Synthesis of various LaMO3 perovskites in molten carbonates

Article abstract of DOI:10.1111/j.1551-2916.2006.01273.x

Lanthanum-containing perovskites, LaMO₃ (M = Al, Sc, Cr, Mn, Fe, Co, Ni, Ga, and In), have been synthesized in molten carbonates from a mixture of lanthanum carbonate and an M-containing oxide or an acetate, or from a co-precipitation mixture of metal (M) and lanthanum. Metal aluminum, chromium, and iron, immersed in a mixture of molten (Li_0.52Na _0.48)₂CO₃ and lanthanum carbonate at 923 K, were covered, respectively, with LaAlO₃, LaCrO₃, and LaFeO₃. Ceramic and single-crystal α-Al₂O ₃, immersed in molten (Li_0.52Na_0.48) ₂CO₃ mixed with lanthanum carbonate, were also covered with LaAlO₃. In situ X-ray diffractometry revealed that La ₂O₂CO₃ reacted with α-LiAlO₃ to yield LaAlO₃; the reaction proceeded faster under N₂ than under a CO₂ atmosphere.

Full text of DOI:10.1111/j.1551-2916.2006.01273.x

J. Am. Ceram. Soc., 89 [12] 3610–3616 (2006)
DOI: 10.1111/j.1551-2916.2006.01273.x
r 2006 The American Ceramic Society
ournal
J
Synthesis of Various LaMO Perovskites in Molten Carbonates
3
w
Toshikatsu Kojima, Katsuhiro Nomura, Yoshinori Miyazaki, and Kazumi Tanimoto
Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology,
Ikeda City, Osaka 563-8577, Japan
Lanthanum-containing perovskites, LaMO (M5 Al, Sc, Cr,
3
II. Experimental Procedure
Mn, Fe, Co, Ni, Ga, and In), have been synthesized in molten
carbonates from a mixture of lanthanum carbonate and an M-
containing oxide or an acetate, or from a co-precipitation mix-
ture of metal (M) and lanthanum. Metal aluminum, chromium,
(
1) Powder LaMO Synthesis
3
Li CO
2
3
2 3
(Nakarai Tesque, 99%), Na CO (Nakarai Tesque,
9
9.5%), and K
2 3
CO (Nakarai Tesque Inc., Kyoto, Japan,
99.5%) were used for preparing molten carbonate mixtures.
These carbonates were mixed to the eutectic compositions of
and iron, immersed in a mixture of molten (Li0.52Na ) CO3
0.48 2
and lanthanum carbonate at 923 K, were covered, respectively,
with LaAlO , LaCrO , and LaFeO . Ceramic and single-crys-
(
Li0.52Na0.48
) CO , (Li0.62K )
2 3 0.38 2
3 2 3
CO , (K0.573Li0.427) CO , or
3
3
3
(
Li0.435Na0.315
0.25 2 3
K ) CO ; after having been melted at 923 K,
tal a-Al O , immersed in molten (Li0.52Na ) CO mixed with
2
lanthanum carbonate, were also covered with LaAlO . In situ
3
0.48 2
3
they were stored in an electric oven at 373 K before use.
CO was mainly used as a flux of perovskite syn-
thesis. For the lower temperature synthesis at 773 K, molten
) CO was used for several lanthanum per-
3
(
Li0.52Na0.48
)
2
3
X-ray diffractometry revealed that La O CO reacted with
2
2
3
a-LiAlO to yield LaAlO ; the reaction proceeded faster under
2
N than under a CO atmosphere.
3
(
Li0.435Na0.315
K
0.25 2
3
2
2
ovskites synthesis. Other carbonates were used for the synthesis
of LaAlO
The following oxides and an acetate were used as the metal
3
.
I. Introduction
(M) source of LaMO : a-Al O (Sigma-Aldrich Co., St. Louis,
3
2 3
MO, 99.998%), Sc
hida Chemical Co. Ltd., Osaka, Japan, 99.97%), Co
2
O
3
(Sigma-Aldrich, 99.999%), Fe
2
O
3
(Kis-
O (Kanto
ANTHANUM-CONTAINING perovskites have been used as ma-
terials for electronic devices, sensors, catalysts, and fuel cell
components. In general, these compounds have been synthesized
through a process involving the sintering of mixed oxides. In
3 4
L
Kagaku, Kanto Chemical Co. Inc., Tokyo, Japan, 99.95%),
Ga O (CERAC Inc., Milwaukee, WI, 99.995%), and
2
3
Mn(CH COO)
3
3
ꢀ 2H
2
O (Merck KGaA, Darmstadt, Germany,
2 3
particular, the usage of the less reactive La O as one of the
98%). The oxide or acetate needed to synthesize 10 mmol of
LaMO
g of the alkali carbonate mixture.
reagents requires a multi-step time-consuming procedure of
mixing and heating at temperatures higher than 1200 K.
Lower temperature syntheses of lanthanum-containing per-
ovskites are favorable because of the advantages of reducing the
production cost and of controlling the particle size and the
composition of the product. The compound LaMnO37d was
3
was mixed with 5 mmol of La (CO
2
3 3
)
2
ꢀ 8H O and about 2
To prepare the precursor through co-precipitation, we used
the following metal nitrates or acetate as starting materials:
Cr(NO ) ꢀ 9H O (Kishida Chemical, 98%), Ni(NO ) ꢀ 6H O
3
3
2
3 2
2
(
Kanto Kagaku, 98%), In(NO ) ꢀ 3H O (Mitsuwa Chemicals
3
3
2
synthesized at 623 K using reactive amorphous precursors ob-
tained from a stoichiometric citrate solution. LaAlO was pre-
1
Co., Osaka, Japan, 99%), and Mn(CH COO) ꢀ 2H O (Merck,
2 3 3 2
3 3 2
3
2
98%). An appropriate amount of La (CO ) ꢀ 8H O and the
pared through a gelation–precipitation technique at 873 K.
Molten salts have been used for a new synthesis method of
metal nitrate or acetate needed for 10 mmol of LaMO synthesis
3
were dissolved in dilute nitric acid. A saturated hot aqueous so-
lution of Na CO was poured into the La–M nitrate or acetate
precise compositions and (or) structure control at relatively low
temperatures. LaCrO was produced in molten KOH at 513 K.
