High-temperature X-ray diffraction and specific heat studies on GdAlO3, Gd3Al5O12 and Gd4Al2O9

Article abstract of DOI:10.1016/j.jssc.2007.06.003

Gadolinium aluminates, GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were synthesized by the solution combustion method. Very fine particles in the nanoparticle range of ～10-20 nm could be prepared by this method as evidenced by surface area measurement by multipoint BET method. Thermal studies on these compounds were carried out using high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC) methods. The thermal expansion coefficients of GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were calculated from the lattice parameter data and specific heats were calculated from DSC data. The lattice parameters of GdAlO₃ and Gd₃Al₅O₁₂ were found to increase linearly with temperature whereas Gd₄Al₂O₉ did not show a linear trend. The specific heats of these compounds show an increasing trend with increase in aluminum atom fraction. Based on the thermodynamic data available in the literature and the specific heat data obtained in this study, oxygen potential diagram was constructed at 1000 K.

Full text of DOI:10.1016/j.jssc.2007.06.003

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 2393–2399
www.elsevier.com/locate/jssc
High-temperature X-ray diffraction and specific heat studies on
GdAlO3, Gd3Al5O12 and Gd4Al2O9
Ã
Satyajeet Chaudhurya, , S.C. Paridab, K.T. Pillaia, K.D. Singh Mudhera
aFuel Chemistry Division, RC & I Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
bProduct Development Section, RC & I Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
Received 28 March 2007; received in revised form 23 May 2007; accepted 3 June 2007
Available online 8 June 2007
Abstract
Gadolinium aluminates, GdAlO3, Gd3Al5O12 and Gd4Al2O9 were synthesized by the solution combustion method. Very fine particles
in the nanoparticle range of ꢀ10–20 nm could be prepared by this method as evidenced by surface area measurement by multipoint BET
method. Thermal studies on these compounds were carried out using high-temperature X-ray diffraction (HT-XRD) and differential
scanning calorimetry (DSC) methods. The thermal expansion coefficients of GdAlO3, Gd3Al5O12 and Gd4Al2O9 were calculated from
the lattice parameter data and specific heats were calculated from DSC data. The lattice parameters of GdAlO3 and Gd3Al5O12 were
found to increase linearly with temperature whereas Gd4Al2O9 did not show a linear trend. The specific heats of these compounds show
an increasing trend with increase in aluminum atom fraction. Based on the thermodynamic data available in the literature and the specific
heat data obtained in this study, oxygen potential diagram was constructed at 1000 K.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Gadolinium aluminate; Solution combustion; Thermal studies; HT-XRD; DSC; Nanoparticle
1. Introduction
In the pseudo-binary system Gd2O3–Al2O3, two stable
compounds, Gd4Al2O9 and GdAlO3 (mole ratios
Gd2O3:Al2O3 ¼ 2:1 and 1:1, respectively), are known [5–8].
Gd4Al2O9 is monoclinic and its structure is related to that of
the mineral cuspidine, Ca4Si2O7F2. At 1406K, Gd4Al2O9
undergoes a reversible, possibly martenstic phase transforma-
tion [9] as do all other RE4Al2O9 phases at specific tem-
peratures between 1273 and 1673 K, followed by congruent
melting at 2224 K. GdAlO3 crystallizes in an orthorhombic
perovskite-type structure and melts congruently at 2342 K.
Two other gadolinium aluminates, Gd3Al5O12 and GdAl11O18
(mole ratios Gd2O3:Al2O3 ¼ 3:5 and 1:11, respectively), have
been reported in the literature [10–14], but their thermo-
dynamic stability has not been thoroughly clarified. Gd3
Al5O12 belongs to the garnet structure type. It has been
reported that Gd3Al5O12 on heat treatment at 1773 K for 24 h
undergoes complete decomposition to GdAlO3 and Al2O3
[15]. GdAl11O18 has magnetoplumbite-type structure. It is
stable only at higher temperatures (41773 K) and eutectically
decomposes to GdAlO3 and Al2O3 at lower temperatures [15].
The aim of the present work, therefore, was to synthesize
the three most stable compounds GdAlO3, Gd3Al5O12 and
Solid solutions of UO2–RE2O3 (where RE ¼ Sm, Eu, Gd
and Er) are being considered as potential fuel pellets
whereas lanthanide sesquioxide acts as a burnable poison
to suppress initial excess reactivity at the beginning of life
in nuclear reactors [1–3]. Among the sesquioxides, Gd2O3
is being used commonly worldwide. Gadolinium aluminate
is currently under development as candidate material for
neutron absorption and control rod applications [4].
Information on thermal expansion coefficient, specific
heat, thermal conductivity, etc. of such materials as a
function of temperature is essential to evaluate the
thermophysical properties under temperature gradients.
As a part of the work program for the development of rare-
earth-based materials of interest in nuclear technology,
thermal studies were carried out on selected compounds of
the Gd–Al–O system.
Ã
Corresponding author. Fax: +91 22 2550 5151.
E-mail address: saty@apsara.barc.ernet.in (S. Chaudhury).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.06.003

