Single crystal growth and characterization of the iron-based superconductor KFe2As2 synthesized by KAs flux method

Article abstract of DOI:10.1143/JPSJ.79.124713

Centimeter-sized platelet single crystals of KFe₂As₂ were grown using a self-flux method. An encapsulation technique using a commercial stainless steel container allowed the stable crystal growth to last for more than 2 weeks. Ternary K-Fe-As systems with various starting compositions were examined to determine the optimal growth conditions. The employment of KAs flux led to the growth of large single crystals with a typical size of as large as 15 × 10 × 0.4 mm³. The grown crystals exhibit a sharp superconducting transition at 3.4K with the transition width 0.2 K, as well as the very large residual resistivity ratio exceeding 450, evidencing the good sample quality.

Full text of DOI:10.1143/JPSJ.79.124713

Journal of the Physical Society of Japan
Vol. 79, No. 12, December, 2010, 124713
#2010 The Physical Society of Japan
Single Crystal Growth and Characterization
of the Iron-Based Superconductor KFe As
2
2
Synthesized by KAs Flux Method
Kunihiro KIHOU1 , Taku SAITO , Shigeyuki ISHIDA , Masamichi NAKAJIMA3;4,
;4
ꢀ
2
3;4
Yasuhide TOMIOKA1 , Hideto FUKAZAWA2;4, Yoh KOHORI2;4, Toshimitsu ITO1;4
;4
,
Shin-ichi UCHIDA3 , Akira IYO1;4, Chul-Ho LEE1;4, and Hiroshi EISAKI1;4
;4
1National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2Department of Physics, Chiba University, Chiba 263-8522, Japan
3Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
4Transformative Research-Project on Iron Pnictides (TRIP), JST, Chiyoda, Tokyo 102-0075, Japan
(
Received September 7, 2010; accepted October 4, 2010; published December 10, 2010)
Centimeter-sized platelet single crystals of KFe2As2 were grown using a self-flux method. An
encapsulation technique using a commercial stainless steel container allowed the stable crystal growth to
last for more than 2 weeks. Ternary K–Fe–As systems with various starting compositions were examined
to determine the optimal growth conditions. The employment of KAs flux led to the growth of large
3
single crystals with a typical size of as large as 15 ꢁ 10 ꢁ 0:4 mm . The grown crystals exhibit a sharp
superconducting transition at 3.4 K with the transition width 0.2 K, as well as the very large residual
resistivity ratio exceeding 450, evidencing the good sample quality.
KEYWORDS: Fe-based superconductor, KFe2As2, single crystal growth
DOI: 10.1143/JPSJ.79.124713
Indeed, recent NMR, penetration depth,10) and thermal
9)
1
.
Introduction
11)
conductivity
measurements suggest that the super-
Soon after the discovery of the superconductivity at 26 K conducting gaps in KFe2As2 have nodes, in contrast to
in LaFeAs(O,F),1) studies on the iron (Fe)-based super- the nodeless gaps termed sꢃ waves proposed for typical
conductors have become one of the main research subjects Fe-based superconductors,12–17) such as LnFeAsO1ꢂxFx,
in the field of material science. To date, a variety of (Ba,K)Fe2As2, and Ba(Fe,Co)2As2. It is also interesting to
9)
superconductors, such as LnFeAs(O,F) (Ln: rare earth), note that the electronic specific heat is large, suggesting a
Ba,K)Fe As , Fe(Se,Te), and LiFeAs, have been found and strong electron correlation and a large carrier mass, which is
(
the superconducting critical temperature (T ) exceeds 50 K, indeed confirmed by dHvA measurements.
2
2
8
)
c
marking the highest record besides cuprates. Worldwide
To sort out the intrinsic properties of materials, systematic
investigations towards developing novel high-Tc supercon- investigations using sizable, high-quality single crystals are
ductors, as well as towards elucidating their superconducting indispensable. In the case of BaFe2As2-related compounds,
2
)
mechanism, are now extensively in progress.
the flux method is commonly used to grow single crystals.