3
2
3
3
4
was produced in molten NaOH at 723 K. Al-
mixture solutions in order to form the precipitates. The precip-
itates were vacuum filtered, washed with distilled water, and
then dried. The cake obtained was crushed and mixed with
about 2 g of the alkali carbonate mixture.
The precursor mixtures were placed in a Au–Pd (80:20 wt%)
alloy crucible (40 mm+ inner diameter, 60 mm high) and
maintained at elevated temperatures.
The mixtures obtained were ground, washed with distilled
water, and filtered in order to remove alkali carbonates. Then,
3
the LaMO products (M5 Al, Sc, Cr, Fe, Co, Ni, Ga, and In)
LaMn0.8Na0.2
O
3
5
3
kali-doped LaNiO was synthesized in molten KOH at 673 K.
Molten carbonates have been used as a flux of perovskite
synthesis. For example, LaNiO was produced with the aid of a
3
6
flux at 1073 K. K
Na
2
CO
3
2
CO
3
was used for the structure-
87 at 1323
K, aiming at the synthesis of 2H-BaRhO . As far as we know,
controlling synthesis of Ba11Rh10
O
30 and Ba32Rh29
O
7
3
however, there are quite a few reports on the synthesis of
LaMO perovskites using molten carbonates.
3
In this paper, we have investigated novel methods for the
synthesis of LaMO3 using mainly molten (Li0.52Na ) CO ,
which has been studied as the electrolyte of molten carbonate
were washed with 12M HCl for further purification.
The materials used and the experimental conditions (reaction
temperature, reaction time, and atmosphere) for powder
0.48 2
3
8
fuel cells, and applied the method to synthesize several lantha-
num perovskites at (or lower than) 923 K. We then present a
3
LaMO synthesis are summarized in Table I.
LaMO coating method on metal or ceramic substrates. Lastly,
3
3
we discuss the reaction mechanism of LaAlO formation in
molten carbonate on the basis of in situ X-ray diffraction (XRD)
data.
(
2) LaMO Production on the Metal and Ceramic Surfaces
3
The following substrates were placed in a Au–Pd (80:20 wt%)
crucible containing the mixture of (Li0.52Na ) CO and La
0.48 2
3
2
(
CO
3
)
3
ꢀ 8H
2
O (1:1 by weight): the metals aluminum (Nilaco
R. Koc—contributing editor
Corp., Tokyo, Japan, 99.999%, 20 mm ꢁ 20 mm ꢁ 0.1 mm),
chromium (Nilaco, 99.9%, 20 mm ꢁ 5 mm ꢁ 5 mm), iron (Ni-
laco, 99.98%, 20 mm ꢁ 20 mm ꢁ 0.5 mm), cobalt (Nilaco,
9
9.98%, 20 mm ꢁ 20 mm ꢁ 0.5 mm), and nickel (Nilaco,
2 3
Manuscript No. 21573. Received March 10, 2006; approved June 30, 2006.
Author to whom correspondence should be addressed. e-mail: kojima-t@aist.go.jp
w
99.7%, 20 mm ꢁ 20 mm ꢁ 0.5 mm), and a-Al
O ceramics
3
610

December 2006
Synthesis of Various LaMO Perovskites in Molten Carbonates
3
3611
Table I. Reaction Conditions and Results of the Synthesis of LaMO (M5 Al, Sc, Cr, Mn, Fe, Co, Ni, Ga, and In) Powder
3
in Alkali Carbonate
Temperature
(K)
Reaction
time (h)
Materials
Alkali carbonates
Precursors
Atmospheres
Product
Al
a–Al O
(Li0.52Na ) CO3
0.48 2
(Li0.52Na ) CO3
0.48 2
Immersionw
Mixture
923
923
923
923
923
773
923
923
923
923
CO2
12 LaAlO3
z
z
CO2
CO2
CO2
CO2
N2
CO2
CO2
CO2
N2
12 LaAlO3
12 LaAlO3
12 LaAlO3
12 LaAlO3
72 LaAlO3
2
3
3
3
3
3
a–Al O
(Li0.62K
0.38 2
(K0.573Li
) CO3
) CO3
0.427 2
Mixture
Mixture
2
z
z
a–Al O
2
a–Al O
(Li0.435Na0.315
(Li0.435Na0.315
(Li0.52Na ) CO3
K ) CO Mixture
0.25 2 3
2
z
a–Al O
K
0.25 2
) CO Mixture
3
2
a–Al O (alumina)
Immersionw
Immersion
12 LaAlO on the surface
3
72 LaAlO on the surface
3
2
3
0.48 2
w
a–Al O (sapphire)
(Li0.52Na ) CO3
0.48 2
(Li0.52Na ) CO3
0.48 2
(Li0.52Na ) CO3
0.48 2
2
3
z
z
a–LiAlO2
a–LiAlO2
Mixture
Mixture
18 LaAlO3
3
LaAlO3
z
z
Sc O
2
(Li0.52Na ) CO3
0.48 2
Mixture
) CO Mixture
923
773
CO2
N2
72 LaScO3
72 Sc O 1La O CO
3
3
Sc O
2
(Li0.435Na0.315
K
0.25 2
3
3
2
3
2
2
Cr(NO ) ꢀ 9H O
(Li0.52Na ) CO3
0.48 2
Co-precipitated
mixture
923
773
CO2
CO2
72 LaCrO3
12 LaCrO3
3
3
2
y
Cr(NO ) ꢀ 9H O
(Li0.435Na0.315
K ) CO Co-precipitated
0.25 2 3
mixture
3
3
2
y
Cr
(Li0.52Na ) CO3
0.48 2
Immersionw
923
923
CO /O 5 2/1 12 LaCrO on the surface
2
2
3
Mn(CH COO) ꢀ 2H O (Li0.52Na ) CO3
Co-precipitated
mixture
CO2
72 LaMnO3
3
3
2
0.48 2
y
z
Mn(CH COO) ꢀ 2H O (Li0.435Na0.315
K
0.25 2
) CO Mixture
3
773
Air
12 LaMnO3
3
3
2
z
z
Fe O
Fe O
(Li0.52Na ) CO3
K
Mixture
) CO Mixture
923
773
923
CO2
CO2
16 LaFeO3
12 LaFeO3
2
3
0.48 2
(Li0.435Na0.315
(Li0.52Na ) CO3
2
3
0.25 2 3
Fe
Immersionw
CO /O 5 2/1 12 LaFeO on the surface