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2394
Gd4Al2O9 by solution combustion method and to investi-
gate their thermal expansion behavior and specific heat as a
function of temperature.
where HFBlank, HFRef and HFSample represent heat flow
during blank, reference and sample steps, respectively.
Cp(T)Sample and Cp(T)Ref represent the specific heats of
sample and reference material, whereas MSample and MRef
represent the masses of sample and reference, respectively.
Accuracy of the measurements was checked by measuring
the specific heat of Fe2O3 (mass fraction 0.998) in the
temperature range from 300 to 860 K and the values were
found to be within 72% compared with the literature
values [18].
2. Experimental
Synthetic compounds of GdAlO3, Gd3Al5O12 and
Gd4Al2O9 were prepared by taking stochiometric amounts
of gadolinium nitrate, Gd(NO3)3 (A.R. grade) and
aluminum nitrate, Al(NO3)3 (A.R. grade) and mixing with
glycerol. The mixtures were heated on a hot plate with
continuous stirring until a brown fine powder (precursor)
was formed. The precursor powders were calcined at
1023 K for 16 h to get crystalline phases of GdAlO3,
Gd3Al5O12 and Gd4Al2O9. The formation of the com-
pounds was examined by X-ray powder diffraction (XRD)
method. High-temperature XRD patterns of these com-
pounds were recorded under reduced pressure
(ꢀ10–5 mbar) in a STOE diffractometer from 298 to
1273 K. The patterns were taken at an interval of 100 K
3. Results and discussion
GdAlO3 (orthorhombic perovskite-type structure and
S.G.: Pbnm (62)). Gd3Al5O12 (cubic garnet-type structure
and S.G.: Ia3d (230)) and Gd4Al2O9 (monoclinic structure
and S.G.: P21/c) could be prepared by solution combustion
(SC) synthesis method in the presence of glycerol. Glycerol
acts as a fuel and serves two purposes:
(a) It is a source of C and H, which on combustion form
CO2 and H2O and liberates heat.
˚
with CuKa-radiation (l ¼ 1.5406 A) in the range
2y ¼ 10–651 using HDK-2.4 Buhler high-temperature
attachment. In the high-temperature chamber, the well-
ground samples, mixed with a drop of collodine were
mounted on a platinum–rhodium sample carrier which is
spot welded at the bottom with a Pt/Pt–13%Rh thermo-
couple and was resistively heated at a programmed rate.
The sample temperature was controlled by a PID
temperature controller within 71 K. The sample was kept
isothermal for about 60 min at each temperature step in
order to ensure thermal equilibrium. The unit cell
parameters at each temperature were determined using
least-squares method using a computer program by
(b) It forms complexes with the metal ions facilitating
homogeneous mixing of the cations in solution.
The basic reactions involved with glycerol could be
MðNO3Þ3 þ 3H2O ! MðOHÞ3 þ 3HNO3,
2HNO3 ! H2O þ 2NO þ 3½Oꢃ,
(2)
(3)
(4)
C3H8O3 þ 7½Oꢃ ! 3CO2 þ 4H2O;
where M stands for gadolinium and aluminum. The overall
reaction can be written as
˚
Wadhawan [16] with an accuracy of 70.001 A.
14MðNO3Þ3 þ 9C3H8O3 ! 14MðOHÞ3
Surface area of these compounds was measured by
multipoint BET method using Sorptomatic-1990, CE
Instruments, Italy.
þ 42 NO þ 27 CO2 þ 15 H2O:
ð5Þ
In reality, the reactions seem to be more complicated.
Glycerol seems to decompose to form, among other
products, carboxylic acids, which in turn, form complexes
with metal ions, thereby preventing the precipitation of
aluminum at lower pH. This is probably the reason for
simultaneous precipitation of aluminum and gadolinium at
a higher pH of around 7. The high viscosity of glycerol may
also be playing a role in preventing the segregation of
aluminum and gadolinium during precipitation. The
exothermicity (adiabatic temperature, Tad) of the redox
reaction is very high. Though these aspects are not fully
understood, the usefulness of the ‘glycerol route’ for
obtaining good-quality gadolinium aluminate powder has
been established. Potential advantages of this wet chemical
route over the conventional solid state reaction method
include better homogeneity, better compositional control
and lower processing temperatures. The surface area found
out by multipoint BET method for GdAlO3, Gd3Al5O12
and Gd4Al2O9 were 56.4, 56.26 and 57.50 m2/g, respec-
tively. Pore volume of the above samples was found to be
Specific heats of these compounds were measured using a
heat flux type differential scanning calorimeter (DSC 131,
SETARAM Instrumentations, France). Temperature cali-
bration, heat calibration and procedure for specific heat
measurement using this calorimeter is extensively discussed
in an earlier publication [17]. The classical three-step
method with continuous heating mode was followed in this
study to measure the specific heat. NIST synthetic sapphire
(SRM-720) in the powder form was used as the reference
material. The specific heat of the sample under investiga-
tion can be calculated by a simple comparison of the heat
flow rates into the sample and into the calibration
substance as illustrated in the literature [17]. The expres-
sion used for the calculation of specific heat of the sample is
given as
HFSample ꢁ HFBlank
HFRef ꢁ HFBlank
MRef
CpðTÞSample
¼
ꢂ
ꢂ CpðTÞRef
,
MSample
(1)

