Sr2GaScO5, Sr10Ga6Sc 4O25, and SrGa0.75Sc0.25O 2.5: A play in the octahedra to tetrahedra ratio in oxygen-deficient perovskites

Article abstract of DOI:10.1021/ic202236h

Three different perovskite-related phases were isolated in the SrGa _1-xSc_xO_2.5 system: Sr₂GaScO ₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5. Sr₂GaScO ₅ (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) A, b = 15.1594(1) A, and c = 5.70926(4) A] with complete ordering of Sc³⁺ and Ga ³⁺ over octahedral and tetrahedral positions, respectively. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. I4 ₁/a, a = 17.517(1) A, c = 32.830(3) A]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ can be described as a stacking of eight perovskite layers along the c axis...[-(Sc/Ga)O_1.6-SrO_0.8-(Sc/Ga)O_1.8- SrO_0.8-]₂.... Similar to Sr₂GaScO₅, this structure features a complete ordering of the Sc³⁺ and Ga ³⁺ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr ₁₀Ga₆Sc₄O₂₅ is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc³⁺ by Ga³⁺ leads to the formation of the cubic perovskite phase SrGa_0.75Sc_0.25O _2.5 (x = 0.25) with a = 3.9817(4) A. This compound incorporates water molecules in the structure forming SrGa_0.75Sc _0.25O_2.5·xH₂O hydrate, which exhibits a proton conductivity of ～2.0 × 10^-6 S/cm at 673 K.

Full text of DOI:10.1021/ic202236h

Article
pubs.acs.org/IC
Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: a Play in the
Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites
Sergey V. Chernov,† Yuri A. Dobrovolsky,† Sergey Ya. Istomin,*,‡ Evgeny V. Antipov,‡ Jekabs Grins,§
Gunnar Svensson,§ Nadezhda V. Tarakina,⊥,¶ Artem M. Abakumov,¶ Gustaaf Van Tendeloo,¶
Sten G. Eriksson,∥ and Seikh M. H. Rahman∥
†Institute of Problems of Chemical Physics, RAS, Acad. Semenov avenue 1, 142432 Chernogolovka, Moscow Region, Russia
‡Department of Chemistry, Moscow State University, Leninskie Gory, 119991 Moscow, Russia
§Department of Materials and Environmental Chemistry, Stockholm University, S-10691 Stockholm, Sweden
⊥Physikalisches Institut, Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
̈
̈
̈
¶EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
∥Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
S
* Supporting Information
ABSTRACT: Three different perovskite-related phases were isolated in the
SrGa1−xScxO2.5 system: Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5.
Sr2GaScO5 (x = 0.5) crystallizes in a brownmillerite-type structure [space
group (S.G.) Icmm, a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å]
with complete ordering of Sc3+ and Ga3+ over octahedral and tetrahedral
positions, respectively. The crystal structure of Sr10Ga6Sc4O25 (x = 0.4) was
determined by the Monte Carlo method and refined using a combination of
X-ray, neutron, and electron diffraction data [S.G. I41/a, a = 17.517(1) Å,
c = 32.830(3) Å]. It represents a novel type of ordering of the B cations
and oxygen vacancies in perovskites. The crystal structure of Sr10Ga6Sc4O25
can be described as a stacking of eight perovskite layers along the c axis
...[−(Sc/Ga)O1.6−SrO0.8−(Sc/Ga)O1.8−SrO0.8−]2.... Similar to Sr2GaScO5, this structure features a complete ordering of the
Sc3+ and Ga3+ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal
structure of Sr10Ga6Sc4O25 is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further
partial replacement of Sc3+ by Ga3+ leads to the formation of the cubic perovskite phase SrGa0.75Sc0.25O2.5 (x = 0.25) with
a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa0.75Sc0.25O2.5·xH2O hydrate, which
exhibits a proton conductivity of ∼2.0 × 10−6 S/cm at 673 K.
the highest oxide-ion conductivity. This is related to a flexibi-
lity of the oxygen coordination environment for these elements,
especially characteristic for the Ga3+ and In3+ cations. Thus,
the oxide-ion conductivity of LSGM outperforms traditional
SOFC materials like YSZ2,3 and is considered as a base for
the intermediate-temperature SOFC (IT-SOFC), operating at
500−700 °C. However, the use of LSGM in SOFCs is re-
stricted by difficulties in obtaining single-phase material, the
volatility of gallium at high temperatures, and its high reactivity
toward metallic nickel, often used as a component of anodes in
SOFCs. Another example is Ba2In2O5 with the brownmillerite
structure, which transforms at ∼930 °C to the disordered
perovskite with high oxide-ion conductivity.4 The cubic
perovskite phase can be stabilized by appropriate doping, and
Ba0.3Sr0.2La0.5InO2.75 is known to exhibit oxide-ion conductivity
up to 0.1 S/cm at 800 °C.5
1. INTRODUCTION
Oxygen-deficient perovskites are known to have various
important industrial applications, including high-temperature
electrochemical devices, such as solid oxide fuel cells (SOFCs).
In SOFCs, electrical energy is obtained through the conversion
of chemical energy released in the redox reaction between the
fuel, e.g., hydrogen (at the anode) and oxygen (at the cathode).
Depending on the utilized type of ionic conductivity, SOFCs
can be classified as proton or oxide-ion conductors. The former
includes different high-temperature proton-conducting materials
like doped cerium or zirconium perovskites ABO3 (A = Sr, Ba;
B = Ce, Zr) or oxygen-deficient perovskite-related niobium and
tantalum oxides like A6−2xB2+2xO11+3x (A = alkaline-earth;
B = Nb, Ta); see, for example, a recent review.1 The latter
includes various oxygen-deficient compounds, e.g., Zr1−xYxO2−x/2
(YSZ) and La1−xSrxGa1−yMgyO3−z (LSGM) with fluorite- and
perovskite-related structures, respectively.
