Moderate pressure synthesis of rare earth nickelate with metal-insulator transition using polymeric precursors

Article abstract of DOI:10.1016/j.jssc.2010.04.019

Rare earth nickelates exhibit a reversible metal-semiconductor phase transition that is, in the infrared range, responsible for a thermo-optical contrast. The state of the art synthesis of these compounds usually requires high oxygen pressure to stabilize Ni in the oxidation state 3⁺. In this work, using polymeric precursor associated with moderate pressure annealing, we show that it is possible to obtain fully oxidized rare earth nickelate with metal-insulator transition. Using thermogravimetric analysis, X-ray diffraction and transmission electronic microscopy we compare different samples synthesized at different oxygen pressures and demonstrate their structural similarity. Thermo-optical properties were measured, in the infrared range, using reflectance measurements and confirmed the metal-insulator transition at 60 °C in both samples.TEM observations lead to the conclusion that the structure commonly obtained at 175 bar is perfectly observed in the 20 bar sample without major structural defects. The two samples exhibit a thermochromic behavior and thermo-optical properties of the two samples are equivalent.

Full text of DOI:10.1016/j.jssc.2010.04.019

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Journal of Solid State Chemistry 183 (2010) 1663–1669
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Moderate pressure synthesis of rare earth nickelate with metal–insulator
transition using polymeric precursors
Ã
C. Napierala a,d, , C. Lepoittevin b, M. Edely c, L. Sauques d, F. Giovanelli a, P. Laffez a, G. VanTedeloo b
a
ꢀ
LEMA, Site de l’IUT de Blois, 3 Place Jean Jaures, 41000 Blois, France
b EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerpen, Belgique
c LPEC, Avenue Olivier Messaien, 72085 Le Mans Cedex 9, France
d DGA, 16bis Avenue Prieur de la Coˆte d’Or, 94116 Arcueil, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Rare earth nickelates exhibit a reversible metal–semiconductor phase transition that is, in the infrared
range, responsible for a thermo-optical contrast. The state of the art synthesis of these compounds
usually requires high oxygen pressure to stabilize Ni in the oxidation state 3+. In this work, using
polymeric precursor associated with moderate pressure annealing, we show that it is possible to obtain
fully oxidized rare earth nickelate with metal–insulator transition. Using thermogravimetric analysis,
X-ray diffraction and transmission electronic microscopy we compare different samples synthesized at
different oxygen pressures and demonstrate their structural similarity. Thermo-optical properties were
measured, in the infrared range, using reflectance measurements and confirmed the metal–insulator
transition at 60 1C in both samples.TEM observations lead to the conclusion that the structure
commonly obtained at 175 bar is perfectly observed in the 20 bar sample without major structural
defects. The two samples exhibit a thermochromic behavior and thermo-optical properties of the two
samples are equivalent.
Received 30 June 2009
Received in revised form
7 April 2010
Accepted 13 April 2010
Available online 7 May 2010
Keywords:
Rare-earth nickelate
Infrared furtivity
Transmisssion electron microscopy
Applied physics
Thermochromism
& 2010 Elsevier Inc. All rights reserved.
1. Introduction
deficit strongly influences transport properties and consequently
may induce changes in thermo-optical properties [10].
Among the different methods that have been developed to
synthesize RNiO3, only those including annealing pressure as high
as 200 bar give samples with sharp MI transition, except the
synthesis of this film using pulsed laser or chemical vapor
deposition [11,12]. Other thin films techniques require high
process annealing. The use of moderate annealing condition leads
usually to poor semiconductive properties, without the expected
MI transition [13].
Rare-earth nickelates exhibiting interesting properties were
synthesized by Lacorre et al. [8] under 150–200 bar oxygen
pressure at 1000 1C using nitrate precursors. More recently, Escote
et al. [14] have prepared polycrystalline samples of RNiO3 (R¼Pr,
Nd, Sm) through different routes (metal oxides mixture, hydro-
xide precipitation and sol–gel process). All powders obtained by
the three different methods were heat-treated several times at
1000 1C and 70 bar oxygen pressure.
In this work, we have synthesized polycrystalline samples of
Nd0.3Sm0.7NiO3 using polymeric synthesis and post-annealing at
20 and 175 bar, respectively. This composition is selected because
it leads to metal–insulator transition around room temperature.
Our intent is to validate the polymeric route for synthesis of rare
earth nickelate powder or ceramics by showing that annealing at
moderate pressure (20 bar) leads to the same structural and
physical properties as the standard annealing at 175 bar. For this
Nowadays smart materials, whose one or more properties can
be significantly changed in a controlled fashion by external
stimuli, such as stress, temperature or electric field, find more and
more industrial applications [1,2]. Among the smart materials,
rare-earth nickelates, which exhibit
a metal–insulator (MI)
transition responsible for optical change in the infrared range,
are one of the promising materials for thermochromic devices or
infrared furtivity [3,4]. Compared to the other infrared switching
materials like VO2 [5] or (Sm,Ca)MnO3 [6,7], rare-earth nickelates
have a sharper transition, whose temperature can be easily
adjusted by an appropriate choice of the rare earth. [3,8,9]. The
main difficulty for its synthesis is the need for high oxygen
pressure, which consequently frustrates large scale applications.
Metal–insulator transition in rare-earth nickelates (space group
Pbnm) requires the nickel to adopt the Ni3+ valence state, which is
normally accomplished at high temperatures and high pressures.
For thermochromic applications, it is necessary to stabilize Ni3+ to
get the oxygen stoichiometry in the RNiO3 compounds and,
consecutively, the optimum metal–insulator transition. An oxygen
Ã
ꢀ
Corresponding author at: LEMA, Site de l’IUT de Blois, 3 Place Jean Jaures,
41000 Blois, France.
E-mail address: cecile.napierala@gmail.com (C. Napierala).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.04.019

