site-specific mapping of transition metal oxygen coordination in
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Site-specific mapping of transition metal oxygen coordination in complexoxides
S. Turner,a) R. Egoavil, M. Batuk, A. A. Abakumov, J. Hadermann, J. Verbeeck,and G. Van TendelooEMAT, Department of Physics, University of Antwerp, B-2020 Antwerp, Belgium
(Received 8 October 2012; accepted 27 November 2012; published online 12 December 2012)
We demonstrate site-specific mapping of the oxygen coordination number for transition metals in
complex oxides using atomically resolved electron energy-loss spectroscopy in an aberration-
corrected scanning transmission electron microscope. Pb2Sr2Bi2Fe6O16 contains iron with a
constant Fe3þ valency in both octahedral and tetragonal pyramidal coordination and is selected to
demonstrate the principle of site-specific coordination mapping. Analysis of the site-specific
Fe-L2,3 data reveals distinct variations in the fine structure that are attributed to Fe in a six-fold
(octahedron) or five-fold (distorted tetragonal pyramid) oxygen coordination. Using these
variations, atomic resolution coordination maps are generated that are in excellent agreement with
simulations. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770512]
In complex transition metal oxides, the bonding and
electronic state of the transition metal cations, i.e., the oxy-
gen coordination, spin state, and oxidation state, is of funda-
mental importance. The spin state, valency, and oxygen
coordination of the transition metal cations are all intricately
related to the structural, electronic, magnetic, catalytic, and
ionic transport properties of the oxide materials.1–3 Informa-
tion on the oxygen coordination of the transition metal cati-
ons (coordination number, bond lengths, bond angles) is
conventionally available from various diffraction or spectro-
scopic methods such as X-ray/neutron/electron diffraction,
extended X-ray absorption fine structure (EXAFS), and
M€ossbauer spectroscopy or their combinations.4 However,
for materials applications, changes in the crystal and elec-
tronic structure often need to be monitored with high spatial
resolution, which is not possible using the above mentioned
methods.5,6 For example, valency changes at surfaces or
under metallic surface particles are of vital importance for
many catalytic processes, while coordination or spin changes
at defects like twin boundaries, grain boundaries, and interfa-
ces can greatly affect the electronic, optical, and transport
properties of bulk materials and thin films.7–9
Atomic resolution elemental mapping by means of spa-
tially resolved electron energy-loss spectroscopy (EELS) and
energy dispersive X-ray spectroscopy in a scanning transmis-
sion electron microscope (STEM-EELS and STEM-EDX) has
become a well-established technique over the past years.10–15
STEM-EDX appears to allow more straightforward data ac-
quisition and interpretation as compared to EELS. However,
recent work has shown that the interpretation of atomic reso-
lution data needs to be combined with simulations for an accu-
rate interpretation in both cases.16 In addition, with the current
commercial spectrometer technology, structural information
like bonding or coordination cannot be obtained from X-ray
spectra.17 Therefore, combining the sensitivity of EELS to
valency, coordination, and spin state through the EELS fine
structure (the energy-loss near-edge structure or ELNES) with
the atomic resolution capabilities of a STEM remains the
most direct method to obtain atomic resolution structure
information.
Oxidation state mapping at atomic resolution was
recently demonstrated, using the correlation between ELNES
and valency.7,18 The main issues hindering atomic resolution
valency and bonding measurements are poor EELS signal-
to-noise ratios due to the need for simultaneous high spatial
and energy resolution of the instrument and problems of sig-
nal intermixing due to elastic scattering in the case of “thick”
crystals.19 These problems are worsened further as, in gen-
eral, changes in the fine structure of the L2,3 edge of transi-
tion metals due to bonding or coordination are supposed to
be far more subtle than changes related to valency.20,21 This
makes studies where bonding and coordination have been
mapped at atomic resolution rare, and in most cases, the
change in bonding coincides with a change in valency which
also affects the EELS fine structure.12,22–24 In recent work,
distinct changes in fine structure of the Fe L2,3 and Co L2,3
edges were found between Fe and Co in octahedral and in
tetrahedral layers in Ca2FeCoO5 brownmillerite using atomic
resolution STEM-EELS. This experiment demonstrated the
problems of atomic resolution oxygen coordination mapping:
even though the octahedral and tetrahedral signatures could
be measured at each distinct plane, the signal to noise ratio
and signal intermixing in the experiments did not allow for a
column by column investigation.25 As stated above, column
by column information is of crucial importance when study-
ing, e.g., point defects or surface sites.