2 2 3
0.48 2
z
z
Co O
3
(Li0.52Na ) CO3
0.48 2
Mixture
) CO Mixture
923
773
923
O /CO 5 1/2 12 LaCoO3
2
4
2
Co O
3
(Li0.435Na0.315
(Li0.52Na ) CO3
K
0.25 2
O2
72 Unidentified1La O CO
2 2 3
4
3
Co
Immersionw
CO /O 5 2/1 12 CoO on the surface
2 2
0.48 2
Ni(NO ) ꢀ 6H O
(Li0.52Na ) CO3
0.48 2
Co-precipitated
mixture
923
773
923
O2
O2
72 LaNiO3
3
2
2
y
Ni O
2
(Li0.435Na0.315
K
0.25 2
) CO Co-precipitated
3
12 LaNiO 1NiO1
3
3
y
mixture
La O CO
3
2
2
Ni
(Li0.52Na ) CO3
0.48 2
Immersionw
CO /O 5 2/1 12 NiO on the surface
2
2
z
z
Ga O
2
(Li0.52Na ) CO3
0.48 2
Mixture
) CO Mixture
923
773
CO2,
N2
72 LaGaO3
72 LaGaO3
3
Ga O
2
(Li0.435Na0.315
K
3
0.25 2 3
In(NO ) ꢀ 3H O
(Li0.52Na ) CO3
0.48 2
Co-precipitated
mixture
1173
CO2,
72 LaInO3
3
3
2
y
wSolid immersed in a mixture of La
and then mixed with alkali carbonate.
(CO
)
ꢀ 8H
O and alkali carbonate. Mixed with La (CO
2
z
)
ꢀ 8H
O and alkali carbonate. Co-precipitated with equimolar lanthanum,
y
2
3
3
2
3
3
2
9
(
Nikkato Corp., Sakai, Japan, SSA-S, 20 mm ꢁ 20 mm ꢁ 3
RIETAN-2000 for the evaluation of phase purity. A split
pseudo-Voigt profile function was used for pattern fitting.
In situ X-ray diffractometry was carried out to investigate the
mm). A single-crystal a-Al (Dalian Danning Opto-electricity
2 3
O
Technology Inc., Dalian, China, 99.99%, C-plane (0001), 10
mm ꢁ 10 mm ꢁ 0.5 mm) was suspended by a gold wire (0.3
mm+) in the mixture of (Li Na ) CO and La (-
reaction mechanism between a-LiAlO
2
(Nippon Chemical In-
dustrial Co. Ltd., Tokyo, Japan) and La (CO ) ꢀ 8H O. In mol-
0.52
0.48 2
3
2
2
3 3
2
CO
)
3 3
2
ꢀ 8H O (10:1 by weight). The single-crystal plane was set
ten carbonate, a-LiAlO
reacts with lithium to produce a-LiAlO
a-Al O but a-LiAlO for the in situ reaction observation. The
2
is more stable than a-Al
2
O
3
, which
1
0
face below in order to prevent the floating particles from accu-
mulating on the plane. These samples were maintained at 923 K.
The samples, after immersion in molten carbonates contain-
ing lanthanum carbonate, were washed with distilled water in
order to remove the carbonate from their surfaces. The materials
used and the experimental conditions are summarized in Table I.
2
. Thus, we did not use
2
3
2
mixture of a-LiAlO2 and La (CO ) ꢀ 8H O (molar ratio
2
3 3
2
2 3
Al:La 5 1:1) was mixed with (Li0.52Na0.48) CO , 1:1 by weight.
Then, the mixture was set in a gold pan (30 mm+, 2 mmt, 0.5
mm depth) in the high-temperature reactor chamber (XRK 900,
2 2
Anton Paar GmbH, Graz, Austria) under CO or N atmos-
phere. The XRD patterns were recorded at several temperatures
up to 923 K.
(3) XRD Analysis and In Situ XRD Observations
XRD patterns of the powder products and the surfaces of the
metals and ceramics immersed in the above-mentioned molten
carbonate mixtures were taken using CuKa radiation at oper-
ating conditions of 45 kV and 40 mA (X’Pert Prosystem, Spec-
tris, Surrey, UK). The XRD patterns of the synthesized powders
were taken using Bragg–Brentano focusing optics, while those of
the pure or purified products were obtained using parallel beam
optics (from 201 to 901, scan step: 0.021, time per step: 2 s). The
data were analyzed using a Rietveld analysis computer program
III. Results and Discussion
(
1) LaMO Powders’ Synthesis
3
The results of the perovskites synthesis in molten carbonates are
summarized in Table I. We were able to obtain the desired
LaMO compounds (M 5 Al, Sc, Cr, Mn, Fe, Co, Ni, Ga, and
3
In). In order to remove (Li0.52Na ) CO , the products were
3
0.48 2
washed with distilled water. As a typical result, the XRD profile

3
612
Journal of the American Ceramic Society—Kojima et al.
Vol. 89, No. 12
reported that Sr- and Mg-doped LaGaO perovskite was pro-
3
duced at 873 K.
The four eutectic carbonates (Li0.52Na0.48
CO CO , or (Li0.435Na0.315
as solvents for the reaction with a-Al O and La (CO ) ꢀ 8H O
2 3 0.38 2
) CO , (Li0.62K )
K ) CO were used
0.25 2 3
3
, (K0.573Li0.427
)
2
3
LaGaO3
2
3
2
3 3
2
La O CO
2
2
3
2
at 923 K under CO for 12 h in order to compare the reactivity
among these carbonates. For all carbonate mixtures, the reac-
tion proceeded completely to produce LaAlO . Every carbonate
3
showed vigorous reactivity to a-Al O for the production of
2
3
LaAlO . Any of these carbonates could be used as solvents for
3
LaMO synthesis.