Among oxide-ion conductors with negligible electronic con-
ductivity, complex oxides of the group 13 elements demonstrate
Received: October 18, 2011
Published: December 22, 2011
© 2011 American Chemical Society
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Perovskite-like oxides A2+(B3+,B′3+)O2.5 (A = Sr, Ba;
B = group 13 element; B′ = other trivalent cation with invariable
oxidation state like Sc3+ or small rare-earth cations) are of
interest as fast oxide-ion conductors used as electrolytes in
SOFCs because they possess a high concentration of oxygen
vacancies. In these oxides, the large B′ cation preferentially
occupies octahedral positions, whereas the group 13 element
data using the PANalytical X’Pert HighScore Plus software. Neutron
powder diffraction (NPD) data were collected on the GEM
diffractometer at the spallation source ISIS, Rutherford Appleton
Laboratory, U.K. Rietveld structure refinements were made with the
GSAS program package.11
Electron diffraction (ED) patterns were recorded with JEOL
JEM2000FX and Philips CM20 transmission electron micrographs
with a LaB6 filament, equipped with a double tilt sample holder 45°,
operating at 200 kV. The sample was fixed on a copper grid with holey
carbon by dipping the grid through an ethanol suspension of the sample.
Electrical conductivity measurements were performed by alternating-
current impedance spectroscopy using a Solartron 1260 frequency
response analyzer in the frequency range of 1 Hz to 1 MHz for
SrGa0.75Sc0.25O2.5. For the electrical conductivity measurements,
pellets of SrGa0.75Sc0.25O2.5 (d = 12.4 mm; h = 1.5 mm) were coated
by platinum paste (Heraeus CL 11-5100) and heated at 1273 K for 1 h.
A Netzsch DIL 402C dilatometer operated in air (298−1173 K,
10 K/min) was used for thermal expansion coefficient measurements
of the Sr2GaScO5 sample. For thermal expansion measurements, oxide
powder was pressed into pellets of 8 mm diameter and 5−5.5 mm
height and annealed at 1473 K.
7
has a coordination number of <6. Ba2AlScO56 and Ba2GaScO5
have ordered structures with hexagonal closed packing of the
BaOx layers (so-called anion-deficient hexagonal perovskites).
In the Sr(Sc,Al)O2.5 system, several compounds have been reported.
SrAl0.75Sc0.25O2.5 has a cubic structure [a = 7.9078(4) Å]
with oxygen vacancies randomly distributed around two B
cation sites.8 Takeda et al.9 have reported the formation of
the cubic SrAl0.5Sc0.5O2.5 perovskite with a = 3.9554(6) Å and
the perovskite-related phases in the compositional range
SrAl0.5−xSc0.5+xO2.5, where 0 ≤ x ≤ 0.15. SrAl0.5Sc0.5O2.5 exhibits
a low oxide-ion conductivity of ∼10−4 S/cm at 1000 °C.
In the present paper, we report on the investigation of the
SrGa1−xScxO2.5 system, where three novel oxygen-deficient
perovskites have been isolated: SrGa0.75Sc0.25O2.5 with the cubic
perovskite structure, brownmillerite-type Sr2GaScO5, and
Sr10Ga6Sc4O25 with a novel type of ordering of the B cations
and oxygen vacancies. The high-temperature conductivity of
SrGa0.75Sc0.25O2.5 in various atmospheres including wet gases
was also studied.
Thermogravimetric analysis (TGA) studies were carried out in
argon (Linde Gas, 99.997%, 5 ppm H2O) or nitrogen (AGA, 5 ppm
H2O) with gas flow rates of 20 mL/min from 298 to 1273 K at a
heating rate of 5 or 15 K/min using a Netzsch STA 409 PC.
3. RESULTS
Three novel compounds with different perovskite-related struc-
tures were found in the SrGa1−xScxO2.5 system. The formation
of single-phase samples was observed for the compositions
SrGa1−xScxO2.5, where x = 0.25, 0.375, and 0.5. The color of the
2. EXPERIMENTAL SECTION
Samples of SrGa1−xScxO2.5, where 0.05 ≤ x ≤ 0.75, were prepared by
polyacrylamide gel synthesis.10 Stoichiometric mixtures of Sc2O3,
Ga2O3, and SrCO3 were dissolved in a minimal amount of
concentrated HNO3. Thereafter, an aqueous solution of ammonium
citrate (NH3)3(cit) was added. The amount of citrate was calculated
according to the chelate formula MLn, where M is the metal, n its
valence, and L a ligand (citric acid). Finally, the pH of the solution was
adjusted to 7 by adding aqueous NH3. Acrylamide and N′,N′-
methylendiacrylamide in quantities of 6 and 1 g/100 mL of solution,
respectively, were dissolved separately in distilled water and mixed
with the citrate solution. The resulting solution was heated to boiling,
whereupon a few grains of α,α′-azoisobutyronitrile were added. The
solution turned into a transparent colorless gel, which was placed into
a furnace and slowly heated in air at a rate of 2 K/min up to 1173 K
and then kept at this temperature for 2 h. The obtained powder was
grinded, pressed into pellets, and heated at 1473−1673 K for 15−24 h.
̀
samples was creme-brown for x = 0.25 and white for x = 0.375
and 0.5. The samples with x ≤ 0.25 contained a significant
amount of Sr3Ga4O9, while for 0.25 < x < 0.35, an unidentified
admixture phase(s) was observed as well. The 0.375 < x < 0.5
samples contained a mixture of the two phases with x = 0.375
and 0.5. Also, the samples with 0.5 < x ≤ 0.75 were not single
phase and contained unknown admixture phase(s), together
with the x = 0.5 phase.
3.1. Sr2GaScO5 with the Brownmillerite Structure.
Single-phase samples of SrGa0.5Sc0.5O2.5 were obtained by
annealing at 1473 K for 15 h followed by cooling to room
temperature with a rate of ∼150 K/h. The PXRD pattern of
SrGa0.5Sc0.5O2.5 was indexed with an orthorhombic unit cell
with a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å,
suggesting the formation of the brownmillerite-type structure,
hereinafter referred to as Sr2GaScO5. The h + k + l = 2n
reflection condition unambiguously suggests a body-centered
orthorhombic unit cell. It is well-known that, depending on
the relative orientation of the tetrahedral chains between the
octahedral layers in brownmillerite, two highly ordered
structures having space groups Ibm2 (No. 46) and Pcmn
(No. 62) can be obtained. A random ordering of the tetrahedral
layers leads to space group Icmm (No. 74).