In this work, we demonstrate the sensitivity of transition
metal L2,3 edges to changes in local oxygen coordination,
which can be used to map out the coordination at atomic
resolution. Pb2Sr2Bi2Fe6O16, a perovskite-based material
with a structural incorporation of crystallographic shear
(CS) planes was selected for the experiment. This type of
complex ferrites, combining magnetic transition metal cations
at the B-position of the perovskite structure with lone pair A-
cations, potentially demonstrate a combination of ferroic prop-
erties such as the antiferroelectric and antiferromagnetica)E-mail: Stuart.turner@ua.ac.be.
0003-6951/2012/101(24)/241910/5/$30.00 VC 2012 American Institute of Physics101, 241910-1
APPLIED PHYSICS LETTERS 101, 241910 (2012)
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orderings found in (Pb,Bi)1�xFe1þxO3�y perovskites.26 Within
the material, the oxygen coordination of Fe changes from an
octahedral (6-fold) coordination in the perovskite blocks to a
distorted tetragonal pyramidal (5-fold) setting at the CS planes,
while keeping the Fe3þ valency constant. For the AnBnO3n-2
(A¼ Pb, Bi, Ba, Sr, B¼Fe, Ti, Sn) family of perovskite-based
oxides with perovskite blocks separated by CS planes, the pre-
sence of iron atoms in two distinct 5 - and 6-fold coordinations
as well as the constant Fe3þ oxidation state were proven by
neutron powder diffraction and M€ossbauer spectroscopy.27–29
Pb2Sr2Bi2Fe6O16 has an orthorhombic crystal structure with
lattice parameters a¼ 5.7199(1) A, b¼ 3.97066(7) A,
c¼ 32.5245(8) A and belongs to the n¼ 6 member of this fam-
ily. The fact that Pb2Sr2Bi2Fe6O16 is a mixed coordination/sin-
gle-valency compound makes it an ideal candidate to
demonstrate coordination mapping at atomic resolution; EELS
fine structure variations between crystallographically distinct
Fe positions can in this way unequivocally be assigned to coor-
dination changes and not to, e.g., valency variation.
The sample with composition Pb2Sr2Bi2Fe6O16 was
synthesized using a high temperature solid state reaction of
PbO, SrCO3, Bi2O3, and Fe2O3. The starting materials were
mixed in the molar ratio 1:1:0.5:1.5, thoroughly ground,
pressed into a pellet and annealed in air at 750 �C for 24 h,
850 �C for 24 h, and 900 �C for 15 h with intermediate
regrinding. The sample was prepared for TEM investigation
by crushing the powder, dispersing it in ethanol, and depos-
iting the dispersion onto a holey carbon grid. The sample
was investigated using a FEI Titan “cubed” microscope
equipped with a probe corrector and a monochromator, oper-
ated at 120 kV. The microscope was operated in STEM
mode using a convergence semi-angle a of 18.5 mrad. The
monochromator was excited to provide an energy resolution
of �250 meV, and the energy slit was chosen to provide a
beam current close to 60 pA for spectroscopy, while keeping
acceptable spatial resolution (probe size of approximately
1.5 A). The high-angle annular dark-field (HAADF) inner
collection semi-angle and spectrometer acceptance semi-
angle b was 160 mrad. The acquisition time per pixel was
80 ms and was chosen to avoid beam damage and provide
the best possible signal-to-noise ratio. The iron elemental
map in Figure 2 was generated by plotting the intensity
under the background subtracted L3 edge using a 9 eV
energy window. The maps in Figure 3 were generated by
back-fitting the 6-fold and 5-fold coordinated Fe EELS ref-
erence components from Figure 2(d) (blue and red spectra)
in a linear combination to the acquired EELS data cube
using the EELS model software package.30,31 A power-law
background (A �E�r) model for the EELS data was used in
the fit. When filtered, a 3� 3 light low-pass filter was used
in the DIGITAL MICROGRAPH software package. The image sim-
ulations were performed with the STEMSIM software package,
a MATLAB based image simulation program capable of han-
dling the complex interplay between elastic and inelastic
scattering in the double channeling approximation.32 Elastic
scattering was simulated with a Bloch wave approach
(max wave vector 2.0 A�1) at a total thickness of 10 nm
using a unit cell sampling of 135*24 pixels per unit cell.
