IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014

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Substitution Mechanism of Mn and Fe Ions in Bi4Ti3O12
Kazuma Nishimura1 , Tsuyoshi Yoshioka1, and Tomoyuki Yamamoto1,2
1 Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan
2 Institute of Condensed-Matter Science, Waseda University, Tokyo 169-8555, Japan

Single-phased polycrystalline Mn- and Fe-doped Bi4 Ti3 O12 were fabricated using a solid-state reaction technique, doping withvarious concentrations of Mn and Fe ions. Substitution mechanism of Mn and Fe ions in Bi4 Ti3 O12 were investigated with X-ray
absorption near-edge structure (XANES) measurements and first-principles calculations. The valence states of the Mn and Fe ionsare 4+ and 3+, respectively, inferred from the L2,3 -edge XANES profiles. From the K-edge XANES analysis, it is determined that Mn
and Fe ions are substituted at one of the Ti sites, i.e., Ti(2a) or Ti(4e) sites. Our first-principles total electronic energy calculations
suggest that Mn ions are likely to substitute at Ti(2a) sites rather than at Ti(4e) sites, whereas the opposite is true for Fe substitution.
Taken together, these results give a clear description of the locations and charge states of the Mn and Fe dopants in Bi4 Ti3 O12 .
Index Terms— Bismuth titanate, first-principles calculation, substitution mechanism, X-ray absorption near-edge structure(XANES).

I. I NTRODUCTION

II. E XPERIMENTAL P ROCEDURESISMUTH titanate (Bi4 Ti3 O12 ) is an attractive lead-freesubstitute for the materials commonly used in ferroelectric random access memory (FeRAM), and its crystal
structure and ferroelectric properties have been extensively
studied [1], [2]. Doping techniques can enhance the ferroelectric properties or alter the Curie temperature of Bi4 Ti3 O12
FeRAM materials, and results indicate that rare-earth-doped
Bi4 Ti3 O12 shows significant improvement in ferroelectric
properties [3]–[5]. In addition to rare-earth doping, simultaneous doping with 3d transition elements, such as Mn, hasalso proven effective [6].
Recently, it was reported that Fe-doped Bi4 Ti3 O12 exhibits
both ferroelectric and ferromagnetic properties, i.e., themultiferroic property, and this property of Fe-doped Bi4 Ti3 O12
has been extensively studied [7]. It is essential to know
the valence states and the local environment of Mn and Fe
ions in Bi4 Ti3 O12 in order to understand their influence on
the ferroelectric and ferromagnetic properties. However, thelocal environment of the substitutional Mn and Fe ions has
not yet been clearly determined. Techniques, such as theL2,3 -edge XANES analysis [8]–[11] and the K-edge XANES
analysis, along with first-principles calculations, are powerfultools for determining the local environment of 3d transition
elements [12], [13]. This combined strategy has been applied
to analyze the dopant structure in various types of functional
materials, and has successfully determined the local environment for some kinds of dilute dopants [14], [15].
In this paper, we investigate the substitution mechanismof Mn and Fe ions using X-ray absorption near-edge
structure (XANES) measurements alongside first-principles
calculations. We determined the valence states using theL2,3 -edge XANES spectra, and the local structure aroundthe Mn and Fe ions by comparing the K-edge XANES
spectra with the calculated XANES spectra. Additionally,
we computed the total electronic energies of the Mn- andFe-doped systems to estimate the most energetically favorablesites for substitution.

Mn and Fe doped Bi4 Ti3 O12 were prepared by thesolid state reaction method changing the concentration of
Mn and Fe ions, i.e., x = 0.00, 0.01, 0.03, and 0.05 in
Bi4 (Ti1−x TMx )3 O12 where TM = Mn or Fe. Our raw materials were reagent-grade Bi2 O3 , TiO2 , α-Fe2 O3 , and Mn2 O3 . We
mixed these powders in an agate mortar, then calcined themat 1023 K for 5 h in air. These calcined powders were groundand mixed again, and pressed into a cylindrical pellet form
(10 mmφ). We sintered these powder aggregates at 1223 K for
5 h in air. The crystal structures of all samples were verified
using the powder X-ray diffraction (XRD) technique with
Cu-Kα X-rays.
Mn- and Fe-L2,3 XANES spectra were measured at
BL-4B in Ultraviolet Synchrotron Orbital Radiation Facility(UVSOR), Okazaki, Japan, using the total electron yield
method. This method is effective to measure low-energy
XANES spectra, because such low-energy X-rays are absorbed
much when the transmission method is used. The X-rays
from the storage ring were monochromatized by a varied-linespacing plane grating (800 lines/mm), and we set the energy
resolution of the incident beams (E/E) to 3000 by tuning
the slit height at the upper and lower reaches of the grating.
Mn- and Fe-K XANES spectra were collected at BL01B1 in
SPring-8, Harima, Japan, using a transmission method. The
incident beam from the storage ring was monochromatizedby a Si(111) double-crystal monochromater. All the XANES
measurements were carried out at room temperature.