Reflection conditions do not allow one to distinguish bet-
ween Ibm2 and Icmm space groups. The Sr2GaScO5 structure
was refined in both space groups using PXRD data. In both
models, we observed complete ordering of the B cations over
two sites in the brownmillerite structure, where the large Sc3+
occupies the octahedral position and Ga3+ is situated in the
tetrahedral one. Refinement in Icmm led to slightly better R
values (Rwp = 0.0796, Rp = 0.0618, and χ2 = 2.23) compared to
Ibm2 (Rwp = 0.0804, Rp = 0.0623, and χ2 = 2.28). Moreover,
some interatomic distances between B cations and O atoms
in Ibm2 were unreasonably short. For example, two distances
For the preparation of the zirconium-doped samples SrGa0.5Sc0.5−x
-
ZrxO2.5+x/2, where x = 0.01−0.12 and Δx = 0.01, stoichiometric
mixtures of Sc2O3 and Ga2O3 were dissolved in a minimal quantity of
concentrated nitric acid. The obtained solution, together with
stoichiometric amounts of SrCO3 and ZrO(NO3)2·5.3H2O, was
added into the melt of citric acid and heated on a hot plate in air to
achieve the formation of a brown viscous mass. Further annealing was
performed at 873 K for 2 h. The obtained powder was grinded,
pressed in pellets, and finally heated in air at 1673 K for 15 h.
The phase purity of the compounds was checked by powder X-ray
diffraction (PXRD) recorded with a Huber G670 Guinier diffrac-
tometer (Cu Kα1 radiation, image foil detector). High-temperature
PXRD data for SrGa0.75Sc0.25O2.5 and room-temperature data for the
Rietveld refinement of the Sr2GaScO5 crystal structure were collected
in air using a Bruker D8-Advance diffractometer (Cu Kα1 radiation,
Vantec PSD) in reflection mode equipped with a high-temperature
camera XRK-900 (Anton Paar). The PXRD pattern for the structural
refinement of the Sr10Ga6Sc4O25 crystal structure was collected with a
PANalytical X'pert PRO MPD diffractometer using Cu Kα1 radiation,
variable slits with a constant area of 4 cm2 irradiated, and a step size of
0.0167°, in the 2θ range 5−125° and with a total measurement time of
30 min, yielding patterns with maximum peak intensities of ca. 50 000
counts. The patterns were converted to corresponding 0.5° fixed-slit
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Table 1. Summary of the Results of the Least-Square Fits, Final Atomic Coordinates, and Displacement Parameters for
Sr2GaScO5
space group
a, Å
Icmm
5.91159(7)
b, Å
15.1526(2)
c, Å
5.70458(7)
Z
8
cell volume, Å3
calcd density, g/cm3
radiation
510.99(2)
4.808
Cu Kα1, λ = 1.54060 Å
5 ≤ 2θ ≤ 100, 0.0081
37
2θ range; step, deg
params refined
χ2, RF , Rwp, Rp
2.23; 0.0406, 0.0796, 0.0618
Uiso × 100/Å2
2
atom
site
x
y
z
occupancy
Sr
8h
4a
8i
0.0200(1)
0.11199(4)
0
0.5
0
1.09(9)
0.9(1)
1.3(1)
0.4(1)
2.2(2)
2.0
1
Sc
0
1
Ga
O1
O2
O3
−0.0718(2)
0.25
0.25
−0.0268(7)
0.25
0.5
1
8g
8h
8i
−0.0140(2)
0.1458(3)
0.25
0.0719(6)
0.833(1)
0
1
0.660(1)
0.5
between Sc and equatorial O atoms were 1.92(1) Å in
comparison with a typical Sc−O distance of ∼2.1 Å. This
indicates that the correct space group for Sr2GaScO5 is Icmm.
The final refined atomic parameters are given in Table 1; the
main interatomic distances are given in Table 2. Experimental,
This, together with disorder in the tetrahedral layers, leads to dis-
tortion of the gallium tetrahedron, with the Ga−O bond length
varying between 1.799(4) and 1.877(8) Å.
The differential scanning calorimetry experiment on
Sr2GaScO5 shows a strong exothermal effect at ∼1663 K,
which can be most likely interpreted as melting. The PXRD
pattern of the Sr2GaScO5 sample quenched from 1723 K
(Figure 2) can be indexed in a cubic perovskite cell with
Table 2. Selected Interatomic Distances (Å) for Sr2GaScO5
O1−Sc
O2−Sc
O2−Ga
O3−Ga
Sr−O1
(×4)
(×2)
(×2)
(×2)
(×2)
(×2)
2.0647(4)
2.250(4)
1.799(4)
1.877(8)
2.605(2)
2.743(3)
2.466(4)
2.9142(7)
2.534(4)
Sr−O2
Sr−O3
(×2)
(×2)
calculated, and difference diffraction profiles are shown in Figure 1.
As mentioned above, in the crystal structure of Sr2GaScO5,
Figure 2. PXRD pattern of the Sr2GaScO5 sample quenched from
1723 K.
a = 3.99382(1) Å because no additional reflections indicating
either superstructure or splitting of the perovskite subcell reflections
are observed. However, one cannot exclude the brownmillerite-
like ordering of the B cations and oxygen vacancies on the level
of the microdomains12 in this metastable phase.
In order to stabilize a disordered cubic perovskite, hetero-
valent substitution of Sc3+ by Zr4+ according to the formula
SrGa0.5Sc0.5−xZrxO2.5+x/2, where Δx = 0.01, was performed. The
samples were annealed at 1673 K and cooled to room
temperature at the rate of 150 K/h. A solid solution with a
cubic perovskite structure was formed in a narrow composition
range of x = 0.03−0.1. For the samples with x = 0.01 and 0.02
Sr2GaScO5 and the cubic perovskite phase were observed,
whereas for the samples with x > 0.1, admixtures of SrZrO3 and
Sc2O3 were found. The compositional dependence of the unit
cell parameter of SrGa0.5Sc0.5−xZrxO2.5+x/2 is shown in Figure 3.
Figure 1. Observed, calculated, and difference between them PXRD
profiles for Sr2GaScO5.
there is a full order of Sc3+ and Ga3+ cations over octahedral and
tetrahedral positions, respectively. The scandium octahedron is
rather distorted with substantially elongated Sc−O axial bonds
[2.250(4) Å] in comparison with equatorial ones [2.0647(4) Å].
