Effect of the synthesis conditions on the crystal, local, and electronic structure of Ce1-x3+ Cex4+ AlO3 + x/2

Article abstract of DOI:10.1134/S0036023616020170

Cerium monoaluminate Ce_1-x³⁺ Ce_x⁴⁺ AlO_3+x/2 powders with low contents of Ce⁴⁺ cations (x ～ 0.052) were synthesized. A set of modern local structure sensitive methods of analysis, including

Full text of DOI:10.1134/S0036023616020170

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2016, Vol. 61, No. 2, pp. 225–231. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © V.V. Popov, A.P. Menushenkov, Ya.V. Zubavichus, A.S. Sharapov, V.A. Kabanova, A.A. Yastrebtsev, L.A. Arzhatkina, N.A. Tsarenko, A.M. Strel’nikova,
V.V. Kurilkin, 2016, published in Zhurnal Neorganicheskoi Khimii, 2016, Vol. 61, No. 2, pp. 238–244.
PHYSICAL METHODS
OF INVESTIGATION
Effect of the Synthesis Conditions on the Crystal, Local,
3+
and Electronic Structure of
4+AlO3 + x/2
Ce1 − xCex
V. V. Popova, A. P. Menushenkova, Ya. V. Zubavichusb, A. S. Sharapova, V. A. Kabanovaa,
A. A. Yastrebtseva, L. A. Arzhatkinac, N. A. Tsarenkoc, A. M. Strel’nikovac, and V. V. Kurilkind
aNational Research Nuclear University MEPhI (Moscow Engineering Physics Institute),
Kashirskoe sh. 31, Moscow, 115409 Russia
bNational Research Center Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia
cJSC All-Russian Research Institute of Chemical Technology, Kashirskoe sh. 33, Moscow, 115409 Russia
d Peoples’ Friendship University of Russia, ul. Miklukho-Maklaya 6, Moscow, 117198 Russia
e-mail: victorvpopov@mail.ru
Received July 2, 2015
Abstract—Cerium monoaluminate
4+ AlO3+x/2 powders with low contents of Ce4+ cations (х ~ 0.052)
Ce13-+xCex
were synthesized. A set of modern local structure sensitive methods of analysis, including X-ray absorption
spectroscopy and Raman spectroscopy, were used to study the crystal, local, and electronic structures of the
synthesized compounds. The degree of reduction and the thermal stability to oxidation of reduced powders
depend not only on the reduction conditions but also on the conditions of heat pretreatment of the initial
samples. It was concluded that the reaction 4CeAlO3 + O2 ↔ 4CeO2 + 2Al2O3 is reversible.
DOI: 10.1134/S0036023616020170
Cerium oxygen compounds are of considerable still no consensus in the literature concerning the type
interest from the standpoint of inorganic synthesis and of symmetry of cerium monoaluminate crystals; there
solid state chemistry owing to the ability of cerium cat- are data about tetragonal [13], orthorhombic [14], and
ions to change the oxidation state depending on the other types of symmetry of CeAlO3 observed both at
preparation conditions. Therefore, the synthesis and
study of the structure of compounds of this class have
been attracting close researchers' attention [1–8]. In
recent years, investigation of the structure of cerium
compounds gained an additional impetus from the
progress of modern methods for local structure inves-
tigation [7, 8]. It is noteworthy that cerium oxide-
based materials are of considerable practical value for
the production of catalysts [9], solid oxide fuel cells
[10], polishing compositions [11], and for other pur-
poses [4, 12].
Cerium monoaluminate CeAlO3, which has some
specific structure details and physicochemical proper-
ties, is a cerium oxide compound that belongs to the
class of lanthanide aluminates. Indeed, among com-
plex cerium oxides containing Ce3+ cations, CeAlO3
(as well as cerium hexaaluminate CeAl11O18) has
enhanced thermal stability against oxidation, being
decomposed during heat treatment in air at tempera-
tures above 1000°C as a result of Ce3+ → Ce4+ oxida-
tion to give CeO2 and Al2O3 owing to the absence of
interaction between components in the CeO2–Al2O3
system [2]. At room temperature, CeAlO3 has a dis-
room temperature and at elevated temperature [15]. It
is noteworthy that CeAlO3 is a promising material for
the manufacture of microwave dielectric ceramics
[13], various types of catalysts [16, 17] (including those
having photocatalytic activity [18]), anodes for solid
oxide fuel cells [10], etc.
