Photon Factory Activity Report 2014 #32 (2015) B

BL-6C/2013G605

Three-dimensional atomic image of Bi2Te3:Mn topological insulator
Shinya Hosokawa1,*, Yuuki Ideguchi1, Kenji Kamimura1, Koji Kimura1, Naohisa Happo2,Kouichi Hayashi3,†, Mamoru Kitaura4, Akimasa Ohnishi4, and Minoru Sasaki4
1
Department of Physics, Kumamoto University, Kumamoto 860-8555, Japan
2
Graduate School of Information Sciences, Hiroshima City University, Hiroshima 731-3194, Japan
3
Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan
4
Department of Physics, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan
To investigate structures around the impurity sites in a Mn-doped Bi2Te3 topological insulator, Mn Kα x-ray fluorescenceholography (XFH) measurements were carried out on Bi2Te3Mn0.1 single crystal at 100 and 300 K. At 300 K, only imagesof the second-neighboring atoms are observed on the (001) plane, indicating that the Mn atoms are substituted with theBi or Te atoms and the nearest-neighboring atoms has strong positional fluctuations in the angular direction. Besides
these images, more intense images are detected at the half positions from the central Mn atoms at 100 K. By combining
with XAFS data, it was suggested that a majority of the Mn impurity atoms are located at the edges of Te triangles
formed on the layer surface.
1 Introduction
In the past, Bi2Te3 was well-known as a thermoelectricmaterial having a large value of thermopower. Nowadays,
this material has also achieved much attention as a
topological insulator (TI), where the so-called Dirac
electrons with an almost zero mass can conduct on thesurface of this material [1]. For this reason, Bi2Te3 ispromising as a raw material for future computer systemsof very fast and energy saving.
Recently, Kim et al. [2] found that Bi2Te3Fex alloys
have an excellent TI behavior at x = 0.03 – 0.10. Theyobserved two interesting TI features that the lifetime of
the TI properties is extremely long and the Dirac
electrons are not scattered by Fe impurities. Mn-doped
Bi2Te3 alloys also show excellent TI properties [3] similar
to Bi2Te3Fex alloys.
Then an important question arises from the structural
point of view; where are the impurity atoms located in theBi2Te3 crystal for showing such TI properties? The
undoped Bi2Te3 has a layer structure with a hexagonal
form [4]. The interlayer positions would be mostplausible, but there is no special reason to exclude thepossibilities of substitutions with Bi or Te atoms orinterstitial positions in the layer.
Structural characterization around impurities is one of
the most difficult issues to study the structure of materials.
Detailed data collections and proper models are necessary
to perform single crystal structure analysis using intense
x-ray or neutron source. XAFS gives mostly onedimensional information on the nearest neighboring
atoms, and the information on the second- or distantneighboring atoms is very poor. A proper model is again
essential.
X-ray fluorescence holography (XFH) is a newly
developed technique for atom-resolved structuralcharacterizations of materials, and enables one to draw
three-dimensional (3D) atomic images around a specificelement emitting fluorescent x-rays [5]. Owing to aninterference between direct incident x-rays and those

scattered by surrounding atoms, the fluorescent x-ray
intensity from the emitter slightly modulates with incident
x-ray angles by about 0.1 %, from which 3D images ofneighboring toms can be obtained by simple Fouriertransforms without any special atomic models. This
technique has, in particular, an excellent potential forinvestigating local structures around impurity atoms [6].
We carried out a Mn Kα XFH measurement on
Bi2Te3Mn0.1 single crystal at room temperature [7]. The
reconstructed atomic images around the central Mn atom
show only a hexagonal configuration of the secondneighboring atoms on the (001) plane. Subsequently, we
have performed the same experiment at a low temperatureof 100 K, which shows a different feature that strong
atomic images appear at positions with a much nearer
distance from the central Mn atoms. In this paper, we
report the results of the recent XFH experiment at 100 K,
and discuss a possible interpretation of the positions ofthe Mn impurity atoms in combination with our recent
XAFS data.
2 Experiment
A single crystal of Bi2Te3Mn0.1 was grown by amodified Bridgeman method, where appropriate amountsof Bi2Te3 and Mn powders were melted and crystallized
in an evacuated quartz ampoule several times by slow
cooling. The sample was cooled from 850 to 550°C witha cooling rate of 10°C/h. The crystal was cut and polishedso as to have a flat (001) surface larger than 3 × 3 mm2.
The crystallinity was examined by taking a Lauephotograph, and the concentration and homogeneity wereconfirmed over the sample within the experimental errors
by an electron-probe micro-analysis.
A Mn Ka XFH measurement on Bi2Te3Mn0.1 wascarried out at 100 K using a cryostream apparatus at BL6C of the Photon Factory in the High Energy Accelerator
Research Organization (PF-KEK), Tsukuba, Japan. Thesample was placed on a two-axes table of adiffractometer. The measurement was performed inPhoton Factory Activity Report 2014 #32 (2015) B