Source size broadening was taken into account using a
Gaussian with 0.7 A FWHM and a Lorentzian with 0.2 A
FWHM.33 Inelastic scattering was simulated using a relativ-
istically corrected dipole approximation with a hard Bethe
ridge cutoff.
Figure 1(a) shows a HAADF-STEM image of the
Pb2Sr2Bi2Fe6O16 sample along the most informative [010]
zone axis orientation. In the image, the a and c directions are
indicated by arrows and the unit cell is marked by a white rec-
tangle. The structural repetition of the perovskite blocks con-
taining Bi, Sr as A cations and 6-fold coordinated Fe as B
cation and the crystallographic shear planes with Bi, Pb
atomic pairs alternating with pairs of 5-fold coordinated Fe is
immediately apparent. The image is taken with the electron
monochromator excited, providing an energy resolution of
approximately 250 meV. Under these conditions, the spatial
resolution is still sufficient to image all the structural details of
the compound. The crystal surface is extremely clean, which
is crucial for high resolution EELS experiments; and when-
ever present, the amorphous surface layer is below 1 nm. Sam-
ple thickness is also likely to be a crucial factor in this type of
experiment. As the EELS signal intermixing due to elastic
scattering in the crystal increases with sample thickness, a
very thin sample region close to the crystal surface was
selected for investigation. A Bloch wave HAADF image sim-
ulation for a 10 nm thick crystal is inserted into Figure 1(a)
and agrees remarkably well with the experimental image. The
projected structural models for the [100] and [010] zone axis
orientations are displayed in (c) and further elucidate the coor-
dination of Fe within the structure.
FIG. 1. Pb2Sr2Bi2Fe6O16 structure. (a) HAADF-STEM image along the
[010] zone axis. The unit cell is indicated by the white rectangle. The inset
image simulation for a 10 nm thick crystal matches well with the experi-
ment. (b) Fourier transform of the image in (a), demonstrating information
transfer beyond 6.0 nm�1 (reflections marked by red circles). (c) Structural
model along the [100] and [010] zone axis orientations. The 6-fold (octahe-
dral) coordinated Fe species are rendered in blue, the 5-fold coordinated (tet-
ragonal pyramidal) Fe species in red, Pb/Bi columns in green, and Bi/Sr
columns in violet.