Although we had successfully obtained the desired products,
after just a water wash the crude products contained unreacted
3
2 2 3
metal oxides and lanthanum oxide carbonate, La O CO . Some
of the crude powders could be purified by washing with 12M
HCl. Pure phase powders of LaMO (M5 Al, Sc, and Fe), and
3
3
the powders of LaMO (M5 Ni, Co, Cr, Ga, and In) with a
2
0
30
40
50
60
70
80
90
small amount of impurity were obtained by HCl wash. Al-
though LaNiO and LaCoO partly dissolved in 12M HCl with
2
ꢀ (Cu-Kα / deg)
3
3
gas evolution, these lanthanum perovskites could be purified by
such an acid wash. On the other hand, LaMnO dissolved
quickly in 12M HCl and purified powder was not obtained
Fig. 1. X-ray diffraction profile of LaGaO and La
3
2
O
2
CO
3
synthesized
2
, and then
3
in molten (Li0.52Na0.48 CO at 923 K for 72 h under CO
)
2
3
washed with distilled water.
through this method.
of crude LaGaO
with distilled water, contained LaGaO
We obtained LaAlO , LaFeO , LaMnO , and LaNiO per-
3
is shown in Fig. 1. The product, after a wash
(a)
3
and La CO
2
O
2
3
.
1
2000
10000
LaAlO3
3
3
3
3
ovskites as powders containing small amount of impurities only
after washing with water. The XRD pattern of LaMnO , shown
in Fig. 2, could be assigned as LaMnO (JCPDS: 35–1353).
measured
calculated
3
8
6
4
2
000
000
000
000
0
3
peak position
difference
The In-containing perovskite, LaInO , could be produced at
3
temperatures higher than 1073 K; almost a pure phase was ob-
tained by keeping the mixture at 1173 K for 72 h and then
washing the powder with distilled water.
(
Li0.435Na0.315
K
0.25
)
2
CO
671 K) of the four molten carbonate mixtures. This carbonate
mixture was used for the LaMO (M5 Al, Sc, Cr, Mn, Fe, Co,
3
has the lowest melting point
(
3
Ni, and Ga) synthesis at 773 K; Table I summarizes the reaction
conditions and results of this work. Our XRD observations
2
0
30
40
50
60
70
80
90
2
2
2
ꢀ (Cu-Kα / deg)
revealed some LaMO
CO , the compounds LaAlO
LaFeO , LaNiO , and LaGaO could be produced at a low
3
in the crude products. In molten (Li0.435
Na0.315K )
0.25 2
3
3
, LaCrO , LaMnO
3
3
,
(b) 4000
000
2000
3
3
3
LaScO3
temperature of 773 K.
From the viewpoint of low-temperature synthesis, LaAlO
3
measured
calculated
3
2
11
3 3
12
773 K), LaScO (773 K), LaGaO (773 K), and LaInO3
(
13
peak position
difference
(1073 K) were synthesized at a temperature lower than that
1
4
3
of the papers reported so far. As for LaGaO , Huang et al.
1
000
0
LaMnO
3
unknown
2
0
30
40
50
60
70
80
90
ꢀ (Cu-Kα / deg)
(c)
1
1
4000
2000
LaFeO3
measured
calculated
1
0000
peak position
difference
8
6
4
2
000
000
000
000
0
2
0
30
40
50
ꢀ (Cu-Kα / deg)
Fig. 2. X-ray diffraction pattern, assigned as LaMnO
synthesized in molten (Li0.52Na0.48 CO at 923 K for 72 h under CO
and then washed with distilled water.
60
70
80
90
2
20
30
40
50
60
70
80
90
ꢀ (Cu-Kα / deg)
3
, of LaMnO
3
)
2
3
2
,
Fig. 3. Rietveld refinement results for X-ray diffraction data of (a)
LaAlO , (b) LaScO , and (c) LaFeO
3
3
3
.

December 2006
Synthesis of Various LaMO Perovskites in Molten Carbonates
3
3613
Table II. Results of Rietveld Analysis of X-ray Distributions for Synthesized LaMO (M5 Al, Sc, Cr, Fe, Co, Ni, Ga, and In)
3
Space Reliability factors, and goodness-of-fit
9
˚
Lattice parameters/A
LaMO
3
group
indicator of Rietveld analysis
Fractional coordinates (x, y, z)
LaAlO3 R-3c Rwp 5 9.27, R 5 6.97,
p
a 5 b 5 5.3637(1), c 5 13.1302(9)
(a 5 b 5 5.3648, c 5 13.1113)
La (0,0,1/4)
Al (0,0,0)
O (0.524(1),0,1/4)
16
R 5 7.62, S 5 1.24,
e
LaScO3 Pnma Rwp 5 14.57, R 5 11.02,
p
a 5 5.7652(1), b 5 8.0556(2), c 5 5.6507(1) La (0.0431(1),1/4,ꢂ0.089(4))
17
R 5 11.92, S 5 1.22
e
(a 5 5.787, b 5 8.098, c 5 5.678)
Sc (0,0,1/2)
O1 (0.473(2),1/4,0.086(2))
O2 (ꢂ0.791(1),ꢂ0.447(1),ꢂ0.794(2))
LaCrO3 Pnma Rwp 5 9.66, R 5 7.65,
p
a 5 5.5107(3), b 5 7.7538(4), c 5 5.4733(3) La (0.0136(3),1/4,ꢂ0.004(1))
17
R 5 6.19, S 5 1.56
e
(a 5 5.514, b 5 7.755, c 5 5.477)
Cr (0,0,1/2)
O1 (0.487(3),1/4,ꢂ0.036(9))
O2 (ꢂ0.728(4),ꢂ0.538(3),ꢂ0.732(5))
LaFeO3 Pnma Rwp 5 4.17, R 5 3.29,
p
a 5 5.5648(2), b 5 7.8527(3), c 5 5.5562(2) La (0.0296(2),1/4,ꢂ0.003(2))
17
R 5 3.73, S 5 1.12
e
(a 5 5.565, b 5 7.862, c 5 5.556)
Fe (0,0,1/2)
O1 (0.513(3),1/4,0.072(7))
O2 (ꢂ0.755(3),ꢂ0.452(2),ꢂ0.767(3))
La (0,0,0)
Co (1/2,1/2,1/2)
O (1/2,1/2,0)
LaCoO3 R-3m Rwp 5 2.96, R 5 2.26,
p
a 5 b 5 c 5 3.8229(1), a 5 90.682(1)
(a 5 b 5 c 5 3.824, a 5 90.70)
17
R 5 2.25, S 5 1.32
e
LaNiO3 R-3c Rwp 5 14.87, R 5 11.00
p
a 5 b 5 5.4596(2), c 5 13.1421(4)
(a 5 b 5 5.4573, c 5 13.1462) [JCPDS
Card No.79-2451]
La (0,0,1/4)
Ni (0,0,0)
O (0.548(1),0,1/4)
R 5 9.28, S 5 1.60
