Synthesis of LaMO3 (M = Fe, Co, Ni) using nitrate or nitrite molten salts

Article abstract of DOI:10.1016/j.jallcom.2010.08.073

Perovskite compound LaMO₃ (M = Fe, Co, Ni) nanocrystals were successfully synthesized in molten nitrates or nitrites from a mixture of lanthanum nitrate and an M-containing nitrate for 2 h. The effect of the various process parameters on the phase purity, crystallite size, specific surface area and morphology of the synthesized nanocrystals were systematically studied by XRD, scanning electron microscopy (SEM), simultaneous TG/DSC and BET measurements. The results showed that salt medium, annealing temperatures (mainly 450-850 °C), oxidising properties and basicity of the melt played an important role in the synthesis of LaMO₃. The addition of Na ₂O₂ facilitated the reaction between La₂O ₃ and NiO or Co₃O₄, leading to the formation of LaNiO₃ and LaCoO₃ at a much lower temperature of 450 °C. Pure hexagonal LaNiO₃ nanocrystals were obtained in molten NaNO₃-KNO₃ eutectic with Na₂O₂ at 550-750 °C.

Full text of DOI:10.1016/j.jallcom.2010.08.073

Journal of Alloys and Compounds 508 (2010) 301–308
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of LaMO3 (M = Fe, Co, Ni) using nitrate or nitrite molten salts
Jun Yanga,∗, Runsheng Lia, Junyi Zhoua, Xiaoci Lia, Yuanming Zhanga, Yulin Longa, Yongwang Lib
a Department of Chemistry, Jinan University, Shipai, Guangzhou, Guangdong Province 510632, People’s Republic of China
b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 2 August 2010
Accepted 21 August 2010
Available online 27 August 2010
Perovskite compound LaMO3 (M = Fe, Co, Ni) nanocrystals were successfully synthesized in molten
nitrates or nitrites from a mixture of lanthanum nitrate and an M-containing nitrate for 2 h. The effect of
the various process parameters on the phase purity, crystallite size, specific surface area and morphol-
ogy of the synthesized nanocrystals were systematically studied by XRD, scanning electron microscopy
(SEM), simultaneous TG/DSC and BET measurements. The results showed that salt medium, annealing
temperatures (mainly 450–850 ◦C), oxidising properties and basicity of the melt played an important role
in the synthesis of LaMO3. The addition of Na2O2 facilitated the reaction between La2O3 and NiO or Co3O4,
leading to the formation of LaNiO3 and LaCoO3 at a much lower temperature of 450 ◦C. Pure hexagonal
LaNiO3 nanocrystals were obtained in molten NaNO3–KNO3 eutectic with Na2O2 at 550–750 ◦C.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Molten salt synthesis
LaMO3
Perovskite
Basicity
1. Introduction
is well known that the molten salt synthesis method is one of the
simplest, most versatile, and cost-effective approaches available for
Lanthanum-containing perovskite-type compounds, LaMO3,
comprising f-transition and d-transition ions have been one of the
most attractive and interesting mixed oxides due to their innova-
tive use in advanced technologies [1]. These oxides have been used
as functional materials in many fields, including solid oxide fuel
cells [2,3], catalysts [4], gas separators [5], giant negative magne-
toresistance [6] and various gas sensors [7,8].
The synthesis of lanthanum-containing perovskite-type oxides
LaMO3 and related compounds have been achieved by many
methods involving solid-state reaction [9], mechanochemical solid
reaction [10], hydrothermal synthesis [11], combustion synthesis
[12,13], sol–gel method [14,15], co-precipitation method [16–19],
reverse micelle method [20], polymerizable complex method [21],
sonochemical method [22] and microemulsion method [23,24].
However, the conventional solid-state reaction method requires
several heating and grinding steps to ensure the homogeneous mix-
ing of the various oxides, while the reverse drop co-precipitation
method requires the use of more chemicals and longer time for the
formation of the LaFeO3 [25]. To obtain desired final powders with
good crystalline structures, all the wet chemical methods need a
very high calcination temperature that wastes time and energy.
