0.8)2[C2O4]3·10H2O
is
representative of the decomposition of all the Nd/Gd
oxalates. Figure 5 shows DTA-TG analysis: many
DTA signals and corresponding different TG steps
can be noticed, meaning that many reactions occur
during the decomposition. It is important to observe
that, for temperatures above about 720°C, the sample
mass is constant and no transformations occur, i.e. the
Table 1 Theoretical and experimental mass losses related to
the transformation of (Gd1–xNdx)2[C2O4]3·10H2O into
(Gd1–xNdx)2O3 between 30 and 750°C
x
Theoretical mass loss/% Experimental mass loss/%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
52.22
52.40
52.58
52.76
52.94
53.13
53.32
53.50
53.69
53.88
54.07
51.94
52.36
52.65
52.63
52.90
53.19
52.35
53.50
53.62
53.98
54.28
Fig. 5 TG-DTA analysis of (Gd0.2Nd0.8)2[C2O4]3
800
J. Therm. Anal. Cal., 91, 2008

MIXED Gd–Nd OXALATES
former is composed of two partially overlapped
different cell parameters, that implies differently
coordinated crystallization water.
signals that can be split in two different signals
lowering the heating rate. For all the samples
isothermal curves at 190°C show the total loss of
water molecules associated with a slight CO2 loss.
The mass variation of (Nd0.8Gd0.2)2[C2O4]3·10H2O
between 25 and 300°C is 24.4%, in agreement with
the calculated value for a complete dehydration.
Furthermore, the detected water loss signals are
independent on the particular reaction atmosphere,
while the subsequent exothermic signals are strongly
affected: changing from O2 to air, the signal at lower
temperature is split, while the second one is shifted at
higher temperatures.
The behavior of the other mixed oxalates is
intermediate between the two previous cases. The
average water coordination in the mixed oxalates
depends on the ratio among the cations. In any case,
the first molecules of water are weakly bound and can
be easily removed from the oxalates. This can explain
the different trend observed for the oven dried Gd rich
oxalates: their total mass loss is smaller than the mass
loss of the aged samples. Their behaviors are very
similar above 300°C, namely where the decomposition
of the anhydrous RE oxalate occurs. The n highest
value for the oven-dried (Gd0.5Nd0.5)[C2O4]3 obtained
in several preparations is 9, in agreement with the fact
that this composition and Nd oxalate show the same
XRD patterns.
For Nd2(C2O4)3·10H2O, the experimental mass
loss between 25°C and 200°C is 24.4%, i.e. very close
to the calculated variation corresponding to the
complete dehydration (24.6%). Associated to this
step there is a large endothermic peak with maximum
at about 150°C.
For x=0.8 there is a mass loss which is over at
about 580°C. The presence of two exothermic DTA
peaks, the first one with an onset at 387 and maximum
at 405°C and the second and greatest one, with an
onset at 465 and maximum at 575°C, allows us to
state that the decomposition of the anhydrous oxalate
goes through the formation of the carbonate,
Gd0.4Nd1.6(CO3)3, and through the formation of
Gd0.4Nd1.6O2+d(CO3)1–d, as reported for other RE
oxalates [15, 17].
Considering
DTA-TG
analyses
of
Gd2[C2O4]3·10H2O, in place of the two steps at low
temperature related to the loss of water, five steps are
detected, as reported in Table 2.
Table 2 Dehydration stages of Gd2(C2O4)3·10H2O
Water
Theoretical
mass loss/%
DT/°C
Mass loss/%
molecules loss
For x=1 the observed mass loss between 25 and
580°C is equal to 47.7%, corresponding to the
theoretical mass variation related to the transformation
of Nd2[C2O4]3·10H2O into Nd2O2CO3, while the value
due to the transformation of the oxalate decahydrated
into Nd2(CO3)3 is 35.8% and the experimental mass
variation between room temperature and 420°C
(corresponding to the offset temperature of the first
exothermal peak and to a slope variation in the TG
curve) is about 34%. We assumed that the initial stage
of the decomposition of the anhydrous Nd oxalate is
the formation of an amorphous Nd carbonate, as
confirmed by our microRaman and XRD analyses.
The formation of amorphous samples occurs between
190 and 550°C: above this temperature, the structures
of the oxycarbonate at first and then of the oxide have
been detected. Finally, between about 590 and 660°C
the last mass loss is detected, corresponding to 5.1%,
which is apparently associated to a weak and large
endothermic DTA peak. In this step the decomposition
of the oxycarbonate occurs.
25–75
2.3
4.3
1
2
2.4
4.8
25–100
25–125
25–200
25–300
>10
18.5
23.5
>4
8
9.4
18.7
23.7
10
Since a prolonged annealing near 100°C causes a
partial dehydration of the product, this fact should be
considered when dealing with samples dried in oven.
On the strength of the data, we can state that the
dehydration path of Gd oxalate is consistent with
the following:
Gd2[C2O4]3·10H2O®Gd2[C2O4]3·9H2O+H2O
Gd2[C2O4]3·9H2O®Gd2[C2O4]3·8H2O+H2O
Gd2[C2O4]3·8H2O®Gd2[C2O4]3·6H2O+2H2O
Gd2[C2O4]3·6H2O®Gd2[C2O4]3·2H2O+4H2O
Gd2[C2O4]3·2H2O®Gd2[C2O4]3+2H2O
It is particularly interesting to note that a
prolonged annealing at constant temperature (550°C)
results in a slight mass decrease with time and an
endothermic DTA peak which develops at a critical
mass loss. In these experimental conditions all the
oxides considered [9, 30, 31] have a C-type structure
that can change increasing the temperature.
In the case of Gd oxalate the complete dehydration
occurs in five steps, meaning that the molecules of
water are not equivalent [16], in contrast to Nd oxalate,
for which it is a two steps process: this different
chemical equilibrium with water is due to their
J. Therm. Anal. Cal., 91, 2008
801