3 Results and Discussion
Figure 1 shows the resonstructed atomic images ofBi2Te3Mn0.1 single crystal at (a) 300 K [7] and (b) 100 Kon the (001) plane around the central Mn atoms marked
by the circles in the figures. The image intensities werenormalized to that of the strongest images at 100 K, andare shown as the color bars besides the figures. As aguide for eyes, intersections of dashed lines indicate ideal
positions of the neighboring atoms obtained fromdiffraction measurement on the undoped Bi2Te3 crystal[4] provided that the Mn atoms are located at a Bi or Teposition. In the crystal structure, all of atoms on the (001)plane are the same element, Bi or Te, depending on the z
values along the c axis.
In Fig. 1(a) at 300 K, clear six atomic images areobserved near the ideal positions of the second
neighboring atoms in the hexagonal lattice. Thus, some ofthe Mn impurity atoms should be located at asubstitutional position of Bi or Te atoms in the original
Bi2Te3 crystal. The local lattice constant around the Mn
impurity is slightly larger than that of the undoped Bi2Te3crystal.
As seen in Fig. 1(b) at 100 K, new and strong atomic
images are observed at the middle positions of thehexagonal lattice from the central Mn atom. The distanceof the atomic images from the central Mn atom is about
0.25 nm, which is much shorter than the shortest Ge-Teinteratomic length of 0.3063 nm. Our new XAFS
experiment near the Mn K edge confirmed such a shortinteratomic distance around the Mn atoms [9]. Therefore,
it is concluded that these Mn impurity atoms are not at
any substitutional positions.
Since the XAFS data indicate that the nearest neighbors
are not light Mn atoms but heavy Bi or Te atoms owing toa characteristic feature of the backscattering amplitudewith k, most plausible positions of the Mn impurities forthese images would be the interstitial position, i.e., theedges of triangles formed on the hexagonal layer. The Bi-Bi or Te-Te second neighboring interatomic length in theundoped Bi2Te3 crystal is 0.4388 nm, and a large lattice
expansion around the Mn impurity is expected by about
0.03 nm. Thus, the most plausible impurity site would beat the Te triangle sites in the surface of the layes, whichallow such large distortions observed by the present XFH
and XAFS data [9].

1.0(a)y (nm)

0.5

1.0
0.8

0Mn

0.6
0.4

-0.5

0.2
-1.0
-1.0
1.0-0.50x (nm)0.5

1.00.0(b)1.0

0.5

0.8y (nm)inverse mode by changing two axes, the exit angle of 0° ≤θ ≤ 75° in steps of 1.00° and the azimuthal angle of 0° ≤ φ≤ 360° in steps of about 0.35°. Incident x-rays werefocused onto the (001) surface of the sample. Mn Kαfluorescent x-rays were collected using an avalanche
photodiode detector with a cylindrical graphite crystal
energy analyzer. The XFH signals were recorded at eight
different incident x-ray energies from 7.0 to 10.5 keV insteps of 0.5 keV. Details of the experimental setup aregiven elsewhere [5].
Holographic oscillation data were obtained bysubtracting the background from the fluorescent x-ray
intensities. An extension of the hologram data was carriedout using the crystal symmetries of the hexagonal
structure [4] and the measured x-ray standing wave lines.
From the hologram patterns, 3D atomic configuration
images were reconstructed using Barton’s algorithm [8]by superimposing the holograms with eight different
incident x-ray energies, which can highly suppress theappearance of twin images.

00.6

Mn

0.4

-0.5

-1.0-1.0

0.2
0.0-0.50x (nm)0.5

1.0

Fig. 1: Atomic images of a Bi2Te3Mn0.1 single crystal at
300 K [7] and 100 K on the (001) plane around thecentral Mn atom obtained from the Mn Kα XFH
measurement.
For the further understandings of the Mn impurity sites
in the Bi2Te3Mn0.1 single crystal, a detailed comparison
between the XANES data and the corresponding FEFF
analysis using cluster models would be helpful, which is
now in progress.
Acknowledgement
The authors thank Professor S. Sasaki and Dr. M.
Okube for the support of the XFH experiments. The
experiments were performed at BL-6C (No. 2013G605).
This work was supported by Grant-in-Aid for ScientificPhoton Factory Activity Report 2014 #32 (2015) B

Research on Innovative Areas “3D Active-Site Science”(No. 26105006).
References[1] H.-J. Kim et al., Phys. Rev. B 84, 125144 (2011).
[2] H.-J. Kim et al., Phys. Rev. Lett. 110, 136601 (2013).
[3] S. Sasaki et al., private communication.
[4] Y. Feutelais et al., Mater. Res. Bull. 28, 591 (1993).
[5] K. Hayashi et al., J. Phys.: Condens. Matter 24,093201 (2012).
[6] S. Hosokawa et al., Phys. Rev. B 87, 094104 (2013).
[7] S. Hosokawa et al., J. Phys.: Conf. Ser. 502, 012024(2014).
[8] J. J. Barton, Phys. Rev. Lett. 67, 3106 (1991). 
[9] S. Hosokawa et al., Phys. Rev. B, in preparation.
*hosokawa@sci.kumamoto-u.ac.jp. †Present address:
Department of Materials Science and Engineering,
Graduate School of Engineering, Nagoya Institute ofTechnology, Nagoya 466-8555, Japan.