Chemical methods of synthesis of materials play a crucial role
in the design and discovery of new materials, and provide better
and less cumbersome methods for preparing known materials. It
obtaining a pure perovskite phase at a relatively low temperature
for a shorter soaking time, in which the molten salts are used as
a reaction medium. The intrinsic scalability, flexibility, and facil-
ity of this technique render it attractive for the fabrication of a
range of ternary metal oxides. LaMn0.8Na0.2O3 was produced in
molten NaOH at 723 K [26]. Alkali-doped LaNiO3 was synthesized
in molten KOH at 673 K [27]. Pb(Mg1/3Nb2/3)O3–PbTiO3 was pro-
duced at 1073 K using molten Li2SO4–Na2SO4 and NaCl–KCl as a
medium of reaction, respectively [28]. [Pb(Fe0.5Nb0.5)O3] was pro-
duced in molten NaCl–KCl at 1073 K for 0.5 h [29]. La-doped NaTaO3
was produced in molten Na2SO4/K2SO4 for 0.5–1 h [30]. Single-
crystalline perovskite BaZrO3 submicrometer-sized particles were
obtained using BaC2O4 and ZrO2 as precursors, NaOH/KOH as the
molten reaction at 993 K [31]. Rare earth orthoferrites of the gen-
eral formula LnFeO3 (Ln = La, Pr and Nd) phases were synthesized in
molten NaOH flux at 673 K [32]. As far as we know, however, there
are quite a few reports on the synthesis of LaMO3 perovskites using
molten nitrates [33,34]. In this paper, we systematically investi-
gate the effect of salt medium, annealing temperatures (mainly
450–850 ◦C) and basicity of the melt (NaNO2, NaNO3–KNO3 and
NaNO3–KNO3 with Na2O2) on morphology, composition, size and
shape of the product in the synthesis of crystalline LaMO3 (M = Fe,
Co, Ni) perovskite particles.
2. Experimental
2.1. Materials and synthesis
∗
Corresponding author. Tel.: +86 020 85220223; fax: +86 020 85220223.
E-mail address: tyangj@jnu.edu.cn (J. Yang).
Materials: La(NO3)3·6H2O (Tianjin Fu-chen Chemical Reagent Factory, 99%),
Fe(NO3)3·9H2O (Shantou Guanghua Chemical Factory, 98.5%), Co(NO3)2·6H2O
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.08.073

302
J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
Table 1
Reaction conditions and results of the synthesis of LaFeO3 powder in molten salts.
No.
Flux
Temperature (◦C)
Products
Color
Crystallite size (nm)
BET area (m2/g)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
NaNO2
NaNO2
NaNO2
NaNO2
450
550
650
750
850
450
550
650
750
800
850
450
550
650
750
800
850
450
800
LaFeO3
LaFeO3
LaFeO3
LaFeO3
LaFeO3, La2O3
La2O3, Fe2O3
LaFeO3, La2O3
LaFeO3
LaFeO3
LaFeO3
LaFeO3, La2O3, Fe2O3
LaFeO3, La2O3
LaFeO3, La2O3
LaFeO3, La2O3
LaFeO3,
LaFeO3
LaFeO3
La2O3, Fe2O3
LaFeO3, La2O3
Yellowish-brown
Yellowish-brown
Yellowish-brown
Yellowish-brown
Gray-black
21.9
25.8
32.3
51.0
–
26.54
21.31
16.23
8.27
–
NaNO2
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
None
Red-brown
Red-brown
–
–
–
–
Yellowish-brown
Yellowish-brown
Yellowish-brown
Yellowish-brown
Black
Black
Black
Black
Black
32.25
47.25
51.15
26.0
–
–
–
36.0
38.4
35.8
–
12.46
7.23
4.12
8.45
–
–
–
1.76
1.48
–
Gray-black
Red-brown
Yellowish-brown
–
7.38
None
–
(Tianjin Fu-chen Chemical Reagent Factory, 99%), Ni(NO3)2·6H2O (Tianjin Damao
Chemical Reagent Factory, 98.5%), KNO3 (Guangzhou Chemical Reagent Factory,
99%), NaNO3 (Tianjin Damao Chemical Reagent Factory, 99%), NaNO2 (Guangzhou
Chemical Reagent Factory, 99%), Na2O2 (Tianjin Dagu Chemical Co. Ltd., 92.5%).