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SAED indicates the reflection conditions hkl (h + k + l = 2n),
hk0 (h, k = 2n), and 00l (l = 4n), consistent with the space
group I41/a (No. 88). In the Rietveld refinement of the crystal
structure (see below), we have found that the composition
of the phase corresponds to the formula Sr10Ga6Sc4O25,
which differs only slightly from the composition of the sample
SrGa0.625Sc0.375O2.5. Therefore, the composition of the phase
will hereinafter be referred to as Sr10Ga6Sc4O25.
3.2.1. Refinement of the Crystal Structure of Sr10Ga6-
Sc4O25. The PXRD pattern of Sr10Ga6Sc4O25 was fully indexed
with the unit cell parameters and space group obtained from
the SAED study [a = 17.517(1) Å; c = 32.830(3) Å]. The struc-
ture was solved using the direct space Monte Carlo simulation
program FOX14 applied to PXRD data. The multiplicity for a
general site in the space group I41/a is 16, and it was assumed
that the unit cell contains 16 formula units of Sr10Ga6Sc4O25.
The solution of the structure was simplified by the fact that it
contains B cations with different crystal chemistry. Large Sc3+
cations are known to possess octahedral oxygen coordination.
A localization of all four ScO6 octahedra and all Sr atoms in the
structure, accounting for ca. 80% of the total scattering power,
would, in the case of an ordered structure, probably enable
the remaining atoms to be located subsequently. Structure
optimization was accordingly carried out using FOX, with 10 Sr
atoms at general 16f positions and four ScO6 semirigid
octahedra introduced randomly and using the dynamical
occupancy correction. Four Sr atoms were found to occupy
special 8e positions, and eight were located at 16f positions. After
the Sr atoms were located, six Ga atoms were randomly
introduced in 16f positions. Probable Sc/Ga atom positions
could at this stage also be inferred from the partial structure
model. The difficulty in assigning the Sc and Ga atoms to
different positions was overcome during a considerably long
optimization. Several O atoms of the ScO6 octahedra atoms
were located by first refining the structure model using the
full PXRD data range. The obtained refined atom positions
were then fixed, and FOX was used to find the positions of
the missing atoms.
Figure 3. Compositional dependence of the unit cell parameters for
SrGa0.5Sc0.5−xZrxO2.5+x/2
.
The minor increase of the unit cell parameter of the perovskite
phase with the zirconium content is most probably associated with
an increase of the oxygen content because the ionic radius of Zr4+
(r = 0.84 Å) is slightly smaller than that of Sc3+ (r = 0.87 Å 13).
3.2. Novel Oxygen-Deficient Perovskite Sr10Ga6-
Sc4O25. The SrGa0.625Sc0.375O2.5 sample was prepared by
annealing in air at 1473 K for 15 h. The PXRD pattern
suggests the formation of a perovskite-related phase. However,
in addition to the perovskite subcell reflections, there were
numerous weak superstructure reflections, which required
selected-area ED (SAED) to identify the supercell. The
SAED investigation revealed the material to be well crystallized
without indications of disorder. A number of tilt series were
recorded, and the relationship between the supercell and
perovskite subcell was found to be a* = 1/10(2a*p − b*p),
b* = 1/10(−a*p + 2b*p), and c* = 1/8c*p. The superstructure
reflections can thus be indexed with a tetragonal unit cell,
a = b ≈ 2√5ap ≈ 17.5 Å and c ≈ 8ap ≈ 33 Å, with ap ≈ 4 Å
(subcell parameter). A number of SAED patterns correspond-
ing to low-index zone axes in the perovskite subcell in addition
to low-index zone axes in the supercell are shown in Figure 4.
The final refinement was performed in the GSAS program
package using simultaneously both the PXRD data and time-of-
flight NPD data. In total, 310 variables were refined. Final
2
reliability factors were χ2 = 3.56 and RF = 0.023 for the
tot
2
PXRD data (4475 reflections) and RF = 0.0339 for the NPD
data (154.4° data bank, 7074 reflections at 1.77 <d < 0.75 Å;
see Table 3). Observed, calculated, and difference PXRD and
NPD profiles are given in Figure 5a,b. Final atomic coordinates
and displacement parameters are given in Table 1 in the
Supporting Information and selected bond distances in Table 4.
The variations in the bond lengths are 1.97−2.28 Å (average
distance 2.105 Å) for the ScO6 octahedra and 1.76−1.93 Å
(average distance 1.831 Å) for the GaO4 tetrahedra. These
values are within the ranges typical for the Sc−O and Ga−O
bond lengths observed in other oxides.
The crystal structure of Sr10Ga6Sc4O25 (Figure 6) represents
the novel type of ordering of the B cations and oxygen
vacancies in perovskites. It can be described as a sequence of
eight perovskite layers along to the c axis. There are only two
unique (Sc/Ga)Ox layers in the structure, named layers 1
(Figure 7a) and 2 (Figure 7b). Other layers are related to them
by the symmetry elements of the I41/a space group. As in
the structure of brownmillerite Sr2GaScO5, Ga and Sc cations
are fully ordered in the structure, with Ga3+ and Sc3+ located in
tetrahedra and octahedra, respectively. However, in comparison
Figure 4. SAED patterns of Sr10Ga6Sc4O25. Selected low-index zone
axes patterns in the perovskite subcell are shown as well in the
supercell. The relationship between the perovskite subcell and
supercell is a = b ≈ 2√5ap and c ≈ 8ap.
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connected to other Ga/Sc polyhedra. In layer 1, the remaining
O atoms of the Ga2O4 tetrahedra are connected with both
tetrahedral and octahedral units, while in layer 2, the Ga5O4
tetrahedra are connected with the octahedral units only. This
leads to different orientations of the Ga2O4 and Ga5O4
tetrahedra [O15 belongs to the (Sc/Ga)Ox layer, while O6
belongs to the SrOy layer]. As a result, the oxygen content of
layer 1 is (Sc/Ga)O1.6, while it is (Sc/Ga)O1.8 for layer 2.
Therefore, the sequence of layers along to the c axis can be
written as ...[−(Sc/Ga)O1.6−SrO0.8−(Sc/Ga)O1.8−SrO0.8−]2....