The purpose of this work was to synthesize cerium
monoaluminate CeAlO3 and study its crystal, local,
and electronic structures using a unique set of local
structure sensitive X-ray diffraction and X-ray absorp-
tion methods, providing information about cationic
ordering of the crystal lattice, and Raman spectros-
copy, which gives information about anionic ordering
of the complex oxide structures.
EXPERIMENTAL
The powders of CeAlO3 were synthesized by
reverse coprecipitation [19] of an aqueous solution of
a Ce(NO3)3 · 6H2O (99.99%) and Al(NO3)3 · 9H2O
(reagent grade) mixture (0.266 mol/L total salt con-
centration; atomic ratio Ce : Al = 1 : 1, pH 3.14) with
aqueous ammonia (3.5 mol/L). The suspension thus
torted perovskite structure [1, 2]. However, there is formed (pH 9.80) was filtered, the resulting precipitate
225

226
POPOV et al.
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EFFECT OF THE SYNTHESIS CONDITIONS
of mixed cerium aluminum hydroxide (precursor) was
227
I
washed with distilled water and dried at 90°C for 8 h.
The cation content in the filtrate was determined by
inductively coupled plasma atomic emission spectros-
copy (ICP-AES) using a Vista-PRO spectrometer
(Varian, USA).
The dried powder (xerogel) was divided into two
portions. One portion of the precursor xerogel was
reduced in a hydrogen flow at 1000°C for 4 h, while
the second one was subjected to isothermal annealing
in air at 1100°C for 3 h. The resulting oxide powder was
then reduced in two ways: in a hydrogen flow for 4 h at
1000°C and by isothermal annealing in vacuo (resid-
ual pressure of 1.33 × 10–3 Pa) for 3 h at 1400°C. The
designations and the color of the samples are summa-
rized in Table 1.
The thermogravimetric analysis (TGA) of the
powders (precursor and reduced samples) was carried
out using an SDT Q 600 thermal analyzer (TA Instru-
ments) by heating at a 10 °C/min rate in a 30–1400°C
temperature range in an air flow. The composition and
the degree of reduction of the synthesized powders
were determined by the thermogravimetric (TG)
method from the change in the sample mass during
heating to 1400°C and subsequent keeping at a con-
stant temperature for 4 h in an air flow.
4
3
2
1
500
1000
1500
2000
2500
ν, cm−1
Fig. 1. Raman spectra of samples: (1) CeAl_P; (2)
CeAl_1100A; (3) CeAl_1100A/1400V; (4) CeAl_1100A/
1400V/1400A.
resolution was 4 cm–1. The laser focusing point was
chosen by means of a built-in optical microscope; the
focused beam diameter on the sample was 50 μm.
The crystal structure and phase composition of the
resulting powders were studied by powder X-ray dif-
fraction at the “Structural Materials Science” station
of the Kurchatov synchrotron radiation source. The
measurements were carried out in the transmission
geometry at λ = 0.68886 Å and 150 mm sample–
detector distance using an Imaging Plate Fuji Film
BAS-5000 2D detector. The 2D patterns were inte-
grated to the standard I(2θ) form using the Fit2D soft-
ware [20]; the Rietveld full-profile analysis of X-ray
diffraction patterns was carried out using the Jana2006
software [21].
The Ce L3-edge X-ray absorption near-edge spec-
tra of the powders were recorded at the “Structural
Materials Science” station of the Kurchatov synchro-
tron radiation source using a Si(111) monochromator
and at the i811 station of the MAX-lab (Lund, Swe-
den). All measurements were carried out in the trans-
mission mode using ionization chambers at room
temperature. The XANES spectra were simulated
using the XANDA software [22].