e
LaGaO3 Pnma Rwp 5 19.00, R 5 13.23
p
a 5 5.5267(2), b 5 7.7792(2), c 5 5.4947(1) La (0.074(5),1/4,0.0043(8))
Ga (0,0,1/2)
O1 (0.429(4),1/4,0.000(14))
17
R 5 11.58, S 5 1.64
e
(a 5 5.524, b 5 7.787, c 5 5.496)
O2 (ꢂ0.719(4),ꢂ0.462(2),ꢂ0.705(4))
a 5 5.9366(1), b 5 8.2099(2), c 5 5.7185(1) La (0.0550(2),1/4,0.0138(4))
LaInO3 Pnma Rwp 5 14.64, R 5 10.76,
p
13
R 5 9.20, S 5 1.59
e
(a 5 5.935, b 5 8.221, c 5 5.726)
In (0,0,1/2)
O1 (0.457(3),1/4, ꢂ0.095(3))
O2 (ꢂ0.682(2),ꢂ0.548(2),ꢂ0.688(2))
In order to verify the phase purity, we analyzed the XRD
data by the Rietveld method using the computer program RIE-
Li/La 5 0.008, Na/La5 0.004). On the other hand, the sample
of LaAlO contained a relatively large amount of lithium (atom-
ic ratio: Li/La5 0.15). Both lithium and sodium can be found in
the sample of LaFeO (atomic ratio: Li/La 5 0.021, Na/
La5 0.015).
These lithium and sodium would be inside perovskite lattice
or constituents of amorphous phases such as LiAlO or LiFeO
Several alkali-doped lanthanum perovskites such as La0.97
3
9
TAN-2000. As examples, the results of LaAlO
, LaFeO , and
3 3
LaScO
3
are shown in Fig. 3. The ‘‘goodness-of-fit’’ indicator
S ( 5 R /R values ideally 1) was under 1.3 and indicated a
3
wp
e
1
5
good fit. These results confirmed that the lanthanum perovs-
kite compounds obtained by our method were single phases.
Although a trace amount of impurity phase can be seen in XRD
2
2
.
K
0.03
5
4
18,19
patterns, the goodness of refinement S for LaCrO (S 5 1.56),
3
NiO , LaMn0.8Na0.2O31d, and La Li TiO
3 2/3ꢂx 3x 3
have been
LaCoO
LaInO
3
(S 5 1.32), LaNiO
3
(S 5 1.60), LaGaO
3
(S 5 1.64), and
(S 5 1.59) indicates successful refinement. These results
synthesized. The condition of incorporated lithium and sodium
needs more precise investigation. Both Li and Na have potential
to affect the transport properties of the resulting perovskites. It
does not matter where and what Li/Na is; if it is believed to be
3
of Rietveld analysis for LaMO (M 5 Al, Sc, Fe, Co, Cr, Ga,
3
and In) are summarized in Table II. The lattice parameters
1
3,16,17
obtained accord well with the reported ones.
For further analysis of composition, LaAlO
LaFeO , in which little impurity phases can be seen in XRD
2
on A-site (La) or to form Li(M)O it will significantly affect the
3 3
, LaScO , and
ionic and electronic properties of these compounds. Even they
are in amorphous state as the stoichiometry/nonstoichiometry
will change in the perovskite structure.
3
(
Fig. 3), were dissolved in nitric acid and elemental analysis was
performed using ICP (SPS-1700HVR: Seiko Instruments Inc.,
Chiba, Japan). The results are shown in Table III. It was found
that the sample of LaScO3 was almost pure (atomic ratio:
For the synthesis of a product with less impurity, further
optimizations of reaction condition and purification method are
needed.
Table III. Elemental Analysis of Synthesized Lanthanum Perovskites, LaAlO , LaScO , and LaFeO after being Purified with
3
3
3
1
2M HCl
Weight percent (%)
Elemental ratio
Perovskites
La
M
Li
Na
La
M
Li
Na
LaAlO3
LaScO3
LaFeO3
62.6
57.9
56.0
12.9
20.1
23.8
0.49
0.02
0.06
0.16
0.04
0.14
1
1
1
1.036
1.073
1.057
0.151
0.008
0.021
0.004
0.004
0.015

3
614
Journal of the American Ceramic Society—Kojima et al.
Vol. 89, No. 12
XRD pattern of powder LaAlO3
XRD pattern of LaAlO on single crystal a-Al O
2 3
3
(
sapphire)
LaAlO3
2
0
30
40
50
60
70
80
90
Fig. 5. X-ray diffraction (XRD) pattern of LaAlO
3
produced on the
surface of single-crystal a-Al (sapphire) immersed in molten (Li0.52
2 3
O
Na0.48) CO added with lanthanum carbonate at 923 K for 72 h under
2 3
Fig. 4. Microscope photograph of the surface of chromium immersed
in molten (Li0.52Na0.48 CO added with lanthanum carbonate at 923 K
for 12 h under O /CO 51/2.
CO2.
)
2
3
2
2
anode catalyst for MCFC internal reforming of methane. It
should be noted that the addition of lanthanum compound to
MCFC would influence not only the electrode but also the elec-
trolyte matrix and stainless-steel housing because of LaMO3
formation. Our preliminary study showed that in molten car-
bonates with lanthanum carbonate added stainless steel was
(
Substrates
2) Surface Coverage of LaMO on Metal and Ceramic
3
The metals aluminum, chromium, iron, a-Al
2 3
O ceramic plate,
and single-crystal a-Al O reacted with lanthanum carbonate in
covered with a fragile LaFeO scale, and the electrode materials
3
LiFeO and LiCoO were transformed to LaFeO and LaCoO ,
2 2 3 3
2
3
molten (Li0.52Na0.48
)
2
CO
3
at 923 K and produced perovskite
compounds on their surfaces. The results are summarized in
Table I.