3.3. SrGa0.75Sc0.25O2.5 with a Cubic Perovskite Struc-
ture. The PXRD pattern of the sample with the nominal com-
position SrGa0.75Sc0.25O2.5 (Figure 8) was indexed with a cubic
perovskite cell with the unit cell parameter a = 3.9817(4) Å.
Table 3. Summary of the Results of the Least-Squares Fits to
the PXRD (NPD) Data Collected for Sr10Ga6Sc4O25
space group
a, Å
I41/c
17.52963(4)
32.85749(9)
16
c, Å
Z
cell volume, Å3
calcd density, g/cm3
PXRD data set:
radiation
10096.71(6)
4.932
Cu Kα1, λ = 1.54060 Å
5 ≤ 2θ ≤ 125, 0.0167
0.023, 0.0374, 0.0294
2θ range; step, deg
2
RF , Rwp, Rp
NPD data set:
d range, Å
0.75−14
0.0339, 0.0273, 0.0231
15
2
The presence of weak (<%1) reflections from Sr3Ga4O9 and
RF , Rwp, Rp
unidentified phases was observed in the PXRD pattern. All
attempts to prepare a single-phase sample by slightly varying
the cation content off the stoichiometric SrGa0.75Sc0.25O2.5
composition were unsuccessful. Thorough analysis of the
profiles of the reflections at the PXRD pattern did not indicate
deviation from the cubic symmetry. The primitive cubic unit
cell was also supported by the absence of superstructure reflec-
tions in the SAED patterns of SrGa0.75Sc0.25O2.5 (Figure 9a).
The B cations as well as oxygen vacancies thus do not exhibit
long-range order, although there is short-range order forming
domains as seen in diffuse scattering in the [001] and [1−30]
SAED patterns and in a corresponding a dark-field image from
diffuse scattering of the former (Figure 9b).
params refined
310
2
χtot
3.56
In order to determine the thermal expansion properties
of SrGa0.75Sc0.25O2.5, a high-temperature PXRD study was
performed. Temperature variation of the unit cell parameter is
shown in Figure 10. Two regions with different thermal expan-
sion coefficients (TECs) are observed. In the low-temperature
region (298−498 K), the TEC value is 16.1 ppm/K, while it
decreases sharply to 11.8 ppm/K in the high-temperature region
(698−1098 K). The latter value is close to 11.2 ppm/K obtained
from dilatometry data for the Sr2GaScO5 brownmillerite.
Combined thermal analysis and mass spectrometry studies
revealed the release of water from the SrGa0.75Sc0.25O2.5 sample
in the temperature range of 500−673 K. The measured weight
loss corresponds to the composition SrGa0.75Sc0.25O2.5·
0.13H2O. The unit cell parameter of SrGa0.75Sc0.25O2.5·xH2O
increases with increasing water content (x). For example, the
unit cell parameter of the SrGa0.75Sc0.25O2.5 sample treated in
moisturized N2 at 488 K for 3 days [a = 3.989(1) Å] is
substantially larger compared to that for the sample annealed in
dry argon at 1173 K for 1 h [a = 3.973(1) Å]. This explains the
existence of two regions with different TEC values shown in
Figure 10. The absorption of water by SrGa0.75Sc0.25O2.5 seems
to be the first step of the decomposition of the compound in
wet air. Holding SrGa0.75Sc0.25O2.5 at 343 K for 2 days in air
saturated with water vapor leads to the complete decom-
position of the compound. Only reflections from SrCO3 are
observed on the PXRD pattern, indicating the formation of
amorphous products containing scandium and gallium. The
decomposition seems to be accelerated by the formation of
the thermodynamically stable SrCO3 due to a reaction between
SrGa0.75Sc0.25O2.5·xH2O and CO2 present in air. However, the
decomposition process is rather slow at room temperature in
air with natural humidity, and SrGa0.75Sc0.25O2.5 samples can
thus be stored in these conditions for a prolonged period of
time.
Figure 5. Observed, calculated, and difference between them PXRD
(a) and NPD (b) profiles for Sr10Ga6Sc4O25.
with the brownmillerite, gallium tetrahedra and scandium
octahedra are present in all layers. Each layer consists of 12
GaO4 tetrahedra and 8 ScO6 octahedra. The arrangement of the
scandium octahedra in layers 1 and 2 is different. In layer 1,
there are units of four corner-sharing ScO6 octahedra, while in
layer 2, only pairs of corner-sharing ScO6 octahedra are present.