RESULTS AND DISCUSSION
Initially, coprecipitation forms a brown-violet sus-
pension, which indirectly attests to the presence of
Ce3+ cation in the mixed cerium aluminum hydroxide
precipitate. However, the dried precipitate was pale
yellow-colored (Table 1), apparently, as a result of oxi-
dation of cerium cations during drying of the precur-
sor, the color being caused by the Ce4+ → O2– charge
transfer [15]. ICP AES analysis of the filtrate showed
the absence of cerium (<10–4 mg/L) or aluminum
(<5 × 10–3 mg/L) cations in the liquid phase, which
attested to complete precipitation of cations and pro-
vided the conclusion that the elemental composition
(Ce : Al atomic ratio) of the synthesized solid oxide
products corresponds to the amounts of initial salts
taken for the reaction.
The results of X-ray diffraction attest to X-ray
amorphous precursor structure. The Raman spectrum
of the dried precursor powder shows two clear-cut
peaks at 452 and 1049 cm–1 (Fig. 1, spectrum 1). The
peak at 452 cm–1 is due to Ce–O vibrations in the
CeO2 fluorite structure [23]. The observed shift of the
Raman spectra were measured at the shared use
center of the Analytical Test Center of the JSC All-
Russia Research Institute of Chemical Technology on
a Nicolet iS50 spectrometer (Thermo Fisher Scientific
Inc.) equipped with an additional Thermo Scientific
iS50 Raman block, suitable for recording and Fourier
peak position relative to the CeO2 reference (465 cm–1)
and large full width at half maximum (FWHM = 62 cm–1)
are likely to be due to the nano-sized state of the sam-
transformation of the Raman spectra. The Raman ple [24]. The peak at 1049 cm–1 is indicative of the
spectra were measured under laser excitation (λ = presence of OH groups in the sample [25]. The cerium
1064 nm) in the 100–3700 cm–1 range. The spectral
L3-edge XANES spectrum is split into two peaks,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 61 No. 2 2016

228
m, %
POPOV et al.
I
104
103
102
101
100
99
3
2
1
2
3
1
10
20
30
2θ, deg
40
0
200 400 600 800 1000 1200 1400
T, °C
Fig. 3. General view of X-ray diffraction patterns of various
types of synthesized samples: (1) CeAl_1100A; (2)
CeAl_1100A/1000H; (3) CeAl_1100A/1400V.
3+
Fig. 2. Mass variation curves of reduced
4+AlO
3 + x/2
Ce1−xCex
samples on heating in air: (1) CeAl_P/1000H; (2)
CeAl_1100A/1000H; (3) CeAl_1100A/1400V.
at 930°C (Table 1). Quantitative treatment of the TGA
data demonstrated that the composition of the
reduced samples described by the general formula
3+
which attests to the presence of Ce4+ cations in the
dried sample. Hence, the dried precursor powder was,
most likely, mixed cerium(IV) aluminum hydroxide.
4+AlO3 +
also depends not only on the
Ce1−xCex
x/2
reduction conditions but also on the conditions of heat
pretreatment of the powders (Table 1). As can be seen
from Table 1, the highest content of Ce4+ cations (i.e.,
the lowest degree of reduction) is inherent in the pow-
der obtained by the reduction of the precursor.
The heat treatment in air affords pale yellow pow-
ders (irrespective of the temperature and duration).
The powder X-ray diffraction study of the sample syn-
thesized by isothermal annealing of the precursor at
1100°C/3 h (Fig. 3, pattern 1) demonstrated that this
powder has a fluorite structure corresponding to CeO2
(JCPDS No. 34-0394; cubic symmetry; space group
A TGA study demonstrated that the decrease in the
precursor weight upon dehydration occurs in several
stages. The first stage corresponding to the removal of
non-structural water present as adsorbed water mole-
cules (–65% of the total mass loss) occurs in the tem-
perature range of 30–235°C. The second stage corre-
sponding to removal of a structure component as OH
groups (–29%) occurs in the 235–482°C range. The
residual water (–6%) is removed in the 482–1000°C
temperature range. Further temperature rise to
1400°C resulted in a minor increase in the powder
mass (+0.6% of the total mass change), probably, as a
result of oxidation of the remaining Ce3+ cations to
(225)) with the lattice parameter a = 5.422 Å
Fm3m
and coherent scattering length (CSL) of 37 nm.