Under a CO atmosphere, the aluminum was covered with a
respectively. The addition of lanthanum compound to MCFC
needs more study for establishing whole system stability.
Following their immersion in the molten eutectic solvent/lan-
2
LaAlO
The compound LaCrO
metal chromium under a CO
sive scale of LaCrO formed, but a further oxidation reaction
3
scale.
thanum mixture, and under a CO
the single-crystal a-Al (sapphire (0001) plane) and a-Al
ceramic plates were found to be coated with LaAlO . The XRD
pattern of LaAlO on the a-Al O ceramic was similar to that
2
atmosphere, the surfaces of
3
was produced on the surface of the
/O 5 2/1 atmosphere. An adhe-
O
2 3
2 3
O
2
2
3
3
3
2
3
2ꢂ
occurred, which introduced yellow-colored CrO
molten carbonate. The coating of LaCrO on the surface of Cr
4
ion into the
of LaAlO
LaAlO produced on single-crystal a-Al
peaks of the LaAlO3 film were much larger than the corre-
3
powder. On the other hand, the XRD pattern of
3
3
2 3
O
was different. Two
metal would be important because it might provide a means of
coating metal interconnect in SOFC to resist oxidation and
2
0,21
to decrease contact resistance.
Microscope (Lasertec,
VL2000D, Lasertec Corp., Yokohama, Japan) observation of
surface morphology is shown in Fig. 4. It was shown that cubic
crystals less than 5 mm of LaCrO covered the metal chromium
3
surface without holes or cracks.
Under the same atmosphere, iron was covered with a LaFeO
scale. The scale was fragile and easily removable.
3
On the other hand, cobalt and nickel plates immersed in mol-
CO containing lanthanum carbonate under a
CO /O atmosphere were covered with CoO and NiO, respec-
ten (Li0.52Na0.48
)
2
3
2
2
tively, not with perovskite compounds. These divalent oxides
existed in a stable form on each metal plate, and did not produce
trivalent oxides, which can react with lanthanum to generate
perovskites. It was confirmed that for the formation of LaMO3
in molten carbonate, M(III) should be stable.
Molten carbonate fuel cells utilized many materials that we
8
mentioned above. A fine powder of a-LiAlO
2
is utilized as the
electrolyte matrix. The eutectic carbonates (Li0.52Na ) CO
0.48 2
3
and (Li0.62K ) CO
0.38 2 3
have been used as the electrolyte. Porous
2 2
ceramics of NiO, LiCoO , and LiFeO are used as the cathode
materials. Stainless steel is used as the separator and current
corrector material and made of iron, chromium, and nickel.
2
2
Matsuzawa et al. reported that La
2 3
O addition reduced the
solubility of NiO cathode into molten carbonate electrolyte.
Fig. 6. Microscope photograph of the surface of single-crystal a-Al
2 3
O
2
3
¨
2 3
Bohme et al. reported that La O addition enhanced the NiO
cathode conductivity. Tagawa et al. tried to use La O as an
(sapphire) immersed in molten (Li0.52Na ) CO added with lanthanum
0.48 2
3
2
4
2
carbonate at 923 K for 72 h under CO .
2
3

December 2006
Synthesis of Various LaMO Perovskites in Molten Carbonates
3
3615
LaAlO3
La LiAlO8
LaAlO3
4
La O CO
La O CO
2 2 3
2
2
3
α-LiAlO2
2 3
powder
98K
La O
powder
298K
2
f
g
9
23K18h
e
9
23K3h
f
e
d
9
23K2h
d
c
923K1h
823K
9
23K
7
73K
23K
b
a
873K
c
7
b
a
823K
773K
2
0
30
40
ꢀ (Cu-Kα / deg)
50
60
2
2
0
30
40
ꢀ (Cu-Kα / deg)
50
60
Fig. 7. X-ray diffraction patterns of a mixture of a-LiAlO
2 2
, La
2
(
CO O, and (Li0.52Na0.48 CO under CO atmosphere at sever-
)
3 3
ꢀ 8H
2
)
2
3
2
al temperatures up to 923 K. (a) 723 K, (b) 773 K, (c) 823 K, (d) 923 K
after 2h, (e) 923 K after 18 h, (f) ex situ, powder pattern at 298 K after 18
h at 923 K.
Fig. 8. X-ray diffraction patterns of a mixture of a-LiAlO , La
2
2
(CO ) ꢀ 8H O, and (Li0.52Na ) CO under N atmosphere at several
3
3
2
0.48 2
3
2
temperatures up to 923 K. (a) 773 K, (b) 823 K, (c) 873 K, (d) 923 K
after 0.5 h, (e) 923 K after 1 h, (f) 923 K after 3 h, (g) ex situ, powder
pattern at 298 K after 3 h at 923 K.
sponding peaks of LaAlO3 powder, as illustrated in Fig. 5.
These two peaks originated from the (012) plane of LaAlO3.
This suggests that the (012) plane of LaAlO covered the crystal
3
pattern at 923 K (Fig. 8(d)), and after 3 h, a large part of the
surface. Microscope (VL2000D, Lasertec) observation is shown
in Fig. 6 of the surface of single-crystal a-Al O (sapphire) im-
3
sample transformed to LaAlO (Fig. 8(f)). XRD pattern of the
sample kept at 923 K under a N atmosphere for 3 h, and after
2
that cooled to room temperature, showed that much of the
2
3
mersed in molten (Li0.52Na0.48
)
2
CO
3
with lanthanum carbonate
added. On the surface, square crystal planes of about 5 mm
size were produced. These crystal planes were parallel to the
substrate and could be the reason the strong reflection of the
3
sample had transformed into LaAlO (Fig. 8(g)) via the follow-
ing reactions:
(
012) plane of LaAlO
on single-crystal Al
set faced below; therefore, La
3
. It seemed that LaAlO
. The plane on which the crystal grew was
CO particles did not pile up
3
grew epitaxially
LiAlO2 þ 2La2O2CO3 ! La4LiAlO8 þ 2CO2
La4LiAlO8 þ 3LiAlO2 ! 4LaAlO3 þ 2Li2O
(3)
(4)
2
O
3
O
2 2
3
on the surface and did not make contact. Lanthanum species
dissolved in molten carbonate should react with the surface of
sapphire to grow LaAlO crystal. It should be noted that
3
lanthanum species dissolved in molten carbonates play an im-
portant role in producing LaMO3.