Among various Fe-based superconductors, KFe2As2 There are a number of reports on the crystal growth of
stands out as a unique material. It is the end member of (Ba,K)Fe2As2 and Ba(Fe,M)2As2 (M ¼ Co, Ni, Rh, Pd, Ru)
1
7–19)
20–25)
the Ba1 K Fe As solid solution system that exhibits the by employing tin (Sn)
ꢂx
flux or FeAs
(self-) flux.
x
2
2
3–5)
highest T of 38 K at x ¼ 0:4.
Therefore, one can con- As for KFe As , several groups have grown single crystals
2 2
c
1
1,26)
sider KFe2As2 as a ‘‘parent’’ compound, such as LaFeAsO using FeAs flux.
The samples show superconducting
or BaFe2As2, in that high-Tc superconductivity emerges transition at 3.0 K, slightly lower than the Tc of 3.8 K
upon its chemical substitution. However, while other parent reported for polycrystalline samples. The in-plane residual
compounds are all nonsuperconducting and exhibit a long- resistivity ratio (RRR) of these crystals, defined by
range antiferromagnetic order, KFe2As2 is a superconductor ꢀabðꢄ300 KÞ=ꢀabð4 KÞ, is estimated to be 86–87.
in itself, with a relatively low Tc of 3.8 K. The formal
It is known that the single crystals grown using Sn flux
valence of Fe ions is 2.5+, much higher than those of other contain several percent of Sn contamination, which mark-
Fe-based superconductors, which are more or less 2+. As a edly affects their physical properties. For example, in the
result, the shape of the Fermi surface (FS) is expected to be case of BaFe2As2, Sn contamination results in the decrease
significantly modified. Band structure calculations predict in its structural/antiferromagnetic phase transition temper-
17)
that while most of the Fe-based superconductors possess ature from 138 to 80 K. Accordingly, the self-flux method
two-dimensional electron- and hole-FS sheets of roughly is more favorable in growing single crystals with better
equal size, FSs of KFe2As2 are dominated by large hole quality. Particularly, in the case of KFe2As2, crystal growth
6)
sheets centered at the ꢀ point. This feature is experimen- using FeAs self-flux is also expected to be difficult because
tally confirmed by angle-resolved photoemission spectrosco- of the difficulty in controlling potassium (K) at high
py (ARPES)7 and de Hass–van Alphen (dHvA) experi- temperatures. The vapor pressure of K increases at around
)
8)
ꢅ
ments. A different FS shape would lead the pairing the melting temperature of FeAs (Tm ¼ 1030 C), and in
interaction distinct from other Fe-based superconductors. some cases, it attacks the sample container. As a result, a
stable single crystal growth cannot be sustained, which
ꢀ
E-mail: k.kihou@aist.go.jp
should degrade the size and properties of the grown crystals.
124713-1

J. Phys. Soc. Jpn., Vol. 79, No. 12
K. KIHOU et al.
(
a)
Fig. 1. (Color) Sealing assembly for the single crystal growth. (1) Cap
union body (1-in. inner diameter), (2) cap union nut, (3) front ring, (4)
back ring, (5) stainless tube (1-in. diameter), (6) alumina crucible, and
(7) starting materials.
(
b)
A possible solution to avoid such difficulty is to employ an
adequate self-flux that allows the crystal growth at lower
temperatures. Here, in this study, we have tested two kinds of
ꢅ
self-flux with low Tm values, namely, K (Tm ¼ 63:7 C) and
ꢅ
KAs (Tm ¼ 625 C), expecting that their lower Tm contrib-
utes to the stable synthesis of single crystal samples. Also, to
prevent the corrosion between the sample container and the K
vapor, we have developed an original encapsulation tech-
nique using a stainless steel sample container assembly. After
some trials, we have found that the mixture of KAs and FeAs
precursors with the mixing ratio KAs : FeAs ¼ 3{6 : 1 is
Fig. 2. (Color) (a) K–Fe–As triangular diagram for the compositions
of the starting materials used to grow KFe2As2 single crystals. The
compositions surveyed in the present study are indexed. (b) Temperature
profile of the single crystal growth.
