Direct observation of the intergrown α-phase in β-TmAlB4 via high-resolution electron microscopy

Article abstract of DOI:10.1016/j.materresbull.2009.03.013

A TmAlB₄ crystal with a ThMoB₄-type (β-type) structure phase related to a hexagonal AlB₂-type structure was studied by electron diffraction and high-resolution electron microscopy. A high-resolution image clearly exhibits

Full text of DOI:10.1016/j.materresbull.2009.03.013

Materials Research Bulletin 44 (2009) 1743–1746
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Materials Research Bulletin
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Direct observation of the intergrown
electron microscopy
a-phase in b-TmAlB4 via high-resolution
b
c
c
d
a
Kunio Yubuta a, , Takao Mori , Andreas Leithe-Jasper , Yuri Grin , Shigeru Okada , Toetsu Shishido
*
a Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
b International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
c Max-Plank-Institut fu¨r Chemische Physik fester Sto¨ffe, 01187 Dresden, Germany
d Department of Science and Engineering, Kokushikan University, Tokyo 154-8515, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
A TmAlB4 crystal with a ThMoB4-type (
b
-type) structure phase related to a hexagonal AlB2-type
Received 16 February 2009
Accepted 26 March 2009
Available online 5 April 2009
structure was studied by electron diffraction and high-resolution electron microscopy. A high-resolution
image clearly exhibits an intergrown lamellar structure of a YCrB4-type ( -type) phase in the matrix of
the -type phase in TmAlB4 crystal. The lamellar structure can be characterized by a tiling of deformed
hexagons, which are a common structure unit in the -type and -type structures. The intergrown
a
b
a
b
Keyword:
nanostructure is considered to be attributed to the origin of low temperature anomalies in physical
A. Inorganic compounds
C. Electron diffraction
C. Electron microscopy
D. Crystal structure
properties.
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
c = 0.37981(3) nm, respectively. The previous work indicates that
the existence of an intrinsic intergrown nanostructure influences
The rare-earth metal borides have yielded intriguing systems to
study fundamental problems in physics and chemistry [1]. The rare-
earth metal aluminoboride system RAlB4 has been attracting
increasing attention with recent discoveries. Crystals with two
different structure types were reportedly obtained from the same
flux forYbAlB4 and LuAlB4 [2] withheavy fermionsuperconductivity
observed for YbAlB4 [3], while multiple magnetic transitions were
reported to occur in TmAlB4 at low temperatures below an
antiferromagnetic transition temperature TN [4]. TmAlB4 takes a
well-known YCrB4-type structure [5] (space group Pbam), and we
havefound thatX-ray diffraction measurements point toanintrinsic
tiling variation existing in the crystals [6] due to the presence of
fragmentsofthecloselyrelatedThMoB4-typestructure(spacegroup
Cmmm) [7]. Both structures have similarities to the AlB2-type
structure, with a planar boron network build of pentagonal and
heptagonal rings having differently sized rare-earth and aluminum
physical properties and is the origin of multiple magnetic anomalies
which are observed in TmAlB4. This is striking because there are
more than 100 compounds reported with this type of structure, and
it is possible that such indicated phenomenon of the intrinsic tiling
variation is actually ubiquitous.
With this interest in mind, we have attempted to observe
directly the crystal structures of a successfully synthesized b-type
TmAlB4 crystal by high-resolution electron microscopy (HREM) to
find a direct proof of the existence of such tiling variations.
2. Experimental
Samples were prepared via a two-step synthesis. First, a TmB4
master alloy was made by reactive sintering (48 h at 1673 K) of
compacted stoichiometric mixtures of thulium (Ames, 99.9 wt.%)
filings and boron powder (crystalline, Chempur, 99.999 wt.%). All
experimental steps were carried out inside an argon glove-box
system (partial pressures of O2 and H2O < 0.1 ppm). In the second
step, the binary tetraboride was powdered and compacted in a
ratio of Tm:Al:B = 1:2:5 with aluminum filings (Chempur,
99.999 wt.%), placed in alumina crucibles, welded in Ta ampoules
which were sealed in evacuated silica tubes. The tubes were then
slowly heated to 973 K, kept there for 1 week, followed by
regrinding and an additional heat treatment at 973 K for 2 weeks.
Single phase polycrystalline samples were obtained. Magnetic
atoms. The difference between the YCrB4-type (
ThMoB4-type ( -type) structures is in the orientation of the pairs
of condensed pentagonal rings. The lattice parameters for - and
a-type) and
b
a
b-
type phases are a = 0.59225 nm, b = 1.14784(5) nm and
c = 0.35224(2) nm, and a = 0.72795(6) nm, b = 0.93248(8) nm, and
* Corresponding author. Tel.: +81 22 215 2199; fax: +81 22 215 2137.
E-mail address: yubuta@imr.tohoku.ac.jp (K. Yubuta).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.03.013

