Magnetic ground state of nanosized b-Fe
2
O
3
and
its remarkable electronic features†
Ond
ˇ
rej Malina,
a
Ji
ˇ
r
´
ı Tu
ˇ
cek,
*
a
Petr Jakubec,
a
Josef Ka
ˇ
sl
´
ık,
a
Ivo Med
ˇ
r
´
ık,
a
Hiroko Tokoro,
b
Marie Yoshikiyo,
b
Asuka Namai,
b
Shin-ichi Ohkoshi
b
and Radek Zbo
ˇ
ril
*
a
To date, iron oxides have been extensively investigated for promising high applicability in various scientific
and industrial fields. In general, several forms can be distinguished with respect to their crystal structure,
which drives their specific physical (in particular, magnetic) properties. In this study, the pure b-Fe
2
O
3
phase, prepared in a nanoparticle form by a solid-state synthetic strategy, was investigated by employing
57
Fe M
¨
ossbauer spectroscopy, magnetization measurements, transmission electron microscopy, X-ray
powder diffraction, heat capacity measurements, and cyclic voltammetry. It is revealed that below the
N
´
eel transition temperature, b-Fe
2
O
3
behaves as a canted antiferromagnet with a small net magnetic
moment. For further possible utilization in photoelectrochemical applications, an estimation of the b-
Fe
2
O
3
band gap by cyclic voltammetry was performed, which was measured to be 2.2 eV.
Introduction
Iron oxides belong to a family of prominent materials that have
been studied for decades. The interest in exploring them stems
from the appealing physicochemical properties they exhibit.
1
Among them, their electronic and magnetic features have
roused eminent attention not only in the scientic world but
have also inspired the development of novel applications and
the introduction of new technologies in which these features
play an irreplaceable role.
1
Moreover, they show interesting
biochemical properties (e.g., biodegradability, biocompatibility,
and non-toxic characteristics) favourable for their exploitation
in various biomedical elds where other materials can hardly
compete with them.
2,3
Moreover, when synthesized as nano-
objects (nanoparticles, thin lms, nanowires, and nanorods),
they become equipped with new material characteristics (e.g.,
superparamagnetism) driven by nite-size and surface effects,
4
remarkably extending their application potential.
Iron(III) oxide shows a polymorphism, a feature connected
with the existence of two or more phases that are isochemical in
nature but possess various physical behaviors, which originate
from different crystal structures they possess.
1,5–9
Apart from the
amorphous Fe
2
O
3
form, four crystalline iron(III) oxide phases
are recognized so far:
1,5–9
(i) a-Fe
2
O
3
(hematite), (ii) b-Fe
2
O
3
, (iii)
g-Fe
2
O
3
(maghemite), and (iv) 3-Fe
2
O
3
. The a-Fe
2
O
3
and g-Fe
2
O
3
phases are the most frequently occurring polymorphs of iron(III)
oxide; they can be readily found in nature and exist in both bulk
and nanosized forms. Conversely, b-Fe
2
O
3
and 3-Fe
2
O
3
phases
are rare forms with scarce abundance, which are stable only in
nanosized dimensions. Upon heating, irreversible poly-
morphous transformations are commonly observed when
b-Fe
2
O
3
, g-Fe
2
O
3
, and 3-Fe
2
O
3
transform directly or via inter-
mediates (i.e., other iron(III) oxide polymorphs, existing in a
certain temperature range) to a-Fe
2
O
3
as the most thermody-
namically stable iron(III) oxide form.
5,10,11
b-Fe
2
O
3
was rst reported in the pioneering work of
Bonnevie-Svensen in 1956.
12
b-Fe
2
O
3
shows a body-centered,
cubic crystal structure of a bixbyite type with a lattice constant
of a ¼ 9.393
˚
A, which falls into the Ia
3 space group.
5,7
The two
crystallographically nonequivalent cation sites are recognized
in the b-Fe
2
O
3
crystal structure, i.e., the b-sites and d-sites, both
occupied with Fe
3+
ions in a high-spin state (S ¼ 5/2). They differ
in the degree of local symmetry;
5,7
whereas cation b-sites show
C
3i
symmetry, cation d-sites are describable in terms of C
2
symmetry. In the b-Fe
2
O
3
unit cell, there are three times the
number of d-sites than b-sites (24 Fe
3+
d-site cations and 8 Fe
3+
b-site cations), with all sites lled and no vacant positions
remaining.
5,7,13,14
The difference in site symmetry implies the
different distortion of cation polyhedron related to the b- and d-
sites; that is, b-sites are more distorted than d-sites.
5
The b-
Fe
2
O
3
phase is thermally unstable and in most cases, trans-
forms directly into a-Fe
2
O
3
when the temperature exceeds 500
C.
5,10
However, for b-Fe
2
O
3
nanoparticles with hollow
a
Regional Centre of Advanced Technologies and Materials, Departments of
Experimental Physics and Physical Chemistry, Faculty of Science, Palacky University,
17. listopadu 1192/12, 771 46 Olomouc, Czech Republic. E-mail: jiri.tucek@upol.cz;
radek.zboril@upol.cz
b
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
† Electronic supplementary information (ESI) available: Temperature evolution of
the hyperne magnetic eld and quadrupole-splitting parameter for both
crystallographically non-equivalent sites, values of the M¨ ossbauer hyperne
parameters derived from spectral tting, and values of the effective mass of the
M¨ ossbauer probed atom and Debye temperature. See DOI: 10.1039/c5ra07484c
Cite this: RSC Adv. , 2015, 5, 49719
Received 24th April 2015
Accepted 19th May 2015
DOI: 10.1039/c5ra07484c
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2015 RSC Adv. , 2015, 5, 49719–49727 | 49719
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