241910-2 Turner et al. Appl. Phys. Lett. 101, 241910 (2012)
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The atomic resolution EELS data acquired at high
energy resolution are plotted in Figure 2. To acquire the
spectroscopic data needed for coordination mapping, the
spectrum imaging (SI) technique was adopted.17 In this tech-
nique, the electron probe (in our setup, the probe has an ap-
proximate size of 1.5 A) is scanned over the sample and an
EELS spectrum is acquired in each point together with a
high-angle annular dark-field signal as image reference. The
overview image in Figure 2(a) shows the region selected for
spectrum image acquisition. The simultaneously acquired
HAADF image is displayed in Figure 2(b). The Fe L2,3
edge was acquired to investigate local changes in Fe coordi-
nation. The L3 and L2 “white lines” arise from transitions of
2p3/2! 3d3/23d5/2 (L3) and 2p1/2! 3d3/2 (L2) and are known
to be sensitive to valency and coordination as their intensity
is related to the density of unoccupied states in the 3d
bands.20,34 Using a simple integration window placed over
the background-subtracted L3 edge in the EELS datacube, an
Fe elemental map was generated and is plotted in Figure
2(c). At this point, it is important to note that no post-
processing such as principle component analysis (PCA/
weighted PCA35) of the data was performed. All analyses
were carried out on raw data. Previous studies have shown
that PCA treatment of atomic resolution data can mask small
details in the acquired EELS data.36
The distinction between the 5-fold coordinated Fe3þ
atomic pairs in the CS plane structures and the 6-fold coordi-
nated Fe3þ B cations in the perovskite blocks is immediately
apparent. Simple inspection of the data summed over Fe
positions in the perovskite blocks (blue region) and the Fe
CS plane structure positions (red region) reveals distinct
changes in the fine structure for the two types of Fe coordina-
tion. The data collected from the octahedral Fe3þ sites (blue)
displays a clear splitting of the L3 peak, which results from
the split of the energy levels in the Fe 3d unoccupied states
into t2g and eg levels. The pre-peak to L3 at 708 eV is associ-
ated with transitions from 2p3/2 ! t2g. The main L3 maxi-
mum, associated with a transition from 2p3/2! eg, is present
at 709.5 eV. The fine structure shape and edge onset values
are therefore both in full agreement with literature data for
Fe3þ in an octahedral coordination.20 The data collected
from the tetragonal pyramidal Fe3þ sites show other distinct
features. A first observation is that, even though the ELNES
shape of the peak varies with respect to the octahedral data,
the edge onset remains the same, confirming that the valency
of Fe is not changing from one crystallographic site to
another.19 The L3 maximum is shifted by 0.3 eV to lower
energy. At the same time, the pre-peak to the L3 is subdued
due to the loss in symmetry and/or a decrease in the crystal
field splitting at these crystallographic positions, similar to
the case of 4-fold coordinated Fe3þ.4,25 The sum of both sig-
nals yields the total Fe L2,3 edge (black spectrum). In all, it is
clear that even though the coordination of the Fe cations
changes by only a single oxygen atom, clear differences are
present in the Fe L2,3 ELNES signatures.
In Figure 3, the average octahedral and tetragonalpyramidal internal reference spectra for Fe, the blue and red
spectra from Figure 2(d), are fitted to the entire acquired
EELS datacube in a linear combination, following the proce-
dure from earlier work on atomic resolution valency map-
ping.7,18,30,31 By fitting these two internal reference spectra
for Fe in different oxygen coordinations to each spectrum
in the datacube, 2D component maps of the spectral
weights are generated. The results of the fit are displayed in
Figure 3(a). It can be seen that each component peaks at the
correct atomic columns. The 6-fold coordinated Fe positions
in the perovskite blocks peak in the map of the octahedral
coordination, while the 5-fold coordinated Fe positions peak
in the map of tetragonal pyramidal coordination, even
resolving the Fe3þ atomic pairs that are only separated by
2.3 A in this projection. In order to confirm the direct
FIG. 2. Site-specific ELNES investigation. (a) HAADF-STEM overview
image of the 44� 25 pixel spectrum image region, indicated by the white
rectangle. (b) Simultaneously acquired HAADF signal, (c) Fe map, gen-
erated by integrating a 9 eV wide energy range under the background
subtracted Fe L3 edge in the spectrum image. (d) Summed Fe L2,3 edges
from the 6-fold coordinated Fe sites (blue spectrum, blue region indi-
cated in (b) and (c)), 5-fold coordinated Fe sites (red spectrum, red
region indicated in (b) and (c)), from the sum of both regions (black
spectrum) and a single pixel spectrum from an 6-fold octahedral position
(grey spectrum).