UBALDINI et al.
Thus, in agreement with Nair et al. [28] it is possible
to propose the decomposition of Nd decahydrated
oxalate as follows
Nd2[C2O4]3·10H2O®Nd2[C2O4]3·8H2O+2H2O
endothermic process - dehydration
Nd2[C2O4]3·8H2O®Nd2[C2O4]3 + 8 H2O
endothermic process - dehydration
Nd2[C2O4]3+3/2O2®Nd2(CO3)3+3CO2
exothermic process - carbonate formation
Fig. 6 Normalized Raman spectra of (Gd1–xNdx)2[C2O4]3.
From top to bottom x=0.1, 0.3, 0.6, 0.8 and 1.0
Nd2(CO3)3®Nd2O2CO3+2CO2
exothermic process - oxycarbonate formation
oxalate it is at about 1468 cm–1 and for the copper(II)
oxalate it is at 1489 cm–1 [32, 33].
Nd2OCO3®Nd2O3+CO2
endothermic process - oxide formation
In all Raman spectra of the mixed oxalates the
positions of the other peaks are similar, meaning that
they slightly depend on the stoichiometry, even if
they are a little different in the form: also in this case
the modifications regularly follow the composition.
All the behaviours reported on Raman spectra
are coherent with the results of the other techniques
used in this work.
The last three stages can be recognized in all the
mixed oxalates, meaning that the behavior of the
anhydrous RE oxalates is similar, even if the
temperatures of all the transformations increase with
the Gd content. The thermal stability of the RE
carbonate and oxycarbonate increases as a function of
the molecular mass of the oxalate.
More significant differences in the oxalates de-
hydration path are observed, in agreement
with [16–18, 23] who reported an ‘inner core’ com-
posed by strongly bound water molecules. It must be
noted, moreover, that the number of strongly bound
water molecules in mixed oxalates depends on the
Nd/Gd ratio.
Conclusions
Several samples of Gd-Nd oxalates were prepared
using a coprecipation method. The obtained mixed
oxalates are decahydrated, but the Gd rich oxalates
easily lose water during the first drying process.
The decomposition of the mixed oxalates is
complex and it is complete below 700°C. (Gd1–xNdx)
A confirmation of the experimental results reported
so far comes from Raman analysis performed on
Nd2[C2O4]3·10H2O and Gd2[C2O4]3·10H2O: in both
cases, in the OH stretching region, there are some sig-
nals, due to crystallization water. Some differences ex-
ist, concerning the number of peaks and shoulders be-
tween Nd and Gd oxalate, confirming that the chemical
bond with water is different in the two cases. In the
mixed oxalates the positions and the forms of peaks in
the OH region depend on the composition, showing a
regular modification going from Nd to Gd oxalate.
The isolated oxalate ion has D2d symmetry and
the group theory predicts 3A1+B1+2B2+3E Raman
active modes [31]. For the various oxalates, the
symmetry depends on the coordination with the
metal [32].
anhydrous
oxalates
decompose
first
in
(Gd1–xNdx)2(CO3)3, then in (Gd1–xNdx)2O2CO3, and
finally in the mixed oxides.
The different coordination of water molecules of
Nd and Gd oxalate is evident in the Raman spectra: by
this technique is also possible to compare the
behaviour of the mixed oxalates.
XRD and DTA/TG analyses allowed to conclude
that a solid solution forms for each x value. This is an
important result for the preparation of Gd–Nd mixed
oxides, because all the properties of the mixed oxides
are intrinsic and depend only on their composition
and thermal treatment temperature.
Figure 6 shows the normalised Raman spectra of
(Gd1–xNdx)2[C2O4]3 (x=0.1, 0.3, 0.6, 0.8 and 1.0): they
are consistent with the spectra of other metallic oxa-
lates. The most intense peak is at about 1480 cm–1 and
can be assigned to the n stretching mode of the CO
group, being comparable with the reported wave num-
ber of the stretching mode of CO of other oxalates. For
instance, it was reported that this band for potassium ox-
alate is at about 1496 cm–1, for the calcium dihydrated
Such results allow to extend the same conclusions
to other rare earth mixed oxides and, in particular, to
perform the study of their pseudobinary phase diagrams.
Acknowledgements
The authors are very grateful to Dr. Giorgio Luciano for his
helpful contribution.
802
J. Therm. Anal. Cal., 91, 2008

MIXED Gd–Nd OXALATES
18 D. Dollimore, Thermochim. Acta, 117 (1987) 331.
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