La(NO3)3·6H2O was mixed with Fe(NO3)3·9H2O, (Co(NO3)2·6H2O and
Ni(NO3)2·6H2O), respectively, with a molar ratio of 1:1. Then NaNO2, NaNO3–KNO3
eutectic (molar ratio 1:1) or NaNO3–KNO3 eutectic with Na2O2 (molar
3. Results and discussion
3.1. Synthesis and characterization of LaFeO3 nanocrystals
Previously, LaFeO3 was successfully obtained in molten NaOH
at 450 ◦C for 2 h [32], molten NaNO2 at 450 ◦C for 2 h [33] and
molten carbonates at 650 ◦C for 16 h or 550 ◦C for 12 h in CO2 atmo-
sphere [34], respectively, using La(NO3)3·6H2O and Fe(NO3)3·9H2O
as reactants.
In order to obtain lanthanum ferrite with La:Fe molar ratio
1:1, four systems were tested: La(NO3)3·6H2O–Fe(NO3)3·9H2O in
NaNO2 (A1); NaNO3–KNO3 eutectic (B1); NaNO3–KNO3 eutectic
with Na2O2 (C1); La(NO3)3·6H2O–Fe(NO3)3·9H2O without molten
salt (D1). The results of the synthesis of LaFeO3 in molten salts are
summarized in Table 1.
ratio 2:2:1) as molten salt was mixed with reactants in
a in a molar ratio
of La3+:Mn+:NaNO2 = 1:1:20, La3+:Mn+:NaNO3–KNO3 eutectic = 1:1:20 and
La3+:Mn+:NaNO3–KNO3 eutectic with Na2O2 = 1:1:25, respectively. The mix-
ture was placed into the ceramic crucible with a layer of corrosion resistant
borosilicate ceramic glaze inside and kept at different temperatures for 2 h in a
muffle furnace. After cooling down to room temperature naturally, the soluble
compounds of the reaction mixture dissolved in distilled water. The resulting solid
product was filtered, washed with distilled water and dried at 120 ◦C.
2.2. Characterization
In the case of system A1, pure yellowish-brown LaFeO3
nanocrystals were obtained in the temperature range of
450–750 ◦C. As shown in Fig. 1, the XRD patterns of samples
could be assigned as orthorhombic LaFeO3 (PDF# 74-2203). No
extra peaks were observed in the powder diffraction patterns,
indicating that the obtained samples were single phase in nature.
The intensity of orthorhombic LaFeO3 peaks increased with an
increase in temperature and the crystallite size increased from
21.9 nm at 450 ◦C to 51.0 nm at 750 ◦C, whereas the BET surface
area decreased from 26.54 to 8.27 m2/g. If the temperature of the
system is higher than the molten salt decomposition, the basicity
of the system is no longer controlled. Above 850 ◦C, gradual trans-
The thermal behavior of the powder mixtures was determined by thermogravi-
metric analysis (TG/DSC) using a SDT Q600 V8.3 Build 101 instrument in an air
stream from room temperature to 1000 ◦C with a heating rate of 10 ◦C/min. Powder
XRD patterns of the LaMO3 samples were recorded on a Siemens D-5000 diffrac-
tometer with monochromatized Cu K␣1 radiation (ꢀ = 0.15405 nm) at 40 kV, 30 mA.
SEM images were taken on a JEOL JSM-6330 F scanning electron microscopy. The
specific surface area of LaMO3 was measured through low-temperature N2 adsorp-
tion method (TriStar 3000).
Fig. 1. XRD patterns of samples prepared from La(NO3)3·6H2O–Fe(NO3)3·9H2O in
Fig. 2. XRD patterns of samples prepared from La(NO3)3·6H2O–Fe(NO3)3·9H2O in
NaNO2 (system A1).
NaNO3–KNO3 eutectic (system B1).

J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
303
Fig. 4. TG and DSC curves of La(NO3)3·6H2O–Fe(NO3)3·9H2O mixture.
Fig. 3. TG and DSC curves of La(NO3)3·6H2O–Fe(NO3)3·9H2O in NaNO3–KNO3 eutec-
tic.
formation of LaFeO3 to La2O3 occurred and a gray-black mixture
of LaFeO3 (black) and La2O3 (white) was obtained.
that the effect of molten NaNO3–KNO3 eutectic could improve the
purity of LaFeO3. By raising the temperature to 850 ◦C, which is over
the decomposition temperature of molten salts [33], we could see
very weak signals of La2O3 and Fe2O3 appeared in the XRD patterns.
