Photon Factory Activity Report 2014 #32 (2015) B

BL-8A, BL-8B/2014S2-003

Structural study of 112-type Ca1-xLaxFeAs2
Hiroshi Sawa1,*, Naoyuki Katayama1
1
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
1 Introduction
Synchrotron x-ray diffraction studies of Ca1-xLaxFeAs2[1]
with monovalent arsenic zigzag chain layers arepresented. While the crystal twins appear in all samples
for the nominal composition of x = 0.25, we successfully
obtained the samples without crystal twins for thenominal composition of x = 0.17.
2 ExperimentSingle crystalline samples of Ca1-xLaxFeAs2 with x =0.17 and 0.25 were synthesized by the conventional solidstate reaction method. Single crystalline x-ray diffraction
data were collected at BL8A beamline at KEK facility(Japan). A typical size of 40 × 40 × 20 µm3 was measured
with the wavelength of 0.6899 Å. The electricalresistivity ρab (parallel to the ab-plane) and magnetization
M were measured using the standard DC four-terminal
method in a physical property measurement system(Quantum Design PPMS) and a SQUID magnetometer(Quantum Design MPMS) equipped at the Institute for
Solid State Physics (ISSP, Japan), respectively.
3 Results and Discussion
Fig.1 shows the single crystal x-ray diffraction patternsof the present material, Ca1-xLaxFeAs2 with nominal
composition of x = 0.25. At a first glance, Bragg peaks
are split by two with streaks in between them, clearly
indicating the appearance of twinned domains. In theexperiments, we checked 40 samples in total and foundthat the twin domains appear in all samples measured.
The twin peaks can be successfully indexed by assumingthat the domains are sharing the plane parallel to bc plane,
as shown in Fig.1. Due to the twin formation in thepresent material with the monoclinic structure of  ~91.4°, the surface of the crystal should be waving, which
can be clearly observed using polarized microscope, as
shown in Fig.1. The structural analysis of the twinned
samples is possible in principle, however, the appearanceof streaks prevent us from obtaining accurate structuralparameters.
On contrast to the samples with nominal composition of
x = 0.25, we successfully obtained samples without any
twinned domains for the nominal composition of x = 0.17.
The single crystal x-ray diffraction patterns clearly
exhibit narrow Bragg peaks without any signs of splitting.
The obtained samples show bulk superconductivity at theonset Tc = 25 K with a shielding volume fraction (VF) of
35% in magnetic susceptibility measurement. The
emergence of the superconductivity can be also
confirmed using electronic resistivity measurement with

Fig.1 (a) Single crystal x-ray diffraction patterns of
Ca1-xLaxFeAs2 with nominal composition of x = 0.25.
Peaks are split due to the crystal twins. (b) Schematicview for the relationship between twinned domains in
the real space. (c) Polarized microscope image for asingle crystal with twinned domains.
the trace superconductivity at the onset Tc = 47 K, whichis the highest record of trace superconductivity among all
112-type superconductors reported so far. The transition
temperatures are gradually suppressed by increasing
magnetic fields. La content x is refined to be 0.182(5)from the single crystal x-ray diffraction experiment,
which is close to the nominal value of x = 0.17. It isnoteworthy that the relationship between bulk Tc
determined from the magnetic susceptibility measurement
and the La content x can be well plotted on the phase
diagram presented in the previous report [2]. Comparedwith the reported lattice parameter of Ca1-xLaxFeAs2 withx = 0.195 [1], we can find the x dependence in lattice
parameters is small. In order to have a further critical
discussion about the deviation of lattice parameters
depending on x, more systematic structural studies shouldbe performed in near future.
References[1] N. Katayama et al., J. Phys. Soc. Jpn. 82 (2013)
123702.
[2] K. Kudo et al., J. Phys. Soc. Jpn. 83 (2014) 093705.
* z47827a@cc.nagoya-u.ac.jp