most effective in growing large single crystals with the prereacted FeAs and KAs as starting materials. FeAs was
3
ꢅ
typical size 15 ꢁ 10 ꢁ 0:4 mm . The crystals have a sharp synthesized by firing the mixture of Fe and As at 900 C for
superconducting transition at 3.5 K, and a large RRR 10 h in a sealed quartz tube. KAs was synthesized by firing
ꢅ
exceeding 500, evidencing the good sample quality.
the mixture of K and As at 650 C for 10 h using a stainless
steel container assembly.
Typically, 30 g of the mixture was put in an alumina
2
.
Experimental Methods
In this study, single crystals of KFe2As2 were grown using crucible. Then, the alumina crucible was sealed in a stainless
K-rich self-flux in a sealed environment. For the growth of steel container. The process is carried out within a globe box
BaFe2As2-related crystals, a quartz glass tube is usually used filled with dried N2. The humidity and oxygen content inside
to seal the samples. However, this glass tube is not adequate the globe box are kept below 1 ppm. The sample was placed
ꢅ
in the present case since it suffers a significant chemical in a box furnace and fired up to 900 C for 10 h. The
reaction with the evaporated K during the crystal growth. To temperature was kept for 10 h, and then slowly cooled down
ꢅ
ꢅ
avoid the reaction between the K vapor and the container, we to 600–650 C at a cooling rate of 1 C/h (lasting for 10
have employed an encapsulation method using a commercial days), as shown in Fig. 2(b). After the crystal growth, flux
stainless steel container, as illustrated in Fig. 1. Here, an was rinsed out of the samples using ethanol.
alumina crucible containing the starting materials is put in a
The composition of the grown single crystals was
steel tube. Tight sealing is accomplished by two caps also determined by energy-dispersive X-ray analysis with the
made of stainless steel, which are screwed to both ends of the scanning electron microscopy (SEM; JEOL JSM-6301F,
ꢅ
tube. The container bears the temperature up to 1000 C. The EPMA; EDAX Phoenix 1M). The crystallinity of the single
entire assembly can be obtained commercially.
crystals was evaluated by X-ray Laue photography using a
The advantages of this encapsulation method are as tungsten target. Lattice parameters were calculated using
follows. (1) Unlike a quartz tube, stainless steel does not X-ray diffraction (XRD) patterns with Cu Kꢁ radiation
react with K vapor. Accordingly, long-time crystal growth (Rigaku ULTIMA IV). Magnetic susceptibility was meas-
can be sustained. (2) The sealing process is simple and easy. ured using a superconducting quantum interference device
In particular, welding is not required to seal the container. (SQUID) magnetometer (Quantum Design MPMS) under a
As a result, all preparation processes can be carried out magnetic field of 10 Oe. The dc resistivity was measured by
inside the globe box, without exposing the reactive starting a conventional four-probe method using Quantum Design
materials (K/KAs) in air. (3) The assembly used in this PPMS.
method is relatively cheaper than those used in other sealing
3.
Results and Discussion
methods.
The starting materials are K (3N, chunk), Fe (4N, 150 mm 3.1 Determination of the optimal growth composition
mesh), and As (6N, 1–5 mm chunk). In some cases, we used In Fig. 2(a), we summarize the compositions we have
124713-2