In order to investigate how the different temperatures affect
the microstructure of LaFeO3, SEM analyses of samples obtained at
650 ◦C, 800 ◦C and 850 ◦C in NaNO3–KNO3 eutectic (system B1) and
800 ◦C without molten salts (system D1) are presented in Fig. 5.
It is clear that the samples synthesized at different temperatures
and molten salts exhibit varied microstructures in terms of particle
size and morphology. SEM pictures show orthorhombic LaFeO3 is
obtained at 800 ◦C in NaNO3–KNO3 eutectic and its SEM micrograph
(micrograph b) shows agglomerates of cubic nanocrystals.
As shown in Fig. 2, in the case of system B1, pure yellowish-
brown LaFeO3 nanocrystals were obtained in the temperature
range of 650–800 ◦C. The TG and DSC curves of the mix-
ture of La(NO3)3·6H2O and Fe(NO3)3·9H2O with and without
NaNO3–KNO3 eutectic are shown in Figs. 3 and 4, respectively.
Below 250 ◦C, removal of the water of crystallization and decompo-
sition of the nitrites cause the weight loss. The weight loss between
300 ◦C and 450 ◦C may be regarded as a result of the formation
of La2O3 on the evidence of the corresponding peaks in the XRD
patterns (Fig. 2). Above 450 ◦C, there is a low change in mass loss
(Fig. 4). At 450 ◦C, the reaction between La2O3 and Fe2O3 occurred
and a very weak signal of LaFeO3 was recognized in the XRD pat-
terns (Fig. 2). In system B1, LaFeO3 was almost completely formed at
800 ◦C, while some signals of La2O3 and Fe2O3 were still observed in
system D1 (without NaNO3–KNO3 eutectic). This strongly suggests
In system C1, by adding Na2O2, the basicity of NaNO3–KNO3
eutectic increased [33]. Black LaFeO3 powders were obtained in
the temperature of 450–850 ◦C. XRD signals (Fig. 6) show that at
temperature lower than 750 ◦C, LaFeO3 powders were obtained as
Fig. 5. SEM photographs of LaFeO3 prepared at (a) 650 ◦C in NaNO3–KNO3 eutectic; (b) 800 ◦C in NaNO3–KNO3 eutectic; (c) 850 ◦C in NaNO3–KNO3 eutectic; (d) 800 ◦
C
without NaNO3–KNO3 eutectic.

304
J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
Fig. 6. XRD patterns of samples prepared from La(NO3)3·6H2O–Fe(NO3)3·9H2O in
NaNO3–KNO3 eutectic with Na2O2 (system C1).
Fig. 7. XRD patterns of samples prepared from La(NO3)3·6H2O–Co(NO3)2·6H2O in
NaNO2 (system A2).
impure. Compare with system B1, the addition of Na2O2 increased
the basicity and the ability to stabilise high oxidation states,
resulting in the formation of LaFeO3 at much lower temperature
(450–550 ◦C). However, by further increasing the temperatures
over 650 ◦C, the effect of Na2O2 adding on the formation of LaFeO3
became insignificant. Some researchers reported that lanthanum
oxide was a strong base and could be obtained either in molten
alkali metal nitrates by adding Na2O2 or by raising the temperature
over the decomposition temperature [33,35].
3.2. Synthesis and characterization of LaCoO3 nanocrystals
Previously, LaCoO3 was obtained as impure powders in molten
NaNO2 at 500 ◦C for 2 h [33] and molten carbonates at 650 ◦C for12 h
in O2:CO2 = 1:2 atmosphere [34], respectively.
Four systems were tested for the synthesis of LaCoO3:
La(NO3)3·6H2O–Co(NO3)2·6H2O in NaNO2 (A2), NaNO3–KNO3
eutectic (B2), NaNO3–KNO3 eutectic with Na2O2 (C2) or
La(NO3)3·6H2O–Co(NO3)2·6H2O without molten salt (D2). The
results of the LaCoO3 synthesis in molten salts are summa-
rized in Table 2. We were able to obtain the desired LaCoO3
compounds.
In system A2, black LaCoO3 powders were obtained in the tem-
perature of 550–750 ◦C. As shown in Fig. 7, the XRD of LaCoO3 could
be assigned as hexagonal LaCoO3 (PDF# 48-0123). The intensity
of LaCoO3 peaks increased with an increase in temperature and
the crystallite size increased from 19.3 nm at 550 ◦C to 28.2 nm at
650 ◦C, whereas BET surface area decreased from 8.13 to 3.33 m2/g.