B

Manuscript received November 9, 2013; accepted January 6, 2014.
Date of current version June 6, 2014. Corresponding author: T. Yamamoto(e-mail: tymmt@waseda.jp).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2014.2301447

III. R ESULTS AND D ISCUSSION
The XRD patterns collected from the Bi4 (Ti1−x Mn x )3 O12
and Bi4 (Ti1−x Fex )3 O12 specimens are shown in
Fig. 1(a) and (b), respectively, together with the establishedXRD pattern of orthorhombic Bi4 Ti3 O12 (JCPDS 35-0795).
All the diffraction peaks could be indexed as orthorhombic
Bi4 Ti3 O12 , and no extra peaks were found in the collected
XRD patterns. The lattice parameters of our samples were
calculated using the conventional least-square fitting method
to the observed XRD patterns. The lattice parameters and thecorresponding volumes of the unit cells are plotted in Fig. 2.
These data are also summarized in Table I. All of the lattice
constants, a, b, and c, of Mn-doped Bi4 Ti3 O12 decreased withincreasing Mn concentration, whereas a decreased and b and c0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014

Fig. 2.
Lattice constants (a) a and b, (b) c, and (c) volume V of
Bi4 (Ti1−x Mnx )3 O12 and Bi4 (Ti1−x Fex )3 O12 obtained from the XRD patterns as a function of dopant content.
TABLE IL ATTICE PARAMETERS AND V OLUMES OF
Bi4 (Ti1−x Mx )3 O12 (M = Mn, Fe)

Fig. 1.
Observed XRD patterns of (a) Bi4 (Ti1−x Mnx )3 O12 and(b) Bi4 (Ti1−x Fex )3 O12 . Calculated XRD pattern of Bi4 Ti3 O12 (JCPDS
35-0795) is included at the bottom of each pattern group.

increased as the Fe concentration increased. The volumes ofboth Mn- and Fe-doped Bi4 Ti3 O12 unit cells decreased as theconcentration of the dopant increased, though this volume
reduction is much smaller in the Fe-doped ones. These results
indicate that both Mn and Fe ions appear to be substituted at
one of the cation sites, i.e., Bi or Ti sites.
Ionic radii for various ions in this system [16] are listed inTable II. If Mn and Fe ions substitute at Bi sites, unit cellvolume should decrease much since all the ionic radii withdifferent valences are much smaller than that of Bi3+ . In thecase of the substitution at Ti sites, only Fe4+ and Mn4+ can
produce the experimentally observed reduction of volumes.
However, the formation of an oxygen vacancy may appear
for charge compensation in the crystal when trivalent Mn andFe ions substitute at Ti4+ site, which may also lead to the
reduction of the volume. Thus, an analysis of the doped crystal
structure based on ionic substitutions alone cannot determine
the substitution sites for Mn and Fe ions in Bi4 Ti3 O12 .

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TABLE III ONIC R ADII OF C ATIONS W ITH S IX -C OORDINATION [16]Fig. 4. Observed K-edge XANES spectra for (a) Mn-doped and (b) Fe-doped
Bi4 Ti3 O12 . The corresponding spectra of Mn and Fe oxides are also plottedfor comparison.

Fig. 3.
Observed L2,3 -edge XANES spectra for (a) Mn-doped and(b) Fe-doped Bi4 Ti3 O12 . The corresponding spectra of Mn and Fe oxides
are also plotted for comparison.

We employed the Mn- and Fe-L2,3 XANES spectra to
determine the valence state of doped Mn and Fe ions; these arein Fig. 3, shown alongside the spectra of Mn and Fe oxides.

