Physica C 470 (2010) S307–S308
Physica C
Superconductivity of ‘‘1 1 1”-type iron arsenide compounds
*
X.C. Wang, Q.Q. Liu, L.X. Yang, Y.X. Lv, W.B. Gao, S.J. Zhang, R.C. Yu, C.Q. Jin
Institute of Physics, Chinese Academy of Science, Beijing 100080, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 16 November 2009
Available online 22 December 2009
‘‘1 1 1”-type iron arsenide compounds AFeAs (A = Li, Na) were synthesized. These compounds crystallize
into Cu2Sb type tetragonal layered structure with space group P4/nmm and show superconductivity with
the transition temperature ꢀ18 K for LiFeAs and ꢀ23 K for NaFeAs. The effect of pressure on LiFeAs sam-
ple was studied. It was found that the Tc decreased almost linearly with increasing pressure and the pres-
sure derivative dTc/dP was about À1.38 K/GPa.
Keywords:
‘‘1 1 1” Iron superconductor
Transition temperature
Pressure effect
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
quartz tubes. They were then sintered at 800–900 °C for 15 h.
The reaction products were pressed into pellets after been ground.
Since the discovery of superconducting LaFeAsO with Tc 26 K
[1], great interests have been stimulated in exploring new super-
conductors and studying the novel superconducting mechanism
in the iron arsenide system. When La atoms are substituted by
other rare earth elements, the Tc has been greatly enhanced up to
more than 50 K. The RFeAsO (R = rare earth elements, termed
‘‘1 1 1 1” type) superconductor has a layered crystal structure
belonging to the tetragonal P4/nmm space group. The [LaO] layers
are sandwiched between the [FeAs] layers. Soon after the discov-
ery, another type iron based superconductors AFe2As2 (A = alkaline
earth metal, termed with ‘‘122” type) are found, which crystallize
into a tetragonal ThCr2Si2-type structure [2]. LiFeAs compounds
are found to be the third type iron arsenide superconductors
(termed ‘‘1 1 1” type) [3]. The crystal structures of ‘‘1 1 1” type is
similar to those of ‘‘1 1 1 1” type superconductors with [LaO] layers
are substituted by Li layers. Here we report the synthesis and the
properties of ‘‘1 1 1” type iron arsenide superconductors.
The pellets wrapped with Ta foil were sealed in quartz tube under
Ar. The samples were annealed at 700–800 °C for 30 h again. All
preparative manipulations were carried out in a glove box pro-
tected with high purity Ar.
The magnetic properties of the samples were measured using a
superconducting quantum interference device (SQUID). The resis-
tance was measured by four-probe electrical conductivity methods
in a diamond anvil cell made of CuBe alloy.
3. Results and discussion
The ‘‘1 1 1”-type AFeAs compounds crystallize into Cu2Sb type
tetragonal layered structure with space group P4/nmm [4], which
is shown in Fig. 1. These ‘‘1 1 1”-type superconductor have a simple
crystal structure that makes them more like an ‘‘infinite layer
structure” comparing with the prototype ACuO2 for high Tc
cuprate.
Fig. 2 shows the dc magnetic susceptibility for samples of LiFeAs
and NaFeAs, which are measured in both zero field cooling (ZFC) &
field cooling (FC) mode with a H = 30 Oe. For LiFeAs sample there is
a sharp diamagnetic transition at 18 K. While for NaFeAs sample
the magnetic susceptibility transition is broad and the diamagnetic
transition temperature is about 11 K. The similar behavior was ob-
served by other report [5]. The inset of Fig. 2 shows the difference
of FC and ZFC susceptibility for the NaFeAs sample. The tempera-
ture where the ZFC and FC curve converges is about 23 K, which
is consistent with the Tc suggested by the resistance measurement
[6]. So the superconducting transition for the NaFeAs sample might
occur at about 23 K.
2. Experimental
The polycrystalline samples of nominal composition AFeAs
were synthesized by solid state reaction method using Li or Na,
FeAs as starting materials. The FeAs precursors were synthesized
from high purity Fe and As powders that were sealed into an evac-
uated quartz tube and sintered at 800 °C for 60 h. The obtained
FeAs powder was pressed into pellets. Then the pellets and Li or
Na pieces were put into alumina crucible according to nominal for-
mula AxFeAs (x is 1.2 for Li and 1.5 for Na) and finally sealed in
The pressure dependence of Tc curve for LiFeAs sample is shown
in Fig. 3. Tc value decreases linearly with increasing pressure and
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0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.