Fig. 8. XRD patterns of samples prepared from La(NO3)3·6H2O–Co(NO3)2·6H2O in
NaNO3–KNO3 eutectic (system B2).
Further increase in temperature to 750 ◦C, the intensity of LaCoO3
peaks decreased and crystallite size declined to 20.6 nm.
In system B2, the XRD patterns are shown in Fig. 8. It is obvi-
ous that black LaCoO3 powders were obtained in the temperature
of 650–850 ◦C. When the temperature increased, the color of the
samples gradually changed from gray to black. Below 550 ◦C, a gray
mixture of Co3O4 and white La2O3 was obtained. At 650 ◦C, the reac-
Table 2
Reaction conditions and results of the synthesis of LaCoO3 powder in molten salts.
No.
Flux
Temperature (◦C)
Products
Color
Crystallite size (nm)
BET area (m2/g)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
NaNO2
NaNO2
NaNO2
NaNO2
450
550
650
750
850
450
550
650
750
800
850
450
550
650
750
850
450
800
La2O3
LaCoO3
LaCoO3
LaCoO3
Black
Black
Black
Black
Gray
–
–
19.3
28.2
20.6
19.2
–
8.13
3.33
–
–
–
NaNO2
LaCoO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
None
Co3O4, La2O3
Co3O4, La2O3
LaCoO3
LaCoO3
LaCoO3
LaCoO3
La2O3, LaCoO3
LaCoO3
LaCoO3
LaCoO3
Gray
Dark gray
Gray-black
Black
Black
Black
Black
Black
Black
Black
Black
Gray
–
–
23.7
29.2
32.5
–
2.43
1.94
1.15
–
–
–
49.5
42.9
37.5
–
–
–
0.67
0.45
0.28
–
–
–
LaCoO3
Co3O4, La2O3
LaCoO3, La2O3
None
Black

J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
305
tion between La2O3 and Co3O4 occurred and XRD signals of LaCoO3
were recognized in the XRD patterns of the samples. In the case of
system B2 (with NaNO3–KNO3 eutectic), LaCoO3 was almost com-
pletely formed at 800 ◦C. In contrast to this, La2O3 was the main
phase observed in system D2 (without eutectic), illustrating the
effect of molten NaNO3–KNO3 eutectic could improve the purity
of LaCoO3. By raising the temperature to 850 ◦C, the intensity of
LaCoO3 peaks decreased, indicating a loss of crystallinity.
In system C2, black LaCoO3 powders were obtained in the tem-
perature of 550–750 ◦C. XRD signals (Fig. 9) show that a single
hexagonal phase of LaCoO3 (PDF# 48-0123) was obtained at 750 ◦C.
Compare with system B2, the addition of Na2O2 facilitated the
reaction between Co3O4 and La2O3, leading to the formation of
LaCoO3 at much lower temperature (450–550 ◦C). By increasing
the temperature, the crystallinity of LaCoO3 slightly improved and
crystallite size increased from 42.5 nm at 550 ◦C to 47.4 nm at 750 ◦C
while BET surface area decreased from 0.67 to 0.28 m2/g. When the
temperature rose up to 850 ◦C, the intensity of LaCoO3 significantly
decreased, demonstrating a loss of crystallinity.
Fig. 9. XRD patterns of samples prepared from La(NO3)3·6H2O–Co(NO3)2·6H2O in
NaNO3–KNO3 eutectic with Na2O2 (system C2).
SEM micrographs of LaCoO3 synthesized with different molten
salts at different temperatures are presented in Fig. 10. SEM pic-
Fig. 10. SEM photographs of LaCoO3 prepared at (a) 550 ◦C in NaNO2 flux; (b) 650 ◦C in NaNO2 flux; (c) 650 ◦C in NaNO3–KNO3 eutectic; (d) 750 ◦C in NaNO3–KNO3 eutectic;
(e) 550 ◦C in NaNO3–KNO3 eutectic with Na2O2; (f) 750 ◦C in NaNO3–KNO3 eutectic with Na2O2.

306
J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
Table 3
Reaction conditions and results of the synthesis of LaNiO3 powder in molten salts.
No.