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K. Yubuta et al. / Materials Research Bulletin 44 (2009) 1743–1746
Fig. 1. Temperature dependence of the magnetic susceptibility
x of b-type TmAlB4.
The inset is an enlarged view of the inverse susceptibility at low temperatures.
susceptibility was measured using a Quantum Design MPMS-XL.
Thin specimens for transmission electron microscopy (TEM) were
prepared by dispersing crushed materials on holey carbon films.
Electron diffraction (ED) patterns and HREM images were obtained
using a 200 kV electron microscope (JEM-2010) at a resolution of
0.19 nm.
Fig. 3. A many-beam bright-field TEM image taken with the incident electron beam
parallel to the [0 0 1] direction.
3. Results and discussion
However, characteristic diffuse streaks, which suggest the
existence of structural modulations, are observed along two
directions dÃ1 and dÃ2, indicated by large arrows in Fig. 2(a). Four
white arrows in Fig. 2(a) correspond to reflection positions of
The magnetic properties of
The magnetic susceptibility
indicative of an antiferromagnetic transition at TN = 9.5 K. As can
be seen in the inset which shows an enlarged view of the inverse
susceptibility, a further change of behaviour is observed below the
b
x
-type TmAlB4 were investigated.
(Fig. 1) exhibits a clear peak
the
a-type phase, as shown in Fig. 2(b), which was taken from a
a
-type TmAlB4 single crystal synthesized by the flux method
´
[6,8]. Similar intensity distributions of strong Bragg reflections
in the ED patterns, as indicated by arrowheads in Fig. 2(a) and
(b), result from the fact that the structures of both phases are
Neel temperature at around T2 = 3.5 K. Therefore, an anomaly
´
below the Neel temperature is observed in
to that observed for -type TmAlB4 [6]. The Curie–Weiss fit of the
data of -type TmAlB4 yields parameters of an effective magnetic
moment eff = 7.1 B/Tm and Curie–Weiss temperature
= À17 K. The value of meff is close to the theoretical value for
the 3H6 multiplet of the free trivalent thulium ion (7.56
B).
b-type TmAlB4 similar
a
x
built by
a unique structural unit, which consists of six
b
heptagonal atomic columns containing edge-shared two penta-
gons (Fig. 5(a)).
Fig. 3 shows a HREM image taken with the incident beam of the
same direction as Fig. 2(a). On the top part of Fig. 3, homogeneous
m
m
a
u
m
Fig. 2(a) shows an ED pattern of the TmAlB4 crystal, taken
with the incident electron beam parallel to the [0 0 1] direction.
lattice fringes of the
b-type structure can be seen, whereas on the
bottom part, a stacking modulation along the vertical direction is
Almost all of diffraction spots are those of the
b-type phase.
Fig. 2. ED patterns of (a)
-type phases. Large arrows in (a) indicate characteristic diffuse streaks. The
respectively.
b
- and (b)
a
-type phases taken with the incident electron beam parallel to the [0 0 1] direction. White arrows in (a) indicate reflection positions of the
a
a
- and -type phases belong to orthorhombic systems, space groups Pbam and Cmmm,
b

K. Yubuta et al. / Materials Research Bulletin 44 (2009) 1743–1746
1745
Fig. 4. An enlarged HREM image from a part of the intergrowth nanostructure. Deformed hexagons in (a) correspond to the structural unit in Fig. 5(a). The upper and lower
rectangles in (a) indicate unit cells of the - and -type structures, respectively. Power spectra in (b) and (c) were obtained from regions of - and -type phases, respectively.
a
b
a
b
observed. In an enlarged HREM image of Fig. 4(a), one can see that
the stacking modulation results from the local appearance of the
type phase, viz., the intergrowth of the -type phase in the -type
phase. The intergrowth structure may cause the low temperature
anomalies in the physical properties. The upper and lower
HREM image, it is confirmed that a crystallographic relationship
between the b- and the a-type phases can be expressed as
a
-
¯
a
b
(1 1 0) k (0 1 0) and [0 0 1] k [0 0 1] .
b
a
b
a
Fig. 5 shows a structure model of the intergrowth of the
a-type
phase in the -type phase. The both structures have a common
b
rectangles in Fig. 4(a) indicate unit cells of the
phases, respectively. As shown by a tiling of deformed hexagons in
the figure, the -type phase can be characterized by a tiling of
hexagons with one direction, whereas the -type phase has a
zigzag stacking of hexagons with two directions. The width of the
lamella -type phase along the b-axis is smaller than five unit cells.
Fig. 4(b) and 4(c) show power spectra obtained from regions of the
- and -type phases, respectively. The intensity distribution of
a
- and
b
-type
hexagonal structural unit formed by connecting Tm atoms, as
shown in Fig. 5(a). As can be seen from a tiling showing the
intergrowth of the a-phase in the b-type matrix in Fig. 5(b), there
are coherent interfaces parallel to the d1 direction. The diffuse
streaks along the direction dÃ1 in Fig. 2(a) corresponds to the
perpendicular direction of the interfaces parallel to the direction
d1. It is reasonable to consider that the diffuse streaks along the
direction dÃ2 originate from the twin-related micro-domains of the
b
a
a
a
b
the observed ED pattern of Fig. 2(a) can be easily understood by
overlapping of those of Fig. 4(b) and 4(c). From the ED pattern and
b
-type phase, because the direction dÃ2 is symmetrically equivalent
to that of dÃ1.