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2395
parameters are best fitted to third-order polynomial
expressions related by Eqs. (12)–(15), given below.
For GdAlO3
negligible indicating non-porous nature of the particles.
Particle size calculated from surface area (d ¼ 6/ra, where
d is the diameter, r the density and a the surface area) were
14.3, 17.87 and 18.56 nm for GdAlO3, Gd3Al5O12 and
Gd4Al2O9, respectively.
The XRD patterns of GdAlO3, Gd3Al5O12 and Gd4
Al2O9 prepared by solution combustion method shown in
Fig. 1 were compared with reported data [19] which
confirmed their formation. The unit cell parameters of
these compounds at room temperature were determined
using least-squares refinement program [16] and are shown
in Table 1.
aT ðAÞ ¼ 5:230 þ 6:422 ꢂ 10ꢁ5T ðKÞ,
(6)
(7)
(8)
(9)
˚
bT ðAÞ ¼ 5:289 þ 3:110 ꢂ 10ꢁ5T ðKÞ,
˚
cT ðAÞ ¼ 7:415 þ 8:491 ꢂ 10ꢁ5T ðKÞ,
˚
3
VT ðA Þ ¼ 205:030 þ 6:220 ꢂ 10ꢁ3T ðKÞ.
˚
For Gd3Al5O12
aT ðAÞ ¼ 12:074 þ 1:121 ꢂ 10ꢁ4T ðKÞ,
(10)
(11)
˚
VT ðA Þ ¼ 1759:538 þ 5:020 ꢂ 10ꢁ2T ðKÞ,
3
3.1. Thermal expansion behavior
˚
For Gd4Al2O9
The HT-XRD patterns of GdAlO3, Gd3Al5O12 and
Gd4Al2O9 recorded at different temperatures upto 1273 K
did not show any phase change except for the shift in the
X-ray line positions to lower values of 2y showing
expansion of the unit cell. The lattice parameters were
aT ðAÞ ¼ 11:209 þ 1:734 ꢂ 10ꢁ4T ðKÞ
˚
ꢁ 2:035 ꢂ 10ꢁ7T2 ðKÞ þ 9:895 ꢂ 10ꢁ11T3 ðKÞ,
ð12Þ
ð13Þ
ð14Þ
˚
obtained within an accuracy of 70.001 A using least-
bT ðAÞ ¼ 10:701 ꢁ 1:906 ꢂ 10ꢁ4T ðKÞ
˚
squares refinement method. The variation of lattice
parameters of GdAlO3, Gd3Al5O12 and Gd4Al2O9 with
temperature is shown in Figs. 2–4, respectively. The lattice
parameters are best fitted to linear expressions for GdAlO3
and Gd3Al5O12 by Eqs. (6)–(11) but for Gd4Al2O9, the
þ 4:866 ꢂ 10ꢁ7T2 ðKÞ ꢁ 2:505 ꢂ 10ꢁ10T3 ðKÞ,
cT ðAÞ ¼ 7:559 þ 3:516 ꢂ 10ꢁ6T ðKÞ
˚
þ 4:249 ꢂ 10ꢁ8T2 ðKÞ ꢁ 1:316 ꢂ 10ꢁ11T3 ðKÞ,
VT ðA Þ ¼ 855:299 þ 3:300 ꢂ 10ꢁ3T ðKÞ
3
˚
Gd4Al2O9
Gd3Al5O12
þ 2:189 ꢂ 10ꢁ5T2 ðKÞ ꢁ 1:165 ꢂ 10ꢁ8T3 ðKÞ.
ð15Þ
These expressions show that the lattice parameters of
GdAlO3 and Gd3Al5O12 increase linearly with temperature
between 298 and 1273 K, whereas Gd4Al2O9 do not show
linear trend. The values of linear thermal expansion
coefficient (aT) at any temperature (T/K) were calculated
using the following relation:
GdAlO3
da
1
Thermal expansion coefficient; aT ¼
ꢂ
,
(16)
dT a298
10
20
30
40
2 (°)
50
60
where da is the change in lattice parameter corresponding
to a change in temperature dT and a298 is the lattice
parameter at 298 K. Average thermal expansion coefficient
of GdAlO3, Gd3Al5O12 and Gd4Al2O9 were theoretically
Fig. 1. Room-temperature X-ray diffraction patterns of GdAlO3,
Gd3Al5O12 and Gd4Al2O9.
Table 1
Lattice parameters refined from room-temperature X-ray powder diffraction data
3
V (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
Compound
Structure
b (deg)
Z
GdAlO3
Gd3Al5O12
Gd4Al2O9
Orthorhombic
Cubic
Monoclinic
5.247(2)
12.108(2)
11.246(2)
5.299(1)
7.438(2)
7.563(2)
4
8
4
206.8(1)
1775.1(3)
857.8(5)
10.679(6)
109.2(2)