After holding at 923 K for 3 h under CO
down, we could observe a very weak LaAlO
2
, and then cooling
peak in the powder
3
sample. On the other hand, after 3 h under N , a large part of
2
the sample transformed to LaAlO . Therefore, we confirmed
3
that the LaAlO
3 2
production reaction will proceed faster in N
(
3) In Situ XRD Observation of the LaAlO Formation
3
than in CO . This means that the reaction proceeds faster with
2
Reaction
Figure 7 shows the XRD patterns of the mixture of a-LiAlO
La (CO ) ꢀ 8H O, and (Li0.52Na ) CO heated up to 923 K un-
the decomposition of carbonate ion.
In general, the reaction between an M-containing oxide and
La O CO is expected to proceed as follows:
2
,
2
3 3
2
0.48 2
3
2
2
3
2 2 3
der a CO atmosphere. At the melting point of (Li0.52Na0.48) CO ,
M2O3 þ La2O2CO3 ! 2LaMO3 þ CO2
(5)
up to 773 K (Figs.7(a) and (b)), the XRD patterns showed car-
bonate peaks. Above the melting point of the carbonate mixture,
the patterns simplified, with the disappearance of the carbonate
peaks, and then, La O CO peaks appeared, which suggests that
2 2 3
the following reaction proceeded up to 823 K (Fig. 7(c)):
It should be noted that the carbonate ion is consumed as the
oxide ion source to produce perovskite at the reaction site. It is
also expected that reaction (5) will proceed faster in an atmos-
phere free from CO
that LaGaO could be produced at 773 K under N
under CO , but no production could be observed at 773 K under
2
, such as N
2
2
or O . Actually, it was found
3
2
, and at 923 K
2
La2ðCO3Þ ꢀ 8H2O ! La2O2CO3 þ 2CO2 þ 8H2O
(1)
3
CO
mosphere without CO
We also confirmed that the reaction producing LaAlO
2
. This means that the condition of high temperature and at-
is favorable for LaMO production.
3
After holding the temperature at 923 K for 2 h (Fig. 7(d)), we
could observe only the La O CO peaks. After 18 h, small
LaAlO peaks appeared (Fig. 7(e)). After the sample cooled to
room temperature, we observed that a large part of it had trans-
formed to LaAlO (Fig. 7(f)). From these results, we suppose
that the following reaction occurs:
2
3
2
2
3
did not proceed without the presence of the (Li0.52Na ) CO
3
0.48 2
3
solvent after 12 h at 923 K. The rate of the LaMO
3
production
reaction in molten carbonates at 923 K appears faster than that
3
2
of a solid-state reaction. Matsuzawa et al. reported that La
2
2 3
O
was transformed to La O CO at 923K and the solubility of La
3
2
2
ꢂ2
2LiAlO2 þ La2O2CO3 ! 2LaAlO3 þ Li2CO3
(2)
was 1.0 ꢁ 10 mole fraction in molten (Li0.52Na ) CO .
0.48 2
3
We also observed that a planar-shaped La
O CO
2 2 3
crystal a
Under a N atmosphere, XRD patterns higher than 873 K
2
few mm in size grew in the molten (Li0.52Na ) CO added with
0.48 2 3
exhibited a different profile assigned as La LiAlO (JCPDS: 01-
4
La (CO ) ꢀ 8H O after 12h at 923 K under air. The crystal
8
2
3 3
2
0
84-1470) (Figs. 8(c)–(g)). LaAlO
3
peaks could be seen in the
growth suggests that vigorous dissolution and precipitation of

3
616
Journal of the American Ceramic Society—Kojima et al.
Vol. 89, No. 12
Alkali-Doped La NiO
2
4
, LaNiO
3
and LaNi0.85Cu0.15
O
3
from Alkali Hydroxide
lanthanum species would occur in molten carbonate containing
La CO . The molten carbonates do not only act as a source
Fluxes,’’ Solid State Sci., 5, 351–7 (2003).
O
2 2
3
6
A. Wold, B. Post, and E. Banks, ‘‘Rare Earth Nickel Oxides,’’ J. Am. Chem.
Soc., 79, 4911–3 (1957).
K. E. Stitzer, A. El Abed, J. Darriet, and H.-C. zur Loye, ‘‘Crystal Growth and
of oxide ion but also as a flux conveying lanthanum to the
reaction site. Consequently, the dissolved lanthanum species can
attack the metal oxide surface to produce lanthanum perovs-
kites more effectively and at lower temperatures than that by
solid-state reactions.
7
Structure Determination of Barium Rhodates: Stepping Stones Toward 2H-
3
BaRhO ,’’ J. Am. Chem. Soc., 126, 856–64 (2004).
8
S. Mitsushima, K. Matsuzawa, N. Kamiya, and K. Ota, ‘‘Improvement
of MCFC Cathode Stability by Additives,’’ Electrochim. Acta, 47, 3823–30
(
2002).
IV. Conclusions
9
F. Izumi and T. Ikeda, ‘‘A Rietveld-Analysis Program RIETAN-98 and Its
Applications to Zeolites,’’ Mater. Sci. Forum, 321-324, 198–204 (2000).
We report a successful new method for the synthesis of lantha-
num perovskites LaMO (M5 Al, Sc, Cr, Mn, Fe, Co, Ni, Ga,
and In) in molten carbonates.
The metals aluminum, chromium, and iron, immersed in the
10
M. Murai, K. Takizawa, K. Soejima, and H. Sotouchi, ‘‘Lithiation of Alu-
mina in Molten Li/K Carbonates,’’ J. Electrochem. Soc., 143, 3456–62 (1996).