Ce4+. From the TGA results, the propotion of Ce3+ in Detailed analysis demonstrated the presence of a minor
the precursor was calculated to be 0.036, while the impurity (~1%) of γ-A12O3 (JCPDS No. 29-0063; cubic
composition of the dried precursor powder was found
symmetry; space group
(227)) (Fig. 4a). These
Fd3m
3+
0.036
4+
0.964
results are in good agreement with published data [2],
which state that heating of the mixed Ce–Al hydroxide
in oxidizing atmosphere at temperatures above
1000°C affords CeO2 and Al2O3. However, in [10], the
Al2O3 phase was detected only after treatment of the
CeAlO3 powder at 1300°C in air. This does not contradict
our results, because the observed Al2O3 reflection inten-
sity for the sample (1100°C/3 h) is very low (Fig. 4a).
to be described as
3.78H2O.
O1.982 · 1/2Al2O3 ·
Ce
Ce
The oxidation onset temperature of the reduced
powders depends on the prehistory, i.e., on the condi-
tions of heat pretreatment of the initial powders (Fig. 2).
The sample prepared by the precursor reduction in a
hydrogen flow at 1000°C/4 h had the lowest oxidation
onset temperature (772°C), whereas the powder
obtained by evacuation at 1400°C/3 h of the sample
The reduced samples were colored in various
preannealed at 1100°C/3 h in air started to be oxidized shades of gray and brown, which attests to partial or
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EFFECT OF THE SYNTHESIS CONDITIONS
229
I, a.u.
(а)
0.015
1
2
0.010
0.005
0
3
4
5
10
20
30
40
2θ, deg
I
(b)
(208)
5700
5720
5740
5760
5780
Photon energy, eV
(134)
(128)
(306)
Fig. 5. Ce L -edge XANES spectra of (1, 2) reference sam-
3
ples and (3–5) synthesized samples: (1) Ce(NO ) ; (2)
3 3
Ce(SO ) ; (3) CeAl_1100A/1400V; (4) CeAl_1100A/
4 2
1000H; (5) CeAl_1100A.
1
2
dizing and reducing conditions was gained by Raman
spectroscopy. The Raman spectrum of the sample
synthesized by isothermal annealing of the precursor
at 1100°C/3 h showed a clear-cut peak at 463 cm–1
(FWHM = 14 cm–1) corresponding to the triply
degenerate F2g mode of the cerium dioxide fluorite lat-
tice (Fig. 1, spectrum 2), caused by symmetric vibra-
tions of oxygen atoms around the cerium ion in the
CeO8 cell [23, 24], and a group of weak peaks in the
29
30
31
32
2θ, deg
33
34
Fig. 4. (a) Detailed X-ray diffraction pattern of the
CeAl_1100A sample (continuous line) and the γ-A1 O
2
3
reference (dashed line). (b) Detailed X-ray diffraction pat-
terns of reduced samples: (1) CeAl_1100A/1000H; (2)
CeAl_1100A/1400V. The reflection indices of the corre-
sponding planes are given in parentheses.
2000–2250 cm–1 region. As the sample is kept in vac-
uum, this characteristic CeO2 line disappeared (Fig. 1,
spectrum 3). The subsequent heating to 1400°C and
keeping at this temperature for 4 h in air gave rise again to
the 462 cm–1 mode in the spectrum (FWHM = 12 cm–1)
(Fig. 1, spectrum 4).
almost complete Ce4+ to Ce3+ reduction (Table 1). It
was found by X-ray diffraction that the reduced sam-
ples mainly consist of cerium monoaluminate CeAlO3
with a minor CeO2 impurity (Fig. 3, patterns 2 and 3).