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214
212
210
208
206
7.52
7.48
7.44
5.34
5.32
5.30
c
b
a
5.28
5.32
5.28
5.24
5.20
200
400
600
800
Temperature (K)
1000
1200
1400
Fig. 2. Variation of lattice parameters with temperature for GdAlO3.
1830
1815
1800
1785
1770
12.24
12.20
12.16
12.12
12.08
a
200
400
600
800
1000
1200
1400
Temperature (K)
Fig. 3. Variation of lattice parameters with temperature for Gd3Al5O12
.
calculated from the lattice parameter values by using
Eq. (16) and the values are given in Table 2. The values
of average thermal expansion coefficient in the pre-
sent study are close to the value of ceramic materials
like UO2 (aa ¼ 10.47 ꢂ 10ꢁ6 Kꢁ1) [20] and ThO2 (aa ¼
9.04 ꢂ 10ꢁ6 Kꢁ1) [21].
3.2. Specific heat measurements
Experimental values of specific heats of these com-
pounds are not reported in the literature. However, the
estimated values of these compounds are reported in the
literature [8,18]. The specific heats of the compounds as a

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109.22
109.20
109.18
876
870
864
858
852
7.62
7.60
7.58
7.56
7.54
c
10.80
10.74
10.68
10.62
b
11.32
11.28
11.24
11.20
a
200
400
600
800
Temperature (K)
1000
1200
1400
Fig. 4. Variation of lattice parameters with temperature for Gd4Al2O9.
0.72
Table 2
Average thermal expansion coefficient of compounds in the temperature
range 298–1273 K
GdAlO [This Study]
3
0.68
0.64
0.60
0.56
0.52
0.48
0.44
0.40
0.36
0.32
0.28
GdAlO [18]
3
Gd Al O [This study]
Compound Average thermal expansion coefficient ꢅ (106) (Kꢁ1
)
3
5 12
Gd Al O [18]
3
5 12
Gd Al O [This study]
4
2 9
aa
ab
ac
av
Gd Al O [18]
4
2 9
GdAlO3
Gd3Al5O12
Gd4Al2O9
13.8
5.04
13.1
31.8
27.2
18.4
9.23
4.42
8.30
5.12
function of temperature are best fitted by the least-squares
method into the following expressions:
For the compound GdAlO3,
C0p ðJ gꢁ1 Kꢁ1Þ ðꢄ2:0%Þ ¼ 0:48827
þ 0:00009T ðKÞ ꢁ 9409:88T2 ðKÞ:
ð17Þ
ð18Þ
ð19Þ
For the compound Gd3Al5O12,
300
400
500
600
T/K
700
800
900
C0p ðJ gꢁ1 Kꢁ1Þ ðꢄ2:0%Þ ¼ 0:61195
þ 0:00005T ðKÞ ꢁ 15374:42T2 ðKÞ.
Fig. 5. Comparison of experimental and estimated specific heats of
GdAlO3, Gd3Al5O12 and Gd4Al2O9.
For the compound Gd4Al2O9,
C0p ðJ gꢁ1 Kꢁ1Þ ðꢄ2:0%Þ ¼ 0:45853
þ 0:00004T ðKÞ ꢁ 9834:53T2 ðKÞ.
These experimental specific heat capacities are compared
with those reported in the literature [18] and are shown in
Fig. 5. It can be seen from the figure that the specific heats