In general, L2,3 -edge XANES spectra of 3d transition metalis quite sensitive to change in valence state, since they canbe obtained from the electric dipole transition from 2p to 3d
states. In addition, profiles of L2,3 XANES spectra canstrongly reflect only nearest neighboring coordinations, since
3d orbital is a localized orbital. Then, we are able to use
the fingerprint type analysis by comparing the spectrumof interest with standard oxides to determine the valence
state of 3d transition elements. From the comparison of
our observed Mn-L2,3 spectra with those of the reference
Mn oxides, we observe that the spectral fine structures of
Mn ions in Bi4 Ti3 O12 [Fig. 3(a)] show a similar profile tothose of MnO2 , which indicates to us that the majority of
Mn ions in Bi4 Ti3 O12 exist as Mn4+ . On the other hand, theFe-L2,3 XANES profile of Fe ions in Bi4 Ti3 O12 [Fig. 3(b)] is

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014

Fig. 5. Comparison of K-edge XANES spectra between experiments andcalculations of (a) MnO and (b) FeO.

similar to that of α-Fe2 O3 , which suggests that most Fe ionsin Bi4 Ti3 O12 are in the 3+ state.
Our observed Mn and Fe K-edge XANES spectra of
Mn- and Fe-doped Bi4 Ti3 O12 are in Fig. 4, alongside those ofstandard Mn and Fe oxides. The spectral fine structures of Mn
and Fe in Bi4 Ti3 O12 are different from any of the spectra ofstandard Mn and Fe oxides, which indicates Mn and Fe ionsdo not exist as simple oxides and the local environments of
Mn and Fe ions are different from those in Mn and Fe oxides.
This type of experimental comparison alone cannot determine
the local environment of the doped ions in the case of K-edge
XANES analysis, because K-edge XANES profile is a reflection of unoccupied p-states, which is rather delocalized thand-states. Then, no standard spectra are available to be compared with the experimental spectrum of interest. Therefore,
the additional information provided by theoretical calculations
is required to resolve the final doped structure. These theoretical fingerprints can be compared with the observed XANES
spectrum of interest to help identify the local environment of
doped ions.
We performed these first-principles calculations to constructtheoretical Mn-K and Fe-K XANES spectra of Mn- andFe-doped Bi4 Ti3 O12 , respectively. All the calculations werecarried out using the augmented plane wave plus local orbital(APW+lo) solution of the Kohn–Sham equations, as implemented in the WIEN2k package [19]. The core-hole effect was
directly introduced to fix the final state of the X-ray absorption

Fig. 6. Comparisons of K-edge XANES spectra between experiments andcalculations of (a) Mn-doped and (b) Fe-doped Bi4 Ti3 O12 .

process by removing one electron from the K-shell of Mn or
Fe and placing an additional electron at the bottom of theunoccupied band, which is approximately correspond to thefinal states of Mn and Fe K-edge X-ray absorption process. We
determined the energies associated with the calculated XANES
spectra by taking the difference in total electronic energy of
the initial (ground) and final (core-hole) states of the Mn and
Fe K-edge X-ray absorption process.
Prior to the calculation of the Mn- and Fe-doped Bi4 Ti3 O12 ,theoretical XANES spectra of MnO and FeO with arock-salt structure were calculated using the 2 × 2 × 2 supercell (64 atoms) of conventional unit-cell. The muffin-tin radii,
RMT s, of Mn, Fe, and O were set to 1.98, 1.98, and 1.30 a.u.,NISHIMURA et al.: SUBSTITUTION MECHANISM OF Mn AND Fe IONS IN Bi4 Ti3 O12