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study, X-ray diffraction, thermogravimetric analysis and trans-
mission electron microscopy were used for structure character-
izations. For thermochromic property evaluation, infrared
reflectivity measurements have been performed.
Thermogravimetric analysis is used to determine the oxygen
level in the structure by reducing the material under a flowing
mixture of H2 3%/Ar 20%/N2. The thermogravimetric data were
obtained on a Perkin Elmer TGA 7 thermogravimetric analyzer
working at a heating rate of 1.5 1C minꢀ1. About 40 mg of the
sample was used in this experiment.
2. Experimental
Specimens for transmission electron microscopy observations
were prepared by grinding the powder in ethanol and depositing
a drop on a carbon coated copper grid. Selected area electron
diffraction (ED) patterns were obtained using a Philips CM20
transmission electron microscope (double tilt7451) operating at
200 kV. High-resolution electron microscopy (HREM) was carried
out on a JEOL 4000EX microscope top-entry microscope (double
tilt7201) operating at 400 kV.
Thermochromic potentialities of each sample were measured
at 5 and 100 1C and infrared reflectivity measurements with a
Nicolet Magna 860 spectrophotometer equipped with an inte-
grating sphere (Hemispherical Directional Reflectometer SOC 100)
including the specular and diffuse parts of reflectivity. For this last
measurement, 1 g of powder was pelletized under 12 ton (surface
area DONNE LA SURFACE DU DISQUE)
2.1. Sample preparation
Polycrystalline samples of Nd0.3Sm0.7NiO3 were prepared by a
modified sol–gel method adapted from Douy [15] using NiO, Sm2O3
and Nd2O3 precursors. The oxides were dissolved in nitric acid,
leading to the corresponding nitrate salts. Appropriate amounts of
each solution were mixed in the stoichiometric ratio and citric acid
was added as a chelating agent. At 80 1C, acrylamide and N–N0–
methylenediacrylamide were added in order to form a three-
dimensional gel. The resulting polyacrilamide gel was calcined at
300 1C for 2h and then at 6501C for 5 h in a furnace, producing a
spongy material that turned into a dark gray fine powder after 12 h
annealing at 700 1C. The samples were heat-treated twice at 790 1C
under 175 and 20 bar oxygen pressure for 12 h.
3. Results
2.2. Characterizations
3.1. X-ray diffraction
Powder X-ray diffraction patterns were collected at room
temperature on a Rigaku Miniflex diffractometer using Cu K
a
y
˚
radiation (
l
¼1.5418 A). Diffraction pattern was recorded in the 2
X-ray diffraction experiments on the powder confirm for each
treatment the synthesis of a single-phase material, showing only
the derived-perovskite structure. All peaks were indexed with the
range 10–1201 (step Dy¼0.021). X-ray data analysis was performed
through the Rietveld method using a pseudo-Voigt function.
Fig. 1. X-ray diffraction patterns of Nd0.3Sm0.7NiO3 samples annealed at 20 and 175 bar.
Table 1
Cell parameters, volume and nominal composition calculated from the thermogravimetric analysis for the Nd0.3Sm0.7NiO3 compounds annealed at 175 or 20 bar of oxygen.
3
Structure
Space
Group
Weight
loss (%)
Composition
˚
˚
˚
˚
dcalculated
a (A)
b (A)
c (A)
V (A )
Nd0.3Sm0.7NiO3,
T¼790 1C,
Orthorhombic Pbnm
5.34470.307 ꢁ 10ꢀ3 5.413670.336 ꢁ 10ꢀ3 7.577970.501 ꢁ 10ꢀ3 219.23 9.32
0.02
Nd0.3Sm0.7NiO2.98
Nd0.3Sm0.7NiO2.97
PO2¼175 bar
Nd0.3Sm0.7NiO3,
T¼790 1C,
Orthorhombic Pbnm 5.350270.386 ꢁ 10ꢀ3 5.407170.413 ꢁ 10ꢀ3 7.582270.615 ꢁ 10ꢀ3 219.34 9.24
0.03
PO2¼20 bar