J. Phys. Soc. Jpn., Vol. 79, No. 12
K. KIHOU et al.
examined to grow single crystals. The gray solid line
corresponds to the solid solution of K and FeAs. Here,
KFe2As2 is located on this line at K ¼ 20 at. %. Along this
line, K less than 20% corresponds to the FeAs self-flux
growth condition and K more than 20% corresponds to the K
self-flux growth condition. In this study, we employed the
compositions K : FeAs ¼ 1:5 : 2, 2 : 2, and 3 : 2, which are
equal to K ¼ 27, 33, and 43%, respectively. Among them, in
the cases of K ¼ 27 and 33%, melting was incomplete and
the resultant material was a spongelike ingot with many
voids. No single crystals were recognized. For K ¼ 43%,
a complete melting was accomplished and the resulting
material was a dense ingot containing KFe2As2 single
crystals with their typical dimensions less than 1 ꢁ 1 ꢁ 0:1
(a)
(b)
(
c)
3
mm . From the magnetic susceptibility and XRD measure-
ments, the single crystals were found to contain ferromag-
netic Fe impurities. These impurities were mainly located at
the grain boundaries and could not be separated from the
crystals. The most likely reason for the formation of Fe
impurities is that a reaction, Liq. (K) þ Solid (FeAs) !
Liq. (K+As) þ Solid (Fe), occurs while heating and the
resultant Fe does not dissolve in the flux during the crystal
growth.
To prevent any possible reaction, we then employed the
flux containing a sufficient amount of K+As from the
beginning. The dashed line in Fig. 2(a) represents the solid
solution of FeAs and KAs. In this study, we examined the
mixture of KAs and FeAs with the molar ratio 1:5 : 1, 3 : 1,
Fig. 3. (Color) (a) Photograph of the KFe2As2 single crystal together with
a millimeter scale. (b) Back-reflected Laue photograph of KFe As . The
2
2
incident X-ray beam was applied normal to the cleaved surface. (c) X-ray
diffraction pattern of the KFe2As2 single crystal. Only (00l) peaks are
recognized.
4
: 1, and 6 : 1, which corresponds to the K concentration
of 30, 38, 40, and 43%, respectively. Among them, the K
concentrations larger than 38% yielded a complete melting.
KFe As single crystals can be found inside the grown ingot.
(
a)
2
2
The size of the crystals increases with increasing KAs flux
3
concentration, reaching 15 ꢁ 10 ꢁ 0:4 mm in the case of
KAs : FeAs ¼ 6 : 1. The photograph of the grown crystal is
shown in Fig. 3(a) together with a millimeter scale. SQUID
and XRD measurements indicate that the grown crystals are
free from Fe impurities. We therefore used these crystals for
the characterization and the physical property measure-
ments.
3
.2 Crystallographic characterization
As seen in Fig. 3(a), the grown crystals exhibit a platelike
morphology with flat, shinny, black surfaces. The sample
easily cleaves along these surfaces. An X-ray Laue back-
scattering photograph from the cleaved surface is shown
in Fig. 3(b). A fourfold symmetry is easily recognized,
indicating that the flat surface corresponds to the (001) plane
of the tetragonal KFe As unit cell. Figure 3(c) shows the
(
b)
2
2
X-ray diffraction pattern with the (00l) reflections. The
˚
c-axis lattice constant c ¼ 13:88 A calculated from the
observed peaks is consistent with the literature value
3
)
reported for the polycrystalline samples.
According to the microprobe analysis, the composition
of the grown crystal is estimated as K : Fe : As ¼ 20:84 :
3
7:57 : 41:59, in accordance with the stoichiometric com-
position of 1 : 2 : 2 within the experimental accuracy. The
result suggests that there is no appreciable K deficiency in
our single crystals.
In Figs. 4(a) and 4(b), we show the SEM micrographs of
the single crystal. As seen in Fig. 4(a), the crystal exhibits a
Fig. 4. (Color) SEM micrographs of KFe2As2. (a) Typical single crystal
possessing well-developed facets. (b) (001) face after the cleavage.
Multilayer stacks with edges arraying along the [100] direction are
recognized.
124713-3

J. Phys. Soc. Jpn., Vol. 79, No. 12
K. KIHOU et al.
Fig. 5. Temperature dependence of the magnetic susceptibility for the
KFe2As2 single crystal. Measurement was carried out under the magnetic
field of 10 Oe applied perpendicularly to the c-axis.