Flux
Temperature (◦C)
Products
Color
Crystallite size (nm)
BET area (m2/g)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
NaNO2
NaNO2
NaNO2
NaNO2
450
550
650
750
850
450
550
650
750
800
850
450
550
650
750
850
450
800
La2O3, NiO
Gray-black
Black
Black
Black
Black
Black
Gray
–
–
–
17.4
–
–
–
–
–
–
–
–
36.8
38.6
38.6
–
–
9.39
–
4.28
–
–
–
–
–
–
–
–
0.05
0.04
0.02
–
LaNiO3, La2NiO4
La2O3, LaNiO3
LaNiO3
LaNiO3, La2NiO4
NiO
NaNO2
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
NaNO3–KNO3–Na2O2
None
La2O3, NiO
La2O3, NiO
Gray
LaNiO3, La2O3
LaNiO3, La2NiO4
LaNiO3, La2NiO4 NiO, NiO2, Ni2O3
LaNiO3, NiO
LaNiO3
LaNiO3
LaNiO3
LaNiO3, La2O3, NiO, Ni2O3
La2O3, NiO
La2O3, LaNiO3
Black
Black
Black
Black
Black
Black
Black
Gray
Black
Black
–
–
–
–
None
tures show the hexagonal phase of LaCoO3 samples with cubic
morphology was obtained at 550–750 ◦C in molten NaNO3–KNO3
eutectic with Na2O2 (Fig. 10e and f). It can be clearly seen that
the addition of Na2O2 affects the phase transformation and mor-
phology of LaCoO3. Obviously, the morphology of LaCoO3 change
from agglomerates of spherical particles (system B2) to cubic par-
ticles (system C2) and the particle size increases sharply from
about 0.63 ␮m in system B2 at 750 ◦C to 4.0 ␮m in system C2 at
750 ◦C.
3.3. Synthesis and characterization of LaNiO3 nanocrystals
Like the synthesis of LaFeO3 and LaCoO3 powders, four sys-
tems were tested: La(NO3)3·6H2O–Ni(NO3)2·6H2O in NaNO2 (A3),
NaNO3–KNO3 eutectic (B3), NaNO3–KNO3 eutectic with Na2O2 (C3)
and La(NO3)3·6H2O–Ni(NO3)2·6H2O without molten salt (D3). The
results of the LaNiO3 synthesis in molten salts are summarized in
Table 3.
Fig. 12. XRD patterns of samples prepared from La(NO3)3·6H2O–Ni(NO3)2·6H2O in
NaNO3–KNO3 eutectic (system B3).
In system A3, LaNiO3 phase was obtained at a temperature over
550 ◦C. The XRD pattern of LaNiO3 (Fig. 11) could be assigned as
hexagonal LaNiO3 (PDF# 33-0711). However, at lower temperature,
the hexagonal LaNiO3 phase did not form completely. It is seen that
at 450 ◦C, only La2O3 and NiO phases were recognized in XRD. In
the temperature range of 450–650 ◦C, gradual transformation of
La2O3 and NiO to hexagonal LaNiO3 phase occurred and a mixture
of LaNiO3 and La2O3 was recognized. Pure hexagonal LaNiO3 could
only be obtained at 750 ◦C. Above 850 ◦C, the intensity of LaNiO3
significantly decreased and the phase transformation of LaNiO3 to
La2NiO4 was observed.
Fig. 12 shows the XRD patterns of the samples prepared in
molten NaNO3–KNO3 (system B3) at different temperature. At
450 ◦C, NiO phase was the main phase found in samples and
very weak signal of La2O3 were observed. In the temperature
Fig. 11. XRD patterns of samples prepared from La(NO3)3·6H2O–Ni(NO3)2·6H2O in
Fig. 13. XRD patterns of samples prepared from La(NO3)3·6H2O–Ni(NO3)2·6H2O in
NaNO2 (system A3).
NaNO3–KNO3 eutectic with Na2O2 (system C3).

J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
307
Fig. 14. SEM photographs of LaNiO3 prepared at (a) 750 ◦C in NaNO2 flux; (b) 550 ◦C in NaNO3–KNO3 eutectic with Na2O2; (c) 750 ◦C in NaNO3–KNO3 eutectic with Na2O2.
of 550–650 ◦C, the intensity of NiO obviously decreased with an
increase in temperature, whereas the intensity of La2O3 increased.