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orthorhombic distorted perovskite in Pbnm space group (Fig. 1).
X-ray data analysis was performed through the Rietveld method
and the refined cell parameters, presented in Table 1, are in good
agreement with those found in literature [16,17]. We observe that
both samples show the same cell parameters, whatever the
annealing conditions, within the experimental precision.
3.2. Thermogravimetric analysis
Thermogravimetric analysis is used to evaluate oxygen rate in
the structure by reducing Nd0.3Sm0.7NiO3 under a H2/N2 flow.
Fig. 2 shows the thermogravimetric profile for Nd0.3Sm0.7NiO3
samples. The experiment suggests that the reduction process
occurs through two consecutive steps (Ni3+-Ni2+-Ni0) adapted
from the equations in Falcon et al. [18] and Alonso et al. [19]:
Nd0.3Sm0.7NiO3ꢀ +(1/2ꢀd)H2-Nd0.3Sm0.7NiO2.5+(1/2ꢀd)H2O
d
Nd0.3Sm0.7NiO2.5+H2-0.15Nd2O3+0.35Sm2O3+Ni+H2O
For both compounds, the first decomposition step occurs at
350 1C and the second step at 450 1C. The decomposition process
was confirmed by X-ray diffraction after each stage. Weight loss
Fig. 2. Thermal analysis (TG) curves of Nd0.3Sm0.7NiO3 samples annealed at 20 and
175 bar under reducing conditions (H2/N2flow).
2.00
175 bars
Heating
Endothermic
Cycle 1
Cycle 1
Cycle 2
Cycle 2
Cycle 3
Cycle 3
Cycle 4
Cycle 4
Cycle 5
Cycle 5
Cycle 6
Cycle 6
Cycle 7
Cycle 7
Cycle 8
Cycle 8
Cycle 9
Cycle 9
Cycle 10
1.75
1.50
-1.75
Cycle 10
Cooling
-2.00
67.59 71.00
40
60
80
100
Température (°C)
2.00
20 bars
Endothermic
Heating
Cycle2
Cycle2
Cycle 3
Cycle 3
Cycle 4
Cycle 4
Cycle 5
Cycle 5
Cycle 6
Cycle 6
Cycle 7
Cycle 7
Cycle 8
Cycle 8
Cycle 9
Cycle 9
Cycle 10
Cycle 10
1.75
1.50
1.25
-1.50
-1.75
-2.00
Cooling
100
66.92 69.00
40
60
80
Température de consigne (°C)
Fig. 3. DSC curves of Nd0.3Sm0.7NiO3 samples annealed at 20 and 175 bar.

