Single crystal growth of the new pressure-induced-superconductor CrAs via chemical vapor transport

Article abstract of DOI:10.1016/j.jallcom.2016.03.236

Mono-arsenide CrAs, endures a helical anti-ferromagnetic order transition at ～265 K under ambient pressure. Recently, pressure-induced-superconductivity was discovered vicinity to the helical anti-ferromagnetic order in CrAs [Wei Wu et al., Nature Communications 5, 5508 (2014).]. However, the size of crystal grown via tin flux method is as small as 1 mm in longest dimension. In this work, we report the single crystal growth of CrAs with size of 1 × 5 × 1 mm³ via chemical vapor transport method and its physical properties.

Full text of DOI:10.1016/j.jallcom.2016.03.236

Journal of Alloys and Compounds 677 (2016) 57e60
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Single crystal growth of the new pressure-induced-superconductor
CrAs via chemical vapor transport
Xiangde Zhu*, Langsheng Ling, Yuyan Han, Junmin Xu, Yongjian Wang, Hongwei Zhang,
Changjin Zhang, Li Pi, Yuheng Zhang
High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, 230031, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mono-arsenide CrAs, endures a helical anti-ferromagnetic order transition at ~265 K under ambient
pressure. Recently, pressure-induced-superconductivity was discovered vicinity to the helical anti-
ferromagnetic order in CrAs [Wei Wu et al., Nature Communications 5, 5508 (2014).]. However, the
size of crystal grown via tin flux method is as small as 1 mm in longest dimension. In this work, we report
the single crystal growth of CrAs with size of 1 ꢀ 5 ꢀ 1 mm3 via chemical vapor transport method and its
physical properties.
Received 2 December 2015
Received in revised form
29 February 2016
Accepted 27 March 2016
Available online 29 March 2016
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Chemical vapor transport
Single crystal growth
Superconductivity
Anti-ferromagnetism
1. Introduction
reported as 260e270 K [8], 265 K [9], and 240 K [10]. The transition
is first order, showing a hysteresis between 260 and 270 K [8].
Since the discovery of superconductivity in rare-earth iron
pnictides [1,2], intensive research efforts have been exerted to
explore new superconductor in transition metal oxide pnictides,
pnictides and chalcogenides [3e7], which leads to booming new
superconductors in the past several years. Superconductivity is
often found to be in the vicinity of the anti-ferromagnetic (AFM) in
newly discovered iron based superconductors. Therefore, it is now
widely accepted that the origin of superconductivity is related to
the magnetism in iron based superconductors as well as the high
temperature cuprates superconductors.
Below the transition, the b-axis lattice parameter b increases by
about 4%, while the a-axis lattice parameter (a) and c-axis lattice
parameter (c) decreases by about 0.3% and 0.5%, respectively [11].
Just recently, superconductivity was observed in CrAs under pres-
sure (P) when its helical AFM transition was suppressed, by two
different groups Wei Wu et al. [12] and Hisashi Kotegawa et al. [13].
The maximum superconducting transition temperature (TC) is
about 2.2 K at critical pressure (Pc) for both groups. The P depen-
dent phase diagram is reminiscent of doped iron based supercon-
ductors. Moreover, nuclear quadrupole resonance results indicate
that it is an unconventional superconductor [14].
The single crystals were grown via tin flux method by both two
groups [13,15]. However, the single crystals were as small as about
0.15 ꢀ 1 ꢀ 0.15 mm3, which hinders some further physics mea-
surements. Another mono-arsnide, FeAs has been grown success-
fully via chemical vapor transport (CVT) method [16]. Therefore, it
is worthy to try to grow CrAs single crystal via CVT method. In this
work, we report the single crystal growth of CrAs with typical size
of 1 ꢀ 5 ꢀ 1 mm3 via chemical vapor transport method and its
physical properties.
CrAs, a mono-arsnide, adopts a structure of the MnP type, which
is tetragonal (space group Pnma, 62) with lattice parameters
a ¼ 5.651 Å, b ¼ 3.465 Å, c ¼ 6.209 Å. The crystal structure viewed
along b-axis and c-axis is shown in Fig. 1a and b, respectively.
Obviously, the crystal structure of CrAs can be regarded as stacking
the unit cells along b-axis. The Cr atoms are six-fold coordinated by
the As atoms, and lie in a zig-zag line along a-axis and c-axis. For
decades ago, neutron diffraction studies demonstrate that CrAs
endures a special double helical AFM order transition with a spiral
ꢀ
propagation vector of 0.353$2pc [8]. Its Neel temperature (TN) was
* Corresponding author.
E-mail address: xdzhu@hmfl.ac.cn (X. Zhu).
http://dx.doi.org/10.1016/j.jallcom.2016.03.236
0925-8388/© 2016 Elsevier B.V. All rights reserved.