At 750 ◦C, the reaction between NiO and La2O3 occurred and LaNiO3
was obtained as impure powders. Above 800 ◦C, transformation of
LaNiO3 phase to La2NiO4 phase occurred. At 850 ◦C, a mixture of
LaNiO3, La2NiO4, NiO, NiO2, and Ni2O3 was obtained, indicating the
decomposition of LaNiO3. In comparison, the phase transformation
of La2O3 to LaNiO3 in system D3 (no flux) was much difficult, and
strong signals of La2O3 are still observed in the XRD patterns in the
sample prepared at 800 ◦C.
4. Conclusions
LaFeO3 nanocrystals were successfully synthesized in molten
NaNO2 at 450–750 ◦C, molten NaNO3–KNO3 at 650–800 ◦C and
molten NaNO3–KNO3 with Na2O2 at 800–850 ◦C for 2 h, respec-
tively.
LaCoO3 nanocrystals were successfully synthesized in molten
NaNO2 at 550–750 ◦C, molten NaNO3–KNO3 at 650–850 ◦C and
molten NaNO3–KNO3 with Na2O2 at 550–750 ◦C for 2 h, respec-
tively.
Fig. 13 shows the XRD patterns of the samples synthesized in
system C3, black LaNiO3 powders were obtained in the temperature
of 450–750 ◦C. Compare with system B3, the addition of Na2O2 facil-
itated the reaction between La2O3 and NiO, leading to the formation
of LaNiO3 at a much lower temperature of 450 ◦C. Pure hexagonal
LaNiO3 nanocrystals were obtained at 550–750 ◦C. No extra peaks
were observed in the powder diffraction patterns, indicating that
the obtained samples were single phase in nature. There is no signif-
icant change in the XRD intensity and crystallite size of LaNiO3 with
an increase in temperature from 550 ◦C to 750 ◦C. Above 850 ◦C, the
intensity of LaNiO3 significantly decreased and a mixture of LaNiO3,
NiO and La2O3 was obtained.
Fig. 14 shows the SEM micrographs of LaNiO3 synthesized
in molten NaNO2 at 750 ◦C and molten NaNO3–KNO3 eutectic
with Na2O2 (system C3) at 550 ◦C and 750 ◦C. It can be seen
that the LaNiO3 prepared in system C3 has a much larger parti-
cle size than that synthesized in molten NaNO2. The particle size
increases sharply from about 0.20 ␮m in system A3 at 750 ◦C to
4.0 ␮m in system C3 at 750 ◦C. In the case of system C3, there
is no significant change in the morphology or crystallite size
of LaNiO3 in the process of temperature increasing from 550 ◦C
to 750 ◦C, which is in good agreement with the results of XRD
(Fig. 13).
LaNiO3 nanocrystals were successfully synthesized in molten
NaNO2 at 750 ◦C, and molten NaNO3–KNO3 with Na2O2 at
550–750 ◦C for 2 h, respectively.
The oxidising properties and basicity of the melt play an
important role in the synthesis of LaMO3. The addition of Na2O2
increased the basicity and the ability to stabilise high oxidation
states. With the addition of Na2O2, the morphology of LaCoO3
synthesized at 750 ◦C changed from agglomerates of spherical
particles (system B2) to cubic particles (system C2) and the
particle size increased sharply from about 0.63 ␮m to 4.0 ␮m.
The LaNiO3 prepared in NaNO3–KNO3 eutectic with Na2O2 had
a much larger particle size than that synthesized in molten
NaNO2.
Acknowledgements
The authors gratefully thank Professor Yulei Zhu and Yu Tang for
the valuable discussion, and acknowledge financial support from
the Team Project of Guangdong Province Natural Science Founda-
tion (Grant No. 05200555).

308
J. Yang et al. / Journal of Alloys and Compounds 508 (2010) 301–308
References
[17] J. Yang, H.Q. Zhong, M. Li, L.Z. Zhang, Y.M. Zhang, React. Kinet. Catal. Lett. 97 (2)
(2009) 269–274.
[18] M. Kumar, S. Srikanth, B. Ravikumar, T.C. Alex, S.K. Das, Mater. Chem. Phys. 113
(2009) 803–815.
[1] Z. Yang, Y. Huang, B. Dong, H.L. Li, Mater. Res. Bull. 41 (2006) 274–281.
[2] C.S. Cheng, L. Zhang, Y.J. Zhang, S.P. Jiang, Solid State Ionics 179 (7–8) (2008)
282–289.