These units are connected through the GaO4 tetrahedra. There
are two tetrahedral groups marked in green and yellow in
Figure 7a,b. The first group (yellow in Figure 7) represents a
pair of tetrahedra with a common corner. They are connected
with a pair of ScO6 octahedra. Such an arrangement is also
observed between octahedra and tetrahedra along the direction
of tetrahedral chains in the neighboring layers of the brown-
millerite structure. In the second group (green in Figure 7),
namely, Ga2O4 in layer 1 and Ga5O4 in layer 2, one of the
tetrahedral vertexes (O6 and O15, respectively) is not
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Table 4. Selected Bond Lengths for the Crystal Structure of Sr10Ga6Sc4O25
Sc1
O8
2.19(1)
2.14(1)
2.02(1)
2.04(1)
2.04(1)
2.05(1)
1.81(1)
1.79(1)
1.86(1)
1.77(1)
1.79(1)
1.85(1)
1.83(1)
1.84(1)
2.95(1)
2.799(9)
3.122(7)
2.82(1)
2.64(1)
Sc2
O9
2.28(1)
2.07(1)
2.13(1)
2.17(1)
2.13(1)
2.10(1)
1.81(1)
1.80(1)
1.93(1)
1.81(1)
1.83(1)
1.76(1)
1.83(1)
1.82(1)
2.773(7)
2.88(1)
2.833(9)
2.923(8)
2.551(9)
3.10(1)
2.57(1)
2.507(8)
2.782(9)
Sc3
O1
2.22(1)
2.16(1)
2.10(1)
1.97(1)
2.07(1)
2.01(1)
1.88(1)
1.85(1)
1.81(1)
1.84(1)
Sc4
O4
2.17(1)
2.04(1)
2.13(1)
2.12(1)
2.11(1)
2.08(1)
1.87(1)
1.89(1)
1.81(1)
1.86(1)
O10
O16
O18
O21
O23
O1
O12
O13
O19
O22
O23
O5
O5
O17
O20
O21
O25
O25
O11
O12
O13
O14
O14
O22
O24
O24
O2
Ga1
Ga5
Sr1
Ga2
Ga6
Sr2
Ga3
Ga4
O2
O6
O9
O3
O7
O10
O11
O4
O8
O15
O16
O17
O18
O17
O18
O21
O25
O25
O3
O7
O19
O20
O1
(×2)
(×2)
(×2)
(×2)
(×2)
Sr3
O4
2.49(1)
2.62(1)
2.66(1)
3.07(1)
2.53(1)
2.60(1)
2.65(1)
Sr4
O2
2.767(9)
2.86(1)
2.81(1)
3.02(1)
3.15(1)
2.837(9)
2.51(1)
2.872(9)
2.81(1)
2.60(1)
2.883(8)
2.70(1)
2.64(1)
2.62(1)
2.952(9)
2.428(9)
2.50(1)
2.79(1)
2.79(1)
2.90(1)
2.412(8)
3.04(1)
2.803(9)
2.95(1)
2.60(1)
2.83(1)
2.554(8)
2.87(1)
O2
O8
O3
O5
O10
O15
O16
O17
O20
O5
O6
O7
O11
O13
O14
O22
O24
O8
O9
O15
O18
O22
O23
O1
Sr5
O7
2.51(1)
2.74(1)
2.73(1)
2.554(9)
(×2)
(×2)
(×2)
(×2)
Sr6
O1
2.479(7)
2.84(1)
2.76(1)
2.63(1)
2.80(1)
3.07(1)
2.937(8)
2.52(1)
2.51(1)
3.05(1)
2.85(1)
2.476(9)
2.84(1)
2.736(9)
2.91(1)
2.812(8)
2.61(1)
Sr7
O6
2.640(9)
3.13(1)
2.73(1)
2.995(9)
2.65(1)
Sr8
O8
O5
O7
(×2)
(×2)
(×2)
(×2)
O3
O16
O18
O8
O12
O13
O19
O5
O10
O12
O14
O22
O23
O24
O3
O9
O11
O13
O14
O19
O19
O2
Sr9
O2
2.88(1)
2.87(1)
3.15(1)
3.36(1)
2.35(1)
2.78(1)
2.70(1)
2.88(1)
2.527(8)
2.70(1)
Sr10
Sr11
O12
O14
O22
O24
O24
2.71(1)
(×2)
Sr12
O3
O4
3.004(9) (x2)
2.98(1)
O6
O4
O9
(×2)
(×2)
(×2)
O10
O11
O12
O19
O21
O21
O23
O10
O15
O16
O17
O20
O21
O25
O17
O18
O19
O21
O25
2.604(9)
2.89(1)
3.3.1. Study of the Proton Conductivity in SrGa0.75
-
samples were heated to 1273 K in dry argon; (2) thereupon,
the samples were cooled to 423 K in dry argon; (3) finally, the
samples were heated to 1273 K in wet argon and the
conductivity was measured in the cooling cycle.
Sc0.25O2.5. Because scandium-containing perovskites, like doped
LaScO3, are known to exhibit high-temperature proton
conductivity,16−18 we have studied the high-temperature con-
ductivity of SrGa0.75Sc0.25O2.5. A water-saturated sample of
SrGa0.75Sc0.25O2.5 was prepared by heating the as-prepared samples
at 488 K for 3 days in a flow of N2 saturated with water vapor at
Complex impedance plots for prehydrated SrGa0.75Sc0.25O2.5
samples in heating and cooling cycles in dry argon are
presented in Figure 2 in the Supporting Information with the
equivalent circuit model used for analysis of the impedance data
shown on the inset. At temperatures higher than 773 K, it was
difficult to separate bulk and grain boundary conductivities;
therefore, only the values of the total conductivities are presented.
The corresponding Arrhenius plots are shown in Figure 11,
from which it is clear that the total conductivity of the
prehydrated samples is higher in the heating cycle than in the
cooling cycle in dry argon, especially at T < 773 K. Therefore,
one can conclude that within this temperature range the proton
conductivity dominates because of the presence of water
363 K, pH O = 0.69 atm (so-called prehydrated samples). The
2
study of the sample by TGA in dry N2 at 298−1273 K (Figure 1
in the Supporting Information) shows that starting from ∼500 K
it exhibits significant mass loss due to the release of water
molecules from the crystal structure. The weight of the sample
stabilizes at ∼1150 K. The composition of the prehydrated sample
was calculated from TGA data as SrGa0,75Sc0,25O2,5·0.043H2O. No
mass gain was observed in the cooling cycle.
Conductivity measurements were performed on the prehy-
drated samples in the following cycle: (1) the prehydrated
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Figure 6. Crystal structure of Sr10Ga6Sc4O25. Red spheres are Sr2+
cations; Sc3+ cations are located in octahedra (blue) and Ga3+ cations
in tetrahedra (yellow and green).
Figure 9. (a) SAED patterns of the cubic perovskite SrGa0.75Sc0.25O2.5
viewed along [001], [11−1], [110], and [1−30]. (b) Dark-field image
from the diffuse intensities in the [001] SAED pattern.
Figure 7. Two unique (Sr/Ga)Ox layers in the crystal structure of
Sr10Ga6Sc4O25 with different oxygen contents: (Sc/Ga)O1.6, layer 1
(a); (Sc/Ga)O1.8, layer 2 (b). GaO4 tetrahedra belong to two groups
with all vertexes shared with other Sc/Ga polyhedra (marked green) or
with some O atoms not connected to other Sc/Ga polyhedra (marked
yellow).
Figure 10. Temperature variation of the unit cell parameter of
SrGa0.75Sc0.25O2.5 determined from high-temperature PXRD data.
TECs for different temperature ranges are given.
Assuming that proton conduction has no effect on other
charge carrier contributions, the pure protonic conductivity σH
can be calculated as the difference between the conductivity in
H2O-saturated argon, σwet, and the conductivity of the water-
free sample obtained during the cooling cycle in dry argon,
σdry:19
Figure 8. PXRD pattern of the SrGa0.75Sc0.25O2.5 sample.
molecules in the crystal structure. This idea is supported by the
fact that the water-free samples obtained after the heating cycle
in dry argon show much higher conductivity at T < 773 K in
wet argon in comparison with dry argon (compare curves 2 and
3 in Figure 11).