3
11
H. Inaba, H. Hayashi, and M. Suzuki, ‘‘Structural Phase Transition of
Perovskite Oxides LaMO and La0.9Sr0.1MO with Different Size of B-Site
Ions,’’ Solid State Ionics, 144, 99–108 (2001).
3
3
mixture of molten (Li0.52Na ) CO and lanthanum carbonate,
0.48 2
12
3
K. Huang, M. Feng, and J. B. Goodenough, ‘‘Sol–Gel Synthesis of a New
were covered with LaAlO
An a-Al
same mixture were also covered with LaAlO
single-crystal a-Al O , the (012) plane of LaAlO predominated.
3
, LaCrO
3
, and LaFeO
immersed in the
. On the surface of
3
, respectively.
Oxide-Ion Conductor Sr- and Mg-Doped LaGaO3 Perovskite,’’ J. Am. Ceram.
Soc., 79, 1100–4 (1996).
H. He, X. Huang, and L. Chen, ‘‘The Effects of Dopant Valence on the
O
2 3
2 3
ceramic and single-crystal a-Al O
13
3
Structure and Electrical Conductivity of LaInO
(2001).
3
,’’ Electrochim. Acta, 46, 2871–7
2
3
3
14
Our in situ XRD investigation of the reaction mechanism
between LiAlO and La (CO O in molten (Li0.52Na0.48
ꢀ 8H
CO under a CO or N atmosphere revealed that La CO
was produced first. Then, however, a reaction ensued that pro-
duced LaAlO . Under a N atmosphere, this latter reaction took
place via La LiAlO , and at a faster rate than the reaction under
K. Huang, R. S. Tichy, and J. B. Goodenough, ‘‘Superior Perovskite Oxide-
Ion Conductor; Strontium- and Magnesium-Doped LaGaO
Spectroscopy,’’ J. Am. Ceram. Soc., 81, 2576–80 (1998).
)
3 3
2
)
2
3
: II, AC Impedance
2
2
3
2
2
O
2 2
3
15
R. A. Young, ‘‘Introduction to the Rietveld Method,’’ pp. 1–38 in The Rietveld
Method, Edited by R. A. Young. Oxford University Press, Oxford, 1993.
16
3
2
T.-Y. Chen and K.-Z. Fung, ‘‘A and B-Site Substitution of the Solid Electro-
lyte LaGaO and LaAlO with the Alkaline-Earth Oxides MgO and SrO,’’ J. Al-
loys. Compd., 368, 106–15 (2004).
4
8
3
3
CO2.
17
R. W. G. Wyckoff, Crystal Structures, Vol. 2, 2nd edition, Interscience Pub-
lishers, New York, 1964.
Y. Inaguma, T. Katsumata, M. Itoh, and Y. Morii, ‘‘Crystal Structure of a
The molten carbonates play an important role to facilitate
LaMO production reaction by dissolving and conveying lan-
18
3
thanum to the reaction site and acting as a source of oxide ion
for perovskite formation.
Lithium Ion-Conducting Perovskite La2/3ꢂxLi3xTiO
3
(x 5 0.05),’’ J. Solid State
Chem., 166, 67–72 (2002).
M. Yashima, M. Itoh, Y. Inaguma, and Y. Morii, ‘‘Crystal Structure and
19
3
Diffusion Path in the Fast Lithium-Ion Conductor La0.62Li0.16TiO ,’’ J. Am.
Chem. Soc., 127, 3491–5 (2005).
N. Q. Minh, ‘‘Ceramic Fuel Cells,’’ J. Am. Ceram. Soc., 76, 563–88 (1993).
21
E. A. Lee, S. Lee, H. J. Hwang, and J.-W. Moon, ‘‘Sol–Gel Derived
References
20
1
F. Licci, G. Turilli, P. Ferro, and A. Ciccarone, ‘‘Low-Temperature Synthesis
and Properties of LaMnO37d and La0.67R0.33Mn37d (R5 Ca, Sr, Ba) from Citrate
3
(La0.8M0.2)CrO (M5 Ca, Sr) Coating Layer on Stainless-Steel Substrate for use
Precursors,’’ J. Am. Ceram. Soc., 86, 413–9 (2003).
P. K. Sahu, S. K. Behera, S. K. Partihar, and S. Bhattacharyya, ‘‘Low Tem-
as a Separator in Intermediate-Temperature Solid Oxide Fuel Cell,’’ J. Power
Sources, 157, 709–13 (2006).
K. Matsuzawa, T. Mizusaki, S. Mitsushima, N. Kamiya, and K. Ota, ‘‘The
2
22
perature Synthesis of Microwave Dielectric LaAlO
3
Nanoparticles: Effect of Chlo-
ride on Phase Evolution and Morphology,’’ Ceram. Int., 30, 1231–5 (2004).
Effect of La Oxide Additive on the Solubility of NiO in Molten Carbonates,’’
J. Power Sources, 140, 258–63 (2005).
3
W. Zheng, W. Pang, G. Meng, and D. Peng, ‘‘Hydrothermal Synthesis and
23
Characterization of LaCrO
3
,’’ J. Mater. Chem., 9, 2833–6 (1999).
N. R. Washburn, A. M. Stacy, and A. M. Portis, ‘‘Low-Temperature, Flux-
Grown, Na-Doped LaMnO : Magnetic Properties,’’ Appl. Phys. Lett., 70, 1622–4
1997).
¨
O. Bohme, F. U. Leidich, H. J. Salge, and H. Wendt, ‘‘Development of Mate-
rials and Production Technologies for Molten Carbonate Fuel Cells,’’ Int. J.
Hydrogen Energy, 19, 349–55 (1994).
4
3
24
(
T. Tagawa, A. Yanase, S. Goto, M. Yamaguchi, and M. Kondo, ‘‘Ceramic
Anode Catalyst for Dry Methane Type Molten Carbonate Fuel Cell,’’ J. Power
Sources, 126, 1–7 (2004).
5
C. Shivakumara, M. S. Hegde, A. S. Prakash, A. M. A. Khadar, G. N. Sub-
banna, and N. P. Lalla, ‘‘Low Temperature Synthesis, Structure and Properties of
&