58
X. Zhu et al. / Journal of Alloys and Compounds 677 (2016) 57e60
Fig. 1. a. The crystal structure viewed from b-axis for CrAs. b. The crystal structure viewed from c-axis for CrAs. c. The experimental and refined powder x-ray patterns of CrAs. The
calculated peak positions are marked as blue bars. The green line below represents the difference between the experimental and refined data. The inset of c shows the photograph
of as-grown CrAs single crystal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Material and methods
high intensity graphite monochromatized CuKa radiation.
Elemental analysis was performed by using energy-dispersive X-
ray spectroscopy (EDS) on an FEI Helios Nanolab 600i. The EDS
results gives a Cr:As ratio of 49.82:50.18, which indicates the good
stoichiometry of the as-grown sample.
2.1. Synthesis
The single crystal of CrAs was successfully grown via chemical
vapor transport method with iodine as the transport agent. Stoi-
chiometric Cr pieces (99.9%, Alfa), As pieces (99.99%, Aladin), and
50 mg iodine (99.99%, Aladin) were mixed and sealed in an evac-
uated quartz tube with diameter of 17 mm, and length of ~20 cm.
Then the tube was put in a two zone horizonal furnace. First, the
2.3. Transport properties and magnetism
Electrical transport measurement was carried out on a Quantum
Design Physical Property Measurement System (PPMS), with the
current applied along b-axis. Data were collected over a tempera-
ture range of 2e300 K. Temperature dependent resistivity mea-
surement were performed using a four-probe configuration. Gold
wires were attached on a polished sample with the electrical
contacts made of silver paint. The magnetization was measured by
a magnetic property measurement system (Quantum Design MPMS
temperature of both zone was slowly heated to 800 ꢁC, and kept for
ꢁ
four days. Second, the source zone was slowly increased to 900 C
for one week before furnace cooling. Single crystals of CrAs in shape
of a helical needle can be found with the size of ~1 ꢀ 5 ꢀ 1 mm3,
which is shown in Fig. 1b. This size is significantly larger than that
of the samples grown via Sn flux method [13,15]. The as-grown
crystal is covered by some black powder, which can be erased
and cleaned. The cleaned single crystal is stable in air, showing
shiny silver-like faces. Arsenic single crystals with hexagonal plate
can also been found as a by-product. The as-grown CrAs crystals
can be easily cleaved along b-axis.
7T-XL) with
(SQUID).
a superconductive quantum interference device
3. Results and discussions
3.1. X-ray diffraction pattern
2.2. X-ray diffraction and elemental analysis
Fig. 1 shows the powder x-ray pattern of the CrAs. The peak
positions are labeled and consistent with the calculated results
from other literature. Some minor peaks can be observed, which
Several single crystals were crushed and ground for powder X-
ray measurement on the Rigaku-TTR3 x-ray diffractometer using