respectively, and the plane wave cutoff Kmax was 3 Ry1/2 .
A 3 × 3 × 3 k-point mesh based on the Monkhorst–Packscheme [20] was sampled in reciprocal space. The calculated spectra were broadened using a Gaussian function withwidth 1.16 and 1.25 eV for Mn and Fe K-edges, respectively. These peak broadenings correspond to the lifetimeof a K core-hole [21]. We employed a generalized gradientapproximation [22] for the exchange-correlation functional.
The calculated XANES spectra of MnO and FeO arecompared to the experiments in Fig. 5(a) and (b), respectively.
Characteristic features of XANES spectra of MnO and FeO
were well reproduced by our calculations when the energy wascorrected by 21.5 eV (E/E = 0.3%) for MnO and 21.6 eV(E/E = 0.3%) for FeO. It is noted here that the calculated
spectra are sharper than the experimental ones. This is because
the calculated spectra were broadened by taking only lifetime
broadening effect but not taking the spectrometer broadening into considerations. We built super-cells for Mn- andFe-doped Bi4 Ti3 O12 by expanding the unit-cell of Bi4 Ti3 O12with a space group of I4/mmm along the b-axis twice, i.e.,a 1 × 2 × 1 super-cell (76 atoms). From the results of Mnand Fe-L2,3 XANES analysis, valence states of Mn and Fe
ions in Bi4 Ti3 O12 have been determined to be 4+ and 3+,
respectively. Then, we constructed the models in which the
Mn4+ and Fe3+ ions were substituted at Bi or Ti sites.
There are two crystallographically independent sites for Bi(i.e., in a perovskite-type layer or a rock-salt-type layer) and
two independent sites for Ti [i.e., Ti(2a) and Ti(4e)]. We
constructed eight types of models by replacing one of the Bi or
Ti ions in one of these configurations by a Mn4+ or Fe3+ ion
and by controlling the total number of electrons in the cell.
When a Bi3+ ion was replaced by Mn4+ ion, for instance,
one electron was removed from the cell at initial condition.
Spectral energies were shifted by the same amounts as for thecase of MnO and FeO, i.e., 21.5 and 21.6 eV, respectively.
Calculated XANES profiles of Mn- and Fe-doped
Bi4 Ti3 O12 are compared with our experimental profiles inFig. 6(a) and (b), respectively. In both figures, the calculated
XANES spectra of Bi site models show significant differencescompared with the observed profiles. On the other hand, bothof the calculated spectra of the two types of the Ti site
substitution models reproduce the experimental data well.
It should be noted here that the spectral signatures of the Ti(4e)and Ti(2a) models are quite similar, which suggests the local
environments of Mn ions are quite similar when they aresubstituted at Ti(2a) and Ti(4e) sites.
Unfortunately, the theoretical XANES fingerprints cannot
determine which Ti site is preferable for the substitution of
Mn and Fe ions in Bi4 Ti3 O12 . To determine the specific sites
for substitution, we compared the total electronic energies
calculated above for the calculations of spectral energies. In
the case of Mn substitution, the total electronic energy is lower
by 0.16 eV/cell when Mn is substituted at Ti(2a) site than atTi(4e), whereas that is lower by 0.30 eV/cell when Fe ion isat Ti(4e) site than at Ti(2a). This is an indication that Mn andFe ions substitute at Ti(2a) and Ti(4e) sites, respectively.
IV. C ONCLUSION
We synthesized polycrystalline Mn- and Fe-doped
Bi4 Ti3 O12 specimens using a solid-state reaction methodwith different concentrations of Mn and Fe ions:x = 0.00, 0.01, 0.03, and 0.05 in Bi4 (Ti1−x Mn x )3 O12 andBi4 (Ti1−x Fex )3 O12 , respectively. The crystal structures of2502306

all samples were verified by powder XRD technique, whichshow all the samples possessed a single-phase orthorhombic
structure equivalent to Bi4 Ti3 O12 . Substitution mechanism of
Mn and Fe ions in Bi4 Ti3 O12 was investigated by XANES
analysis with the aid of the first-principles calculations.
Mn- and Fe-L2,3 XANES measurements determined thevalence states of the Mn and Fe ions as 4+ and 3+,
respectively. K-edge XANES analysis with the aid of thefirst-principles calculations revealed that Mn and Fe ionsshould substitute at Ti site. Finally, the first-principles total
electronic energy calculations suggest that Mn and Fe ionsshould substitute at Ti(2a) and Ti(4e) sites, respectively.
ACKNOWLEDGMENT
The authors would like to thank E. Shigemasa, H. Iwayama,
E. Nakamura, and N. Kondo of UVSOR for their assistance with the Mn- and Fe-L2,3 XANES measurements. The
Mn- and Fe-L2,3 XANES measurements was supported by
BL-4B in UVSOR through the Institute of Molecular Science under Proposal 24-517, 25-519. The Mn-K and Fe-K
XANES measurements was supported by BL01B1 in SPring- 8
through the Japan Radiation Research Institute under Proposal
2013A1034.
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