σH = σwet − σdry
The corresponding Arrhenius plot for pure protonic
conductivity is shown in Figure 12.
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Figure 13. Proton-transfer number (tH) in wet argon for
SrGa0.75Sc0.25O2.5.
measurements were performed for SrGa0.75Sc0.25O2.5 in dry
oxygen and dry argon. The corresponding Arrhenius plots
are given in Figure 3 in the Supporting Information. The
conductivity of SrGa0.75Sc0.25O2.5 in dry oxygen is similar to
that in dry argon, indicating a predominance of the oxide-ion
conductivity in this compound under these conditions.
Moreover, taking into account the total conductivity values at
high temperature (8.5 × 10−3 S/cm at 1273 K) and expectedly
low electronic conductivity for such types of oxides, one can
propose moderate oxide-ion conductivity for SrGa0.75Sc0.25O2.5.
Figure 11. Temperature dependences (Arrhenius plots) of the total
conductivities of the prehydrated SrGa0.75Sc0.25O2.5 samples in heating
and cooling cycles in dry argon. Curve 1 corresponds to the heating of
the prehydrated sample in dry argon; curve 2 corresponds to the
cooling of the water-free sample in dry argon; curve 3 corresponds to
the heating of the water-free sample in wet argon.
4. DISCUSSION
The A2(B,B′)2O5 oxygen-deficient perovskites with only one
type of A cation crystallize in different structure types,
depending on the stable coordination number of the B cation.
One type comprises oxides with Jahn−Teller B cations such as
Ca2Mn2O5,21 Sr2Mn2O5,22−25 and La2Cu2O5.26 In their crystal
structures, BO5 square pyramids are connected by vertexes,
forming a 3D framework (Figure 14a). In the crystal structure
Figure 12. Temperature dependence (Arrhenius plot) of the proton
conductivity (σH) for SrGa0.75Sc0.25O2.5 calculated as the difference
between the conductivity in H2O-saturated argon (σwet) and the
conductivity of the water-free sample (σdry).
The transference number for the proton conductivity (tH) in
wet argon can be calculated from the equation17,20
σ
H
t =
H
Figure 14. Crystal structures of different oxygen-deficient perovskites
A2B2O5: (a) Sr2Mn2O5; (b) La2Ni2O5; (c) brownmillerite
Ca2(Fe,Al)2O5.
σ
wet
The temperature dependence of tH is given in Figure 13. The
proton conductivity dominates in the low-temperature region,
and a substantial decrease is observed only at T > 623 K.
Higher activation energies (EA) are observed for the high-
temperature part of the total conductivity in the heating cycle
in dry argon and the cooling cycle in wet argon and in the
whole temperature range for the cooling cycle in dry argon
(Figure 11) in comparison with the low-temperature part
(e.g., for the SrGa0.75Sc0.25O2.5 sample cooled in wet argon, EA =
1.21 eV at T = 873−1273 K, while at T = 573−773 K, it is only
0.71 eV). Such a high value of EA at high temperature indicates
predominant hole or oxide-ion conductivity. To reveal the
nature of the high-temperature conductivity, conductivity
of La2Ni2O5, Ni2+ (3d8) cations are distributed over an equal
amount of corner-shared squares and octahedra (Figure 14b).27,28
The brownmillerite structure type (Ca2(Fe,Al)2O5) is the most
widely spread among the A2(B,B′)2O5 compounds. It contains
layers of corner-shared octahedra alternating with tetrahedral
layers (Figure 14c). Brownmillerites with a single type of B cation
or with several B cations are known. In the former case, the B
cation should be stable in both tetrahedral and octahedral
environments like in the case of Sr2Fe2O5,29 Ca2Fe2O5,30
Ba2In2O5,31 or Sr2In2O5.32 When different B cations are present,
they can be fully or partially ordered over the octahedral and
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tetrahedral positions in the brownmillerite structure for geo-
metrical and/or electronic reasons. Full ordering is usually
observed in compounds with Jahn−Teller cations, for example, in
LaSrCuGaO533 and M2GaMnO5, where M = Ca, Sr.34,35 Partial
ordering of B cations can be found in Ca2Ga(Al)FeO536,37 or
Sr(Ca)2(Co,Ga)2O5,38,39 where the 3d metal cations preferen-
tially occupy octahedral positions. In the above-listed structures,
all corners of the BOx polyhedra are connected with each other.
To our knowledge, however, there is at least one example of the
anion-deficient perovskite Ba3RM2O7.5 (or Ba2R2/3M4/3O5, where
R = rare-earth elements and M = Al, Ga)40 in which structures
there are tetrahedral units with some vertexes not connected to
other B cations.
The studied SrGa1−xScxO2.5 system provides a rare example
of a variation of the B/B′ cation ratio with fixed oxygen content,
leading to three different perovskite-related phases. The basis
for such a behavior is a substantial difference in the size of Sc3+
and Ga3+ [r(Sc3+) = 0.73 Å; r(Ga3+) = 0.62 Å for CN = 6, and
r(Ga3+) = 0.47 Å for CN = 413] and a strong preference of Sc3+
for an octahedral environment. As a consequence, variation of
the Sc/Ga ratio in SrGa1−xScxO2.5 is equivalent to variation of
the octahedra/tetrahedra ratio. In Sr2GaScO5 (x = 0.5) with the
brownmillerite structure, the octahedra and tetrahedra are
present in equal amounts; thus, oxide represents an example
of the structure in which a complete ordering of the B cations
occurs mainly because of coupling with the ordering of the
oxygen vacancies.
One can imagine several ways of transforming an ordered
brownmillerite phase into a disordered cubic perovskite phase.