Fig. 6. Temperature dependence of the in-plane dc resistivity for the
KFe2As2 single crystal. The inset shows the same data around the
superconducting transition.
tetragonal shape, reflecting the layered, tetragonal crystal temperature (T < 30 K) resistivity follows the Fermi-liquid
2
structure of KFe2As2. The large plane corresponds to the dependence, namely, ꢀabðTÞ ¼ ꢀ0 þ AT . Its physical im-
(
(
001) face and the edges of the crystals are identified as plication is depicted in refs. 9 and 10.
112) facets, which are running along the [110] crystalliza-
The electrical resistivity has decreased gradually at
around 3.8 K. Tc(midpoint) = 3.5 K, and Tc(zero resistivi-
On the (001) face, one can recognize terrace structures ty) = 3.4 K, respectively. The Tc determined by the zero
tion axis.
comprised of multiple layers with a thickness of 5 mm resistivity is consistent with the onset of a diamagnetic
[Fig. 4(b)]. The terrace is formed upon the cleavage of the signal. Accordingly, we employ the Tc ¼ 3:4 K defined by
surface layer. The figures show that KFe2As2 selectively the magnetic susceptibility as the bulk Tc of the obtained
cleaves along the [100] direction (besides the [001] single crystals.
ꢅ
direction), 45 off the growth facets running along the
[
4.
Conclusions
110] axis.
We have successfully grown centimeter-sized platelet
3
.3 Physical properties single crystals of KFe2As2 using a self-flux method. An
Figure 5 shows the temperature dependences of the zero- encapsulation technique using a stainless steel container
field-cooling (ZFC) and field-cooling (FC) magnetic sus- was developed, which allowed the stable crystal growth
ceptibilities (ꢂ) under the magnetic field of 10 Oe applied to last for more than 2 weeks. Ternary K–Fe–As systems
perpendicularly to the c-axis. Both ZFC and ZF curves show with various starting compositions were examined to
a well-defined drop at 3.4 K, indicative of the onset of establish the optimal growth conditions. The employment
superconducting transition at this temperature. The 10–90% of KAs flux led to the growth of large single crystals with
3
transition width is as small as 0.2 K. The Tc value and a typical size of as large as 15 ꢁ 10 ꢁ 0:4 mm . The grown
superconducting transition width of the present sample crystals exhibit a sharp superconducting transition at 3.4 K
1
1,26)
are superior to those reported in the literature.
The with the transition width 0.2 K as well as the residual
volume fraction estimated from the ZFC value below 3 K resistivity ratio exceeding 450, evidencing the good sample
is approximately 100%, indicating the bulk nature of the quality.
superconductivity. The sharp superconducting transition,
Acknowledgements
higher T , and large volume fraction signify the good quality
c
of the present samples. We also note that T is precisely
The authors thank P. M. Shirage, H. Kito, Y. Yoshida,
reproducible. There is no appreciable difference in Tc M. Ishikado, T. Terashima, M. Kimata, S. Uji, T. Yoshida,
within 0.2 K) among the grown samples. The signal above A. Fujimori, K. Hashimoto, T. Shibauchi, Y. Matsuda,
Tc is nearly zero, indicating that the sample is free from and H. Kawano-Furukawa for valuable discussions and
magnetic impurities. K. Shimada for SEM measurements. This work was sup-
c
(
Figure 6 shows the typical in-plane dc resistivity [ꢀabðTÞ] ported by Grants-in-Aid for Specially Promoted Research
of the grown crystal. The resistivity shows metallic (20001004) and for Scientific Research on Innovative
(
dꢀ =dT > 0) behavior in the entire temperature range. Areas ‘‘Heavy Electrons’’ (No. 20102005) from the
ab
The RRR, which is defined by ꢀ ð300 KÞ=ꢀ ð5 KÞ, is as Ministry of Education, Culture, Sports, Science and
ab
ab
large as 458, indicating that the present sample is extremely Technology of Japan and by a Grant-in-Aid for Scien-
clean, almost free from elastic impurity scattering. The tific Research C (No. 22540380) from the Japan Society
number is much larger than the value of 87–88 reported on for the Promotion of Science and by Mitsubishi Foun-
1
1,26)
the crystals grown by the FeAs flux method.
The low- dation.
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