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T = 790°C, PO2 = 175 bars
T = 790°C, PO2 = 20 bars
Fig. 4. Images of the two samples showing an inhomogeneous grain size with the formation of small agglomerated particles.
c**
c*
[001]
b*
[110]
[010]
[100]
020
002
a*
b*
a*
200
[110]*
Fig. 5. Electron diffraction patterns of the Nd0.3Sm0.7NiO3 samples.
compositions for both compounds are considered to be the
same within the experimental error.
c
3.8 nm
According to these first characterizations, both kinds of
annealing allow obtaining of Nd0.3Sm0.7NiO3 powders with
similar structural properties. In order to check the influence of
annealing pressure on these compounds, we performed
a
microstructural investigation using transmission electron micro-
scopy.
3.3. Transmission electron microscopy
Few rare-earth nickelate characterizations by transmission
electron microscopy are reported in literature. The only TEM
studies on such compounds are the ones carried out on NdNiO3
thin films deposited on silicon substrates in order to link sample
preparation conditions to physical properties [20–22], and
structural study at low temperature in order to characterize the
structural transition [23]. In the present work, transmission
electron microscopy is used to determine the influence of oxygen
pressure on microstructure during post-annealing. Both samples
exhibit an inhomogeneous grain size, small agglomerated parti-
Fig. 6. HRTEM image showing a twinning in the Nd0.3Sm0.7NiO3 sample.
cles forming aggregates of size ranging between 1 and 10
(Fig. 4a and b).
mm
and nominal composition of oxygen, calculated from the previous
equations, are presented in Table 1. The calculated composition
gives Nd0.3Sm0.7NiO2.98 at 175 bar annealing and Nd0.3Sm0.7
NiO2.97 at 20 bar annealing (Fig. 3). Nominal oxygen
Fig. 5 shows typical ED patterns obtained for samples annealed
either at 20 or 175 bar. The positions of reflections are
pffiffiffi
characteristic of an orthorhombic lattice with a ꢂ b ꢂ ap 2, c ꢂ

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1667
2ap (ap¼0.38 nm, the suffix p referring to the perovskite unit cell).
Fig. 5b and c shows typical ED patterns illustrating twinning
or 901 oriented domains resulting from geometric relation-
ships between the unit cell parameters (c¼2d[1 1 0] and a¼b).
Reconstruction of the reciprocal space was carried out by tilting
around crystallographic axes, but determination of the conditions
limiting the reflections was difficult to perform due to overlap of
different twin domains. In order to solve this problem, HRTEM
images showing oriented domains were recorded and fast Fourier
transforms (FFTs) were performed on selected areas images. The
HREM image in Fig. 6 was obtained on the 20 bar annealed sample
and displays the [0 1 0] and [1 0 0] oriented areas with the corres-
ponding FFTs. Combining these FFTs with the experimental [0 0 1]
ED pattern (Fig. 5a), the conditions limiting the reflections
become 0kl: k¼2n, h0l: h+l¼2n, consistent with a Pbnm space
group. After such preliminary microscopy results about the
average structure, one may consider both samples are identical
However, considering that the aim of this microscopy work is
to pick out the possibility that microstructural differences may
appear, depending on the annealing condition, we investigated
fine structure of the sample using high resolution imaging. We
carefully looked for stacking defects, twinning, shearing or other
defect commonly observed in the perovskite structure. According
to the high resolution images recorded along different crystal-
lographic axes on both materials ([0 0 1], [1 1 0], [0 1 0] and [1 0 0]
zone axes), one can conclude that the structure commonly
obtained at 175 bar is perfectly maintained on 20 bar sample
without major structural defects; [0 0 1] and [0 1 0] high resolu-
tion images are presented as examples in Fig. 7 for both materials.
Only very slight contrast differences can be observed in the [0 1 0]
oriented images for the material prepared at 20 bar, leading us to
perform FFTs on these small areas to understand if it is
significantly correlated to a structural phenomenon (Fig. 8). It
appears that a Fourier Transform realized inside or outside these
areas gives the same diffraction pattern corresponding to the
[0 1 0] zone axis, but at the interface one can observe diffuse
scattering along the c*-axis characteristic of local disorder.
According to Ref. [24] the diffuse scattering may be related to
oxygen vacancies created by lower oxygen pressure conditions in
the 20 bar sample. However, the defects appear only time to time,
and the average macroscopic structure seems independent of the
annealing conditions.
3.4. Infrared measurement
Other studies have already described phonon behavior in rare-
earth nickelates [25,26]. At low temperature, the spectra present
insulator type reflectivity with well-defined phonon bands. Above
the transition temperature, Nd0.3Sm0.7NiO3 becomes metallic and
the phonon bands are screened by delocalized mobile electrons.
To characterize optical reflectivity switching efficiency, it is
usual to define a contrast factor in the 8–12 mm range, where
[001] 175 bars
[001] 20 bars
b
b
a
a
[010] 175 bars
[010] 20 bars
c
c
a
a
Fig. 7. HRTEM images of the two samples.