The first one includes a temperature-induced phase trans-
formation. An example of such a transformation is Ba2In2O5
with the brownmillerite structure, which reversibly transforms
to a disordered perovskite phase at T > 930 °C due to a disorder
in the oxygen sublattice. The ordering of Ga3+ and Sc3+ in
Sr2GaScO5 brownmillerite prevents a disorder of the oxygen
vacancies without a disorder in the B cation sublattice. A com-
plete disorder at both sublattices was achieved only by quenching
the melt of this compound. Another common way to introduce
disorder in the oxygen sublattice is an increase of the oxygen
content in tetrahedral layers by the heterovalent substitution of
A and/or B cations. This was done for Sr2GaScO5 by par-
tial replacement of Sc3+ by Zr4+ according to the formula
SrGa0.5Sc0.5−xZrxO2.5+x/2. For Sr2GaScO5, a third way to transform
to the cubic phase is the partial replacement of octahedral Sc3+ by
Ga3+, as achieved in the cubic perovskite SrGa0.75Sc0.25O2.5.
The substitution introduces disorder in the octahedral layers of
the brownmillerite structure because of the substantial size difference
between Ga3+ and Sc3+ cations and also allows disorder in the
oxygen sublattice because Ga3+, in comparison with Sc3+, has a
rather high flexibility of its coordination environment.
The transformation of the Sr2GaScO5 brownmillerite to a
cubic perovskite by substitution of Sc3+ by Ga3+ does not occur
continuously, and at intermediate composition, the novel
Sr10Ga6Sc4O25 compound with a previously unknown type of
ordering of B cations and O anions was isolated. Despite the
resemblance of the lattice metrics and space group (a = b ≈
2√5ap and c ≈ 8ap; S.G. I41/a) to some elpasolite-type
compounds like α-K3AlF6,41 the crystal structure is different. In
elpasolites, A2BB′F6, the B and B′ cations are ordered in a
chessboard manner because of the substantial size and charge
difference. Moreover, because one of the B cations represents a
large alkaline cation, which prefers coordination numbers >6,
sometimes large displacements of the anions take place. This
leads to an increase of the coordination numbers of alkaline
cations to CN = 7 and 8, like, e.g., in α-K3AlF6. In the crystal
structure of Sr10Ga6Sc4O25, there is a complete ordering of B
cations over the octahedral (Sc3+) and tetrahedral (Ga3+) sites.
In comparison with brownmillerite Sr2GaScO5, the number of
tetrahedra in Sr10Ga6Sc4O25 exceeds the number of octahedra
by 50%. The oxygen stoichiometry O2.5 is maintained by the
fact that one-third of the tetrahedra in the Sr10Ga6Sc4O25 crystal
structure have one vertex not connected with other Sc/Ga
cations. A similar effect is also observed in the already mentioned
Ba3RM2O7.5 (or Ba2R2/3M4/3O5, where R = rare-earth elements
and M = Al, Ga) anion-deficient perovskites.40 In Ba3RM2O7,
the R3+ and M3+ cations, substantially different in size, are
fully ordered over octahedral and tetrahedral sites. However, the
arrangement of these polyhedra in Ba3RM2O7.5 is completely
different in comparison with that of Sr10Ga6Sc4O25.
Scandium-containing oxygen-deficient perovskites are known
to exhibit proton conductivity because of the partial filling of
the oxygen vacancies by water molecules and the formation of
the hydroxyl groups according to the equation
••
×
•
H O + V + O → 2OH
2
O
O
O
Thus, La0.6Ba0.4ScO2.8 exhibits high proton conductivity
∼1.5 × 10−4 S/cm at 673 K.42 We have shown that SrGa0.75
-
Sc0.25O2.5 is capable of conducting protons. However, its proton
conductivity is much lower (∼2.0 × 10−6 S/cm at 673 K) in
comparison with that of the doped LaScO3. One can explain it
by a high concentration of the remaining oxygen vacancies in
the studied SrGa0.75Sc0.25O2.5 samples, which are not filled by
OH groups.
5. CONCLUSIONS
The SrGa1−xScxO2.5 system provides a unique example of the
substantial difference in the crystal chemistry of Sc3+ and Ga3+
cations, for which variation of the Sc/Ga ratio leads to three
different perovskite-related phases with fixed oxygen content.
In brownmillerite-like Sr2GaScO5, Sc3+ and Ga3+ cations are
fully ordered over an equal amount of the octahedral and
tetrahedral positions of the crystal structure. Partial substitution
of scandium by gallium leads to the formation of Sr10Ga6Sc4O25
having a perovskite-like structure with a large unit cell (a = b ≈
2√5ap and c ≈ 8ap, where ap = perovskite subcell parameter).
Like in the case of brownmillerite, in its structure, a full
ordering of Ga3+ and Sc3+ over tetrahedral and octahedral
positions takes place. However, the number of tetrahedral
positions in Sr10Ga6Sc4O25 is by 50% higher in comparison with
the number of octahedral ones. This leads to the fact that one-
third of the tetrahedra have one vertex not connected with
other Sc/Ga cations. Further substitution of Sc3+ by Ga3+ leads
to the complete disorder of the B cations and oxygen vacancies
and to the formation of SrGa0.75Sc0.25O2.5 with the cubic
perovskite structure. This compound is capable, as are other
scandium-containing perovskites, of being a proton conductor
with σH = 2.0 × 10−6 S/cm at 673 K.
ASSOCIATED CONTENT
■
S
* Supporting Information
Table of final atomic coordinates and displacement parameters
for Sr10Ga6Sc4O25, X-ray crystallographic files of the refined
structures in CIF format, and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Fernandez, M. T. Physica B 1997, 234−236, 18−19.
(29) Greaves, C.; Jacobson, A. J.; Tofield, B. C.; Fender, B. E. F. Acta
Crystallogr. 1975, B31, 641−646.
AUTHOR INFORMATION
Corresponding Author
*E-mail: istomin@icr.chem.msu.ru. Tel.: +7 495 9393490. Fax:
+7 495 9394788.
■
(30) Bertaut, F.; Blum, P.; Sagnieres, A. C. R. Acad. Sci. 1957, 244,
2944−2946.
Author Contributions
All authors have given approval to the final version of the
manuscript.
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134−148.
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ACKNOWLEDGMENTS
■
This work was partially supported by RFBR (Grant 11-03-
01225) and by the Swedish Research Council and Swedish
Institute (Visbyprogramme). The authors gratefully acknowl-
edge Professor A. V. Shevelkov (Moscow State University) for
valuable discussions.
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