X. Zhu et al. / Journal of Alloys and Compounds 677 (2016) 57e60
59
should be the black oxides covered on the surface.
0.00137 mUcmKꢂ2. With the electronic specific heat coefficient
g
¼ 9.1 from Ref. [15], the Kadowaki-Woods (KW) ratio A/
g
2 can be
evaluated as 1.65 ꢀ 10ꢂ5 mU cm K2 mJꢂ2, which is slightly larger
than the universal line a0 ¼ 1 ꢀ 10ꢂ5 of correlated electron systems.
This demonstrates that CrAs is a correlated electron system.
3.2. Resistivity
Fig. 2 shows the temperature dependence of b-axis resistivity
(rb) with cooling and warming processes. The curves show metallic
behavior with abrupt hysteresis transitions at T ¼ 247 K and
T ¼ 257 K for cooling and warming processes, respectively. The
3.3. Magnetization
residual resistivity ratio (RRR, here we use r(250 K)/r(5 K)) is ~9,
which is smaller than the crystal grown via tin flux method [15].
The transition temperature 247e257 K is slight smaller than
260e270 K for the crystal grown via tin flux method. For the first
cooling process, the rb increases abruptly at T ¼ 246 K with ~80%
Fig. 3 shows the temperature dependence of magnetization
(MꢂT) measured by zero-field cooling process and field-cooling
process with applied field (H) perpendicular and parallel to b-
axis, respectively. Obviously, a transition with hysteresis between
250 K and 258 K can be observed on the MꢂT curves, which is
consistent to the resistivity results. Interestingly, the AFM transi-
tion leads to the decrease of magnetization M for Hkb, while the
AFM transition leads to the increase of M for H⊥b. Below the helical
AFM transition, the magnetization decrease with decreasing tem-
perature, and then reach the minimum around 30 K for both Hkb
and H⊥b. These results are consistent with that reported on the
single crystal grown via Sn flux method [15]. The inset of Fig. 3
shows the magnetic hysteresis of CrAs single crystal measured at
2 K for both Hkb and H⊥b. The MꢂH curves show linear field
dependent relation both for Hkb and H⊥b. Generally, the magne-
tization of our sample shows similar behavior to that of the crystal
grown via tin flux method.
(Drb/rb). For the second warming process, the rb increases with
~68%, rather than return to the previous value for W. Wu et al.'s
sample [12]. It is interesting that rb of W. Wu et al.'s sample de-
creases with decreasing temperature during the transition [12],
which is different from our samples and H. Kotegawa et al.'s sam-
ples [13]. The b-axis lattice parameter expands about 4% after the
helical AFM transition [11], which leads to cracks in the crystal. The
cracks will result in the decrease of cross-section of electrical
transport, which is the origin of resistivity jump at the AFM tran-
sition. Although the RRR and transition temperature is different
from that of the tin-flux grown sample, the resistivity jump ratio
80% of our sample is significantly larger than ~35% for the crystal
grown via tin flux method [13,15]. This indicates our sample should
have larger change on cell volume. In CrAs1ꢂx Px [11], it was found
that the TN is related to the b-axis lattice parameters. The different
single crystal growth condition, growth temperature and cooling
procedure, may lead to different stress in as-grown crystal. The
slight difference on stress and b-axis lattice parameter should be
the origin of these difference mentioned above.
4. Conclusions
In summary, large single crystals of CrAs with dimensions of
~1 ꢀ 5 ꢀ 1 mm3 were successfully grown via the chemical-vapor-
transport method. The phase was identified by powder X-ray. The
electrical resistivity and magnetization were measured. The elec-
trical resistivity results indicate that CrAs grown via chemical-
vapor transport method has some different behavior from the
crystal grown via tin flux method, while magnetization results
indicate that they have similar behavior except the difference on
As shown in the inset of Fig. 2, the r
beT2 curve of CrAs shows
linear relation below T < 20 K, which can be well described as
r
¼
r0þAT2, where r0 is the residual resistivity, and A is a constant.
This indicates that CrAs has a Fermi-liquid ground state, in which
the electron-electron interactions play the major role in the scat-
tering mechanism. Since the cracks occur in the crystal below TN,
the A is not intrinsic. We can estimate the intrinsic A by a correction
factor of rb(247 K)/rb(244 K) ~0.57, since the cracks only reduce the
cross-section of the electrical transport. From the obtained fitting
ꢀ
Neel temperature.
values of
A
¼
0.00241 mUcmKꢂ2
, real A is estimated as
Fig. 3. The temperature dependence of magnetization measured by zero-field cooling
(ZFC, solid symbol) and field-cooling (FC, open symbol) process with applied field (H)
perpendicular (rectangle symbol) and parallel (circle symbol) to b-axis. The H is
1000 Oe in all measurements. The inset shows the magnetic hysteresis of CrAs single
crystal measured at 2 K for both Hkb and H⊥b.
Fig. 2. The temperature dependence of resistivity measured with cooling and warming
process, which is marked as arrows. The inset shows the T2 dependence of resistivity.
The solid line represents the linear fitting results.

60
X. Zhu et al. / Journal of Alloys and Compounds 677 (2016) 57e60
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We are very grateful to and Mrs Hui Han and Dr. Lei Zhang for
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lab (Hefei) was supported by National Natural Science Foundation
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