Chemical Science

Research Frontiers 2015

Oxygen storage capability of BaYMn2O5+δ studied
by high-temperature X-ray diffraction
under precisely controlled oxygen pressures
The so-called “oxygen storage materials (OSMs)”
have attracted increased attention in the materials
research community. OSMs are categorized asoxides with fast and reversible oxygen intake/releasecapability. Such materials may be applicable tovarious oxygen-related technologies owing to theirpotential ability for a precise control of redox reaction,and hence contribute to solutions of energy andenvironmental issues. Developments of novel OSMs
are highly desirable to open up the possibility ofvarious future applications.
We recently reported the remarkable oxygenstorage capability of BaYMn2O5+δ [1]. This oxide
crystallizes in a double-perovskite type structure,
which contains a layered arrangement of smaller
yttrium and larger barium ions at the perovskite A-site.
The oxygen site within the yttrium plane is readily
filled/unfilled in response to variations in temperatureand the surrounding atmosphere, resulting in large
oxygen nonstoichiometry ranging 0 ≤ δ ≤ 1, see aschematic illustration in Fig. 1. The oxygen intake/release behaviors of BaYMn2O5+δ are clearly beyond
those of conventional perovskite oxides in termsof the magnitude and sharpness of the processes.
This emphasizes the importance of the characteristic
crystal structure involving the layered cationic order of
Ba2+ and Y3+. Understanding of key structural factorsof the remarkable oxygen intake/release is particularly
important to develop novel/better OSMs. Motivatedby such a research background, we examined in situ
high-temperature X-ray diffraction (XRD) experiments
on BaYMn2O5+δ [2].

BaYMnO

δ=1“O6” phaseδ = 0.5“O5.5” phase

δ= 0“O5” phaseFig. 1. Schematic illustration of the crystal structures ofBaYMn2O5+δ with oxygen contents δ = 1, 0.5, and 0 (“O6”,“O5.5”, and “O5” phases, respectively). The illustration was
drawn with VESTA software [5] on the basis of the roomtemperature structural models reported in the literature.

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Our preliminary investigations revealed that
BaYMn2O5+δ crystallizes in either the oxygenated “O6”
(Fig. 1(a)) or the intermediate “O5.5” form (Fig. 1(b))
at 750°C depending on oxygen pressures P (O2),enabling us to study these distinct forms selectivelyupon varying P (O2) at a constant temperature. Insitu powder XRD measurements were performed at
BL02B2 beamline. A large Debye-Scherrer camera
equipped with a gas/vapor pressure control system[3] was used for the data acquisition at 750°C underprecisely controlled oxygen pressures of P (O2) = 103
and 10 Pa.
Figures 2(a) and 2(b) present XRD patterns for
BaYMn2O5+δ taken at 750°C under P (O2) = 103 and10 Pa, respectively. The data clearly indicate that thisoxide indeed undergoes a distinct structural change
upon lowering oxygen pressure, from a slightly
oxygen-deficient “O6” phase (BaYMn2O5.89; P (O2) =103 Pa) to an oxygen-vacancy ordered “O5.5” phase(BaYMn2O5.51; P (O2) = 10 Pa). The BaYMn2O5.89
structure was reasonably refined assuming anorthorhombic Cmmm unit cell with lattice constants a,b, c ≈ 2a p, where a p denotes the lattice constant of thecubic perovskite structure. The refined BaYMn2O5.89structure is depicted in the inset of Fig. 2(a). Ourcareful refinement suggests that yttrium resides ina split site at a less symmetrical position with 50%
occupancy, see the [001] projection shown in Fig.
2(c). The site splitting for yttrium leads to its random
displacements within the yttrium plane. Since theoxygen sites within the yttrium plane are slightly
unfilled, nonequivalent distances of shorter Y-O andlonger Y-VO (VO denotes an oxygen vacancy) may be
more favorable. We thus interpret the large yttrium
displacement as a consequence of oxygen vacancy
formation at the surrounding oxygen sites.
Meanwhile, the pattern for BaYMn2O5.51 wasfitted well with the orthorhombic Icma model (Fig.
2(b); a, b ≈ 2a p, c ≈ 4a p ) that is proposed for a roomtemperature “O5.5” structure [4]. The BaYMn2O5.51
structure contains alternate ordering of oxygen andvacancy arrays within the yttrium plane (Fig. 2(d)),
giving rise to two distinct manganese sites: one withsquare pyramidal coordination and the other withoctahedral coordination. It is thus evident that the “O6”to-“O5.5” transformation involves drastic modificationsin the Mn polyhedra. Moreover, a closer look at thecoordination geometry around manganese hasrevealed important structural features underlying theResearch Frontiers 2015