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K. Yubuta et al. / Materials Research Bulletin 44 (2009) 1743–1746
the
b
-type matrix. Through TEM observations, slivers of the
a
b
-type
-type
‘‘tiling’’ were clearly observed to be insinuated into the
TmAlB4 structure. The crystallographic relationship between the
¯
b
- and the
a
-type phases can be expressed as (1 1 0) k (0 1 0)
b
a
and [0 0 1] k [0 0 1] . We have also observed an antiferromag-
b
a
netic transition at TN = 9.5 K in
b-type TmAlB4 with a further low
´
temperature anomaly below the Neel temperature at 3.5 K. This
result agrees with our direct observation of the intergrowth
structure and the hypothesis that the intergrowth is the origin of
low temperature anomalies in the physical properties.
It still remains an interesting question of just how ubiquitous
this phenomenon of the intergrown nanostructure is among such
‘‘tiled’’ compounds other than just TmAlB4. Interesting behaviour
has also been found in other b-type compounds such as the heavy
fermion superconductivity recently discovered in YbAlB4 [3] and it
should be further investigated whether such intergrown nanos-
tructure exists and might be playing any role in the physical
properties of the Yb system also.
Acknowledgement
This study was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
References
[1] T. Mori, in: K.A. Gschneidner, Jr., J.-C. Bunzli, V. Pecharsky (Eds.), Higher Borides,
Handbook on the Physics and Chemistry of Rare Earths, Vol. 38, North-Holland,
Amsterdam, 2008, pp. 105–173.
Fig. 5. (a) Structural unit of the deformed hexagon consisting of edge-shared two
pentagonal and six heptagonal atomic columns. (b) Schematic illustration of a tiling
[2] R.T. Macaluso, S. Nakatsuji, K. Kuga, E.L. Thomas, Y. Machida, Y. Maeno, Z. Fisk, J.Y.
Chan, Chem. Mater. 19 (2007) 1918–1922.
[3] S. Nakatsuji, K. Kuga, Y. Machida, T. Tayama, T. Sakakibara, Y. Karaki, H. Ishimoto, S.
Yonezawa, Y. Maeno, E. Pearson, G.G. Lonzarich, L. Balicas, H. Lee, Z. Fisk, Nature
Phys. 4 (2008) 603–607.
of the deformed hexagons in the structure of an intergrowth
phases. The direction of arrows with letters d1 is perpendicular to the dÃ1 direction in
the observed ED pattern of Fig. 2(a). Shade and open regions show the - and -type
a-phase in the b-type
b
a
phases, respectively. Rectangles with dotted lines indicate unit cells of both phases.
[4] T. Mori, S. Okada, K. Kudou, J. Appl. Phys. 97 (2005), 10A910(1)–10A910(3).
[5] Y. Kuz’ma, Sov. Phys. Crystallogr. 15 (1970) 312–314.
[6] T. Mori, H. Borrmann, S. Okada, K. Kudou, A. Leithe-Jasper, U. Burkhardt, Yu. Grin,
Phys. Rev. B 76 (2007), 064404(1)–064404(10).
[7] P. Rogl, H. Nowotny, Monatsh. Chem. 105 (1974) 1082–1098.
[8] S. Okada, K. Kudou, Y. Yu, T. Lundstro¨m, Jpn. J. Appl. Phys. 33 (1994) 2663–2666.
4. Conclusions
In this report we have unequivocally demonstrated/proved the
existence of an intergrowth nanostructure of the
a-type phase in