[3] S. Pathak, J. Kuebler, A. Payzant, N. Orlovskaya, J. Power Sources 195 (11) (2010)
3612–3620.
[4] N. Russo, D. Fino, G. Saracco, V. Specchia, Catal. Today 137 (2–4) (2008) 306–
311.
[5] J.A. Ruud, M.J. Bowman, K.V. Sarathy, M. Manoharan, A.Y. Ku, V. Ramaswamy,
P.R.L. Malenfant, US Patent 2008134895(A1).
[19] W. Li, M.W. Zhuo, J.L. Shi, Mater. Lett. 58 (2004) 365–368.
[20] J. Chandradass, Y.J. Cha, K.H. Kim, Met. Mater. Int. 15 (6) (2009) 1045–1048.
[21] M. Popa, J. Frantti, M. Kakihana, Solid State Ionics 154–155 (2002) 437–445.
[22] N. Das, D. Bhattacharya, A. Sen, H.S. Maiti, Ceram. Int. 35 (2009) 21–24.
[23] Z.Q. Tian, W.J. Huang, Y.J. Liang, Ceram. Int. 35 (2009) 661–664.
[24] A.E. Giannakas, A.K. Ladavos, P.J. Pomonis, Appl. Catal. B: Environ. 49 (2004)
147–158.
[25] S. Farhadi, Z. Momeni, M. Taherimehr, J. Alloys Compd. 471 (2009) L5–L8.
[26] N.R. Washburn, A.M. Stacy, A.M. Portis, Appl. Phys. Lett. 70 (1997) 1622–1624.
[27] C. Shivakumara, M.S. Hegde, A.S. Prakash, A.M.A. Khadar, G.N. Subbanna, N.P.
Lalla, Solid State Sci. 5 (2003) 351–357.
[6] D. Hammer, J. Wu, C. Leighton, Phys. Rev.
134407/1–134407/11.
B
69 (13) (2004),
[7] T. Chen, Z.L. Zhou, Y.D. Wang, Sens. Actuators B: Chem. 143 (1) (2009) 124–131.
[8] D. Wang, X.F. Chu, M.L. Gong, Nanotechnology 17 (21) (2006) 5501–5505.
[9] K. Rida, A. Benabbas, F. Bouremmad, M.A. Pen˜a, A. Martínez-Arias, Catal. Com-
mun. 7 (2006) 963–968.
[28] S.X. Zhao, Q. Li, F.B. Song, C.H. Li, D.Z. Shen, Key Eng. Mater. 336–338 (1) (2007)
10–13.
[29] C.C. Chiu, C.C. Li, S.B. Desu, J. Am. Ceram. Soc. 74 (1) (1991) 38–41.
[30] D.G. Porob, P.A. Maggard, J. Solid State Chem. 179 (6) (2006) 1727–1732.
[31] H.J. Zhou, Y.B. Mao, S.S. Wong, Chem. Mater. 19 (2007) 5238–5249.
[32] C. Shivakumara, Solid State Commun. 139 (2006) 165–169.
[33] C. Matei, D. Berger, P. Marote, S. Stoleriu, J.P. Deloume, Prog. Solid State Chem.
35 (2007) 203–209.
[34] T. Kojima, K. Nomura, Y. Miyazaki, K. Tanimoto, J. Am. Ceram. Soc. 89 (2006)
3610–3616.
[35] D.A. Habboush, D.H. Kerridge, S.A. Tariq, Thermochim. Acta 65 (1) (1983) 53–60.
[10] F. Saito, Q.W. Zhang, J. Kano. J. Mater. Sci. 39 (2004) 5051–5055.
[11] W. Zheng, R. Liu, D. Peng, G. Meng, Mater. Lett. 43 (2000) 19–22.
[12] Z.Q. Tian, H.T. Yu, Z.L. Wang, Mater. Chem. Phys. 106 (2007) 126–129.
[13] X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, Ceram. Int. 29 (2003) 347–349.
[14] P.V. Gosavi, R.B. Biniwale, Mater. Chem. Phys. 119 (1–2) (2010) 324–329.
[15] K. Rida, A. Benabbas, F. Bouremmad, M.A. Pen˜a, E. Sastre, A. Martínez-Arias,
Appl. Catal. A: Gen. 327 (2007) 173–179.
[16] H.Q. Zhong, J. Yang, Y.M. Zhang, Trans. Nonferr. Metal. Soc. 19 (1) (2009)
160–166.