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atmospheric transparency allows thermal detection (for more
details, see [4,6]):
Infrared emissivity for the sample annealed at 20 bar varied
from 82% at 5 1C to 60% at 100 1C, whereas emissivity values for
the sample synthesized at high pressure are 81% at 5 1C and 57% at
100 1C.
Z
l2
1
RðDl,TÞ ¼
Rðl,TÞdl
Dl
l1
In the infrared-detection field (8–12 mm), a thermal imaging
D
R ¼ RðDl,T2ÞꢀRðDl,T1Þ
ð1Þ
camera measures emittance of the object. This emittance,
currently denoted by M, is the thermal power emitted by a unit
surface in half space. This value is determined by
Both samples show large change of IR reflectivity in band III
with temperature and thus exhibit a thermochromic behavior
(Figs. 8 and 9). The calculated infrared contrasts are 0.25 (25%) for
the sample annealed at 175 bar and 0.2 (20%) for the 20 bar
sample. Taking into account experimental error and surface state
of each sample, these contrasts can be considered as equivalent
for both samples. Such reflectivity data can be used for estimating
spectral emissivity of the samples in the considered spectral band.
The sample thickness causes transmittance to be null. Applying
conservation principle of flux and Kirchoff’s law, emissivity can be
easily deduced as el¼1ꢀrl (see Ref. [4] fore more details).
M ¼ esT4
where
ð2Þ
and T
s
is the Stefan’s constant (5.67 ꢁ 10ꢀ8 W mꢀ2 Kꢀ4),
e
are, respectively, the emissivity and real temperature of the
sample (in K). Using the previous equation, emittance of each
sample can be deduced. At 5 1C, emittance of each sample is
277.4 W mꢀ2 (20 bar) and 274.3 W mꢀ2 (175 bar). In a detection
situation, the default value of emissivity is fixed and equal to one.
Hence it allows determination of temperature observed by the
infrared camera. The samples appear to be at ꢀ8.5 1C (20 bar) and
ꢀ9.3 1C (175 bar) whereas the real temperature is 5 1C. Similarly
the samples heated at 100 1C appear at 55.3 and 51.1 1C for,
respectively, sample annealings under 20 and 175 bar.
c*
[010] 20 bars
4. Conclusions
a*
In this study, we compare the influence of annealing condi-
tions on microstructure and thermochromic behavior of
Nd0.3Sm0.7NiO3 powders. It appears that Nd0.3Sm0.7NiO3 synthe-
sized at 20 or 175 bar shows similar properties, with the same
optical contrast and same metal–insulator transition. Oriented
domains or twinnings are observed in both cases.
c*
The decrease of oxygen pressure in the annealing process by
nearly one decade for Nd0.3Sm0.7NiO3 synthesis opens a large
scale field application and allows considering the rare-earth
nickelates as a good candidate for infrared coating or thermal
detection.
a*
c*
c
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