Intensity (counts)

350000(a)a300000400000

cBa
B
aY
Mn
O

250000200000150000350000b

100000300000Intensity (counts)

400000250000200000150000(b)

being triggered by the relative stabilization of thetwo phases. That is to say, the abrupt jump in theoxygen content and thereby the remarkable oxygen
intake/release behaviors are attributed to theexceptional stability of the “O5.5” structure consistingof particular Mn3+ orbital ordering, and the instabilityof the “O6” structure disrupted by the increased
number of oxygen vacancies and associated random
displacements of yttrium.

0.20020.
1954

0.2258aY

Mnc

0.19720.
1916

dMn-O / nm

Ba0.19110.19040.
19610.1982

0.2123

BaYMn2O5.51
0.1890BaYMn2O5.89

0.1904

remarkable oxygen intake/release. As demonstratedin Fig. 3, both the MnO5 /MnO6 polyhedra in theBaYMn2O5.51 structure are significantly distorted: theMn-O bond parallel to the c -axis is much longer thanthose within the ab plane, while octahedral MnO6 islargely elongated along the a -axis. This feature is incontrast to the BaYMn2O5.89 structure, in which theMn octahedra are much more isotropic having similar
Mn-O bond lengths for all directions.
The peculiar atomic arrangement in BaYMn2O5.51
can be understood as a consequence of alternate
orbital ordering for the Jahn-Teller active Mn3+
ions consisting of dz2-type orbital along the c -axis
at the pyramidal site and that along the a -axis
at the octahedral site. It should be noted that thishigh-temperature structure is essentially identicalto the room-temperature structure, indicating therobustness of the orbital-ordered “O5.5” structure
in a wide temperature range. As demonstrated
by the comparison of Figs. 2(c) and 2(d), theabrupt “O6”- to -“O5.5” transformation can also beviewed as discontinuous structural reconstruction
within the yttrium plane, involving oxygen/vacancy
redistribution and yttrium displacement organization,bO

Mn3+ orbital orderingFig. 3. Atomic arrangements of BaYMn2O5.89(upper left) and BaYMn2O5.51 (upper right) at
750°C. The schematic illustration of the Mn3+
orbital ordering is also given (bottom right).

cabBaYMn
O

10000050000
0500008 12 16 20 24 28 32 36 40

8 12 16 20 24 28 32 36 40

2θ (λ = 0.0354 nm) (deg)(c)

Teruki Motohashi a,*, Yoshiki Kubota band Hiroshi Kageyama c02θ (λ = 0.0354 nm) (deg)(d)

Department of Materials and Life Chemistry,
Kanagawa Universityb
Department of Physical Science,
Osaka Prefecture UniversitycDepartment of Hydrocarbon Chemistry,
Kyoto Universitya*E-mail: t-mot@kanagawa-u.ac.jp
abbaFig. 2. High-temperature X-ray diffraction patternsfor (a) BaYMn2O5.89 and (b) BaYMn2O5.51 measured
at 750°C under P(O2) = 103 and 10 Pa, respectively.
The crystal structures were refined by the Rietveld
method using the RIETAN-FP program [6]. The
resultant crystal structure is visualized in the insetof each figure. The [001] projections of the twostructures, showing local atomic arrangements within
the yttrium plane are presented in (c) and (d).

References[1] T. Motohashi et al.: Chem. Mater. 22 (2010) 3192.
[2] T. Motohashi, T. Takahashi, M. Kimura, Y.
Masubuchi, S. Kikkawa, Y. Kubota, Y. Kobayashi, H.
Kageyama, M. Takata, S. Kitagawa, and R. Matsuda:
J. Phys. Chem. C 119 (2015) 2356.
[3] K. Kato et al.: AIP Conf. Proc. 1234 (2010) 875.
[4] C. Perca et al.: Chem. Mater. 17 (2005) 1835.
[5] K. Momma and F. Izumi: J. Appl. Crystallogr. 44(2011) 1272.
[6] F. Izumi and K. Momma: Solid State Phenom.
130 (2007) 15.

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