The synthesized CeAlO3 samples have a rhombohe-
dral structure (JCPDS No. 21-0175; trigonal symme-
Thus, integrated analysis of powder X-ray diffrac-
tion data and Raman spectra provided the conclusion
that the following reaction is reversible:
4CeAlO3 + O2 ↔ 4CeO2 + 2Al2O3.
try; space group
(166)). This is clearly seen while
R3m
considering X-ray diffraction patterns in detail, espe-
cially for the sample obtained in vacuum (Fig. 4b).
The CSLs are 40 nm for the sample reduced in a
hydrogen flow and 46 nm for the sample synthesized
in vacuum. The subsequent heat treatment of reduced
samples at 1400°C/4 h in air resulted in oxidation to
give again CeO2 and α-Al2O3.
Data on the local structure of the powders were
obtained from cerium L3-edge XANES spectrum cor-
responding to electron transition from the core 2p3/2
level to the valence 5d level (Fig. 5). As can be seen
from Fig. 5, the powder obtained by heat treatment in
air at 1100°C/3 h showed a typical Ce L3-edge XANES
spectrum as a split white line corresponding to com-
pounds containing Ce4+ cations, in particular, CeO2
More information about the structure of com-
pounds formed upon heat treatment under both oxi- [26, 27]. For the reduced samples, the absorption peak
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 61 No. 2 2016

230
POPOV et al.
Table 2. Results of determination of the composition of
a superposition of the Ce(NO3)3 and Ce(SO4)2 refer-
ence spectra corresponding to the Ce3+ and Ce4+
reduced samples by calculations of XANES spectra
states. The proportions of Ce3+ and Ce4+ in the sam-
ples estimated from the obtained weighting coeffi-
cients are presented in Table 2. As can be seen from
Table 2, the chemical composition of the reduced
3+
Designation
CeAl_1100A/1000H
CeAl_1100A/1400V
Composition
3+
4+
Ce
Ce
Ce
Ce
AlO3.038
0.925
0.075
3+
4+
AlO3.059
0.883
0.117
4+AlO3 + x/2 powders calculated from XANES
Ce1−xCex
spectra is in qualitative agreement with the TG data
(Table 1) but is somewhat higher in magnitude.
(“white line”) was split because of excitation along
two possible channels due to the difference between
the electronic structures of Ce3+ (4f1) and Ce4+ (4fo)
cations. This spectral pattern is a distinctive feature of
XANES spectra of intermediate-valence compounds
[28]. A decrease in the cation oxidation state shifted
the absorption edge to lower energy (Fig. 5). It is note-
worthy that the XANES spectral patterns for reduced
samples of cerium aluminate coincide with the
reported CeAlO3 spectra [7, 29].
Thus, our study demonstrated that hydrogen reduc-
tion at 1000°C and annealing in vacuum at 1400°C yields
3+
cerium monoaluminate
4+AlO3 +
powders
x/2
Ce1−xCex
with 0.052 < x < 0.204 content of Ce4+cations. The
degree of reduction and the thermal oxidation stability
of reduced powders depend not only on the reduction
conditions but also on the heat pretreatment condi-
tions of the initial powders. Relying on integrated
The experimental curves were simulated in the analysis of the results, we propose the following
5710–5740 eV region by representing each spectrum as scheme of reactions for the system in question:
OH−
T = 80°C
Ce3+Al(OH)6
3+
0.036
4+
0.964
Ce3++ Al3+
(susp.)
Ce
Ce
O1.982 ⋅ 1 2Al2O3 ⋅3.78H2O
H2 1000°C
T = 1100°C
Air
Vacuum 1400°С
or H2 1000°C
Ce3+ Ce4x+ AlO3 +x 2
CeO2 + Al2O3
1−x
T > 1000°C
Air
8. E. M. Moroz, Russ. Chem. Rev. 80, 293 (2011).
ACKNOWLEDGMENTS
9. J. Kašpar, M. Graziani, and P. Fornasiero, Handbook
on the Physics and Chemistry of Rare Earths: The Role of
Rare Earths in Catalysis, Ed. by K. A. Gschneidner, Jr.
and L. Eyring (Elsevier Science, Amsterdam, 2000),
Vol. 29, p. 159.
This work was supported by the Russian Science
Foundation (grant no. 14-22-00098).
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