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2398
obtained in this study are higher than the values taken
from Ref. [18]. This discrepancy could not be explained
unless a third set of specific heat capacity data is available.
It can be inferred from the comparison plot that the
specific heats of these compounds follow the trend: specific
heat (Gd3Al5O12)4specific heat (GdAlO3)4specific heat
(Gd4Al2O9). This trend is in order with the atom fraction
of aluminum for these compounds: Gd3Al5O12 (atom
fraction Al ¼ 0.25)4GdAlO3 (atom fraction Al ¼ 0.20)4
Gd4Al2O9 (atom fraction Al ¼ 0.133). It is imperative to
mention here that a complete data set from very low
temperature (near zero) to high temperature (above
1000 K) is required in order to explain this trend by
formulating suitable thermodynamic models.
evident from the diagram that these ternary oxides are
stable at 1000 K at oxygen potentials far below that
prevailing over the two phases: Al (Liq.)+Al2O3 (s). It is
also apparent from the diagram that on gradually
decreasing the oxygen partial pressure at 1000 K, the
compound Gd3Al5O12 will decompose to GdAlO3 and Al
(Liq.), GdAlO3 will decompose to Gd4Al2O9 and Al (Liq.)
and Gd4Al2O9 will decompose to Gd2O3 and Al. (Liq.).
Similar diagrams can be constructed at other temperature
of interest which aids valuable information on phase
stability.
4. Conclusion
Based on the available estimated thermodynamic data
reported in the literature [8] and the specific heat data
obtained in the present study, the standard molar Gibbs
energies of formation (Df Gom) of these ternary oxides have
been calculated by second law method at 1000 K. For this
calculation, the specific heats for all these compounds
measured in this study in the temperature range 300–850 K
have been extrapolated to 1000 K assuming absence of any
phase transition in all the three compounds GdAlO3,
Gd3Al5O12 and Gd4Al2O9. The oxygen chemical potentials
corresponding to various equilibrium reactions in the
system Gd–Al–O have been calculated at 1000 K using
the calculated values of Df Gom of different phases. In the
ternary system containing two metallic elements (Gd and
Al) and one gas phase (O2), it is interesting to observe the
change in phase relations by changing the activity of gas
phase. In generalized oxygen potential diagram, the values
of oxygen potentials are plotted against the composition
parameter which is generally the cationic fraction. The
oxygen potential diagram for the system Gd–Al–O con-
structed in this study is shown in Fig. 6. The composition
parameter is defined as ZGd/(ZGd+ZAl). The alloy–oxide
equilibria are not shown in the figure because thermo-
dynamic data for Gd–Al alloys are not available. It is
Synthetic samples of GdAlO3, Gd3Al5O12 and Gd4Al2O9
were synthesized by solution combustion route. Particle
size found out from surface area was in nanoparticle range
of 10–20 nm. Thermal expansion behavior of these samples
was studied by HT-XRD method. The lattice parameters
of GdAlO3 and Gd3Al5O12 were found to increase linearly
with temperature whereas Gd4Al2O9 did not show a linear
trend. Specific heats were measured by DSC. The specific
heats of these compounds showed increasing trend with
increase in atom fraction of aluminum. Isothermal oxygen
potential diagram was constructed at 1000 K, which reveals
that on gradually decreasing the oxygen partial pressure,
the compound Gd3Al5O12 will decompose to GdAlO3 and
Al (Liq.), GdAlO3 will decompose to Gd4Al2O9 and Al
(Liq.) and Gd4Al2O9 will decompose to Gd2O3 and Al.
(Liq.).
Acknowledgments
The authors are thankful to Dr. V. Venugopal, Director,
Radiochemistry & Isotope Group and Dr. S.K. Aggarwal,
Head, Fuel Chemistry Division, for their keen interest and
encouragement in this work.
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