241910-3 Turner et al. Appl. Phys. Lett. 101, 241910 (2012)
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interpretation of the generated coordination maps, detailed
image simulations were carried out. These simulations are
plotted in Figure 3(c) and agree very well with the experi-
mental maps. When a light smoothing is applied to the
experimental data (Figure 3(b)), the similarity between
experiment and simulation is even more striking. To judge
the match between experiment and simulation more quantita-
tively, a line profile through both is overlaid in Figure 3(d)
for both the octahedral and the tetragonal pyramidal maps.
In all, experiment and simulation are in excellent agreement.
A slight discrepancy in the 5-fold line profile is present
around 2 nm, which is probably caused by a small residual
sample or instrumental instability. Both simulations and
experiment indicate that even though the crystal selected for
experiments was thin and almost free of amorphous surface,
beam spreading due to elastic scattering and inelastic deloc-
alisation is still a significant effect which leads to a large part
of the EELS signal leaking into the background. In all, these
maps clearly demonstrate that oxygen coordination of the
transition metal cations can be mapped in complex oxide
structures down to atomic resolution. Even though the coor-
dination of the two distinct Fe positions only differs by a sin-
gle oxygen atom, and the resulting ELNES variations in the
Fe L2,3 edge are small, the high signal-to-noise ratio in the
individual spectra allow the coordination to be mapped
atomic column by atomic column.
In conclusion, we have demonstrated the principle of ox-
ygen coordination mapping by atomically resolved, high reso-
lution EELS in an aberration-corrected electron microscope,
through the example of single valency/mixed coordination
compound Pb2Sr2Bi2Fe6O16. Detailed analysis of the Fe-L2,3
edge showed subtle and distinct variations in the fine structure
that could be attributed to Fe3þ in tetragonal pyramidal or
octahedral coordination; the results are in excellent agreement
5-fold
(d)
(a)
(b)
(c)
6-fold Overlay
FIG. 3. Column by column coordination mapping. (a) EELS maps obtained by point by point fitting of the internal reference Fe L2,3 data for 5-fold and 6-fold
coordinated iron to the EELS datacube. The colour overlay displays the octahedral iron columns in blue and the tetragonal pyramidal iron columns in red.
(b) EELS maps after low-pass filtering. (c) Simulated inelastic maps. (d) Line profiles over the low-pass filtered data from the positions indicated by the arrows
in (b) at 4 pixel width, with overlaid line profiles from the simulated maps.
241910-4 Turner et al. Appl. Phys. Lett. 101, 241910 (2012)
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with literature. Using the spectral data as an internal reference,
the coordination of the Fe cations in the compound could be
mapped column by column. These experiments open the gate
for coordination determination at surfaces, defects, and grain
boundaries in a plethora of complex oxide materials that have
been out of reach in the past due to the lack of suitable analy-
sis techniques, and for the study of valence and coordination
as input for structure solving.
D. Batuk is acknowledged for fruitful discussions. S.T.
gratefully acknowledges the Fund for Scientific Research
Flanders (FWO). Part of this work was supported by funding
from the European Research Council under the FP7, ERC
Grant No. 246791 COUNTATOMS and ERC Starting Grant
No. 278510 VORTEX. The EMAT microscope was partially
funded by the Hercules fund of the Flemish Government.
The authors acknowledge financial support from the Euro-
pean Union under the Framework 7 program under a contract
for an Integrated Infrastructure Initiative (Reference No.
312483 ESTEEM2). This work was funded by the European
Union Council under the 7th Framework Program (FP7)
Grant No. NMP3-LA-2010-246102 IFOX. M. B., J. H., and
A. A. acknowledge funding from the FWO under grant num-
ber G.0184.09N.
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241910-5 Turner et al. Appl. Phys. Lett. 101, 241910 (2012)
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