Preparation of REFeAsO1-xFx (R E=Sm and Gd) superconductors at a relatively low temperature

Article abstract of DOI:10.1016/j.jpcs.2010.10.041

A series of the SmFeAsO_1-xF_x and GdFeAsO _1-xF_x (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF₃ as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO_1-xF_x and GdFeAsO_1-xF _x, respectively. These temperatures are at least 5060° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in T_c proved that this low temperature process was more effective to introduce fluorine into R_EFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO_1-xF_x with x=0.05 and at 22 K for GdFeAsO_1-xF_x with x=0.1. The highest T_c was detected to be 54 K in SmFeAsO_0.8F_0.2 and 40.2 K in GdFeAsO_0.75F_0.25. The use of the nano-scaled ReF₃ compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.

Full text of DOI:10.1016/j.jpcs.2010.10.041

Journal of Physics and Chemistry of Solids 72 (2011) 449–452
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Preparation of REFeAsO1ꢀxFx (RE¼Sm and Gd) superconductors at
a relatively low temperature
n
Y.J. Cui a, Y.L. Chen a, C.H. Cheng b, Y. Yang a, Y.Z. Wang a, Y.C Li a, Y. Zhao a,b,
a Key Laboratory of Advanced Technology of Materials, Ministry of Education of China, Superconductivity R&D Center (SRDC), Mail Stop 165#, Southwest Jiaotong University, Chengdu
610031, China
b School of Materials Science and Engineering, University of New South Wales, Sydney 2052 NSW, Australia
a r t i c l e i n f o
a b s t r a c t
Available online 15 October 2010
A series of the SmFeAsO1ꢀxFx and GdFeAsO1ꢀxFx (x¼0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared
using nano-scaled ReF3 as the fluorine resource at a relatively low temperature. The samples have been
sintered at 1100 and 1120 1C for SmFeAsO1ꢀxFx and GdFeAsO1ꢀxFx, respectively. These temperatures are
at least 50–601 lower than other previous reports. All of the so-prepared samples possess a tetragonal
ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in Tc proved that
this low temperature process was more effective to introduce fluorine into REFeAsO. Superconducting
transition appeared at 39.5 K for SmFeAsO1ꢀxFx with x¼0.05 and at 22 K for GdFeAsO1ꢀxFx with x¼0.1.
The highest Tc was detected to be 54 K in SmFeAsO0.8F0.2 and 40.2 K in GdFeAsO0.75F0.25. The use of the
nano-scaled ReF3 compounds has improved the efficiency of the present low temperature method in
synthesizing the fluorine-doped iron-based superconductors.
Keywords:
A. Superconductors
D. Crystal structure
D. Superconductivity
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
fluorine, which would be more serious with the increase in sintering
temperature would accelerate the cracks of the quartz tube.
The discovery of superconductivity at 26 K in LaFeAsO1ꢀxFx
compounds [1] has raised a great many interests in the iron-based
superconductors. The superconducting transition temperatures of the
doped iron-based superconductors have promptly increased from 26
[1] to 56 K [2], but no further increase in the following two years.
Consequently, more and more groups have focused on the
experimental and theoretical researches on the iron-based super-
conductors [3–16]. In the experimental researches, more efficient and
convenient preparation process is needed for synthesizing samples
with high quality. F-doped REFeAsO samples are usually sealed into the
quartz tube and then heated at temperatures higher than 1150 1C, even
to 1200 1C [3–8,13–17], under ambient pressure. The temperature of
1150 1C was critical for the quarz tube continuously used under high
temperature. In that case, it would be very likely that the quartz tubes
crack during the preservation or cooling process, leading to the failure
of the experiments. This is an incontrovertible problem in the solid
state preparation process for REFeAsO systems. For the F-doped
REFeAsO compounds, another problem during preparation process
should also be taken into consideration. The vaporization of the
On the other hand, a large departure between the real and
nominal F doping content usually exists in REFeAsO1ꢀxFx samples
prepared using the traditional methods [2–4]. This departure of the
fluorine content could be attributed to the vapors of fluorine in the
raw materials and the incomplete reactions between the fluorine
and other raw materials. Consequently, reducing the sintering
temperature and improving the reactions between the raw
materials would improve the quality of the sample.
In the present work, a series of the REFeAsO1ꢀxFx (x¼0.05, 0.1,
0.15, 0.2, 0.25) samples has been prepared using a new developed
low temperature method. The nano-scaled REF3 has been used as
the source of fluorine in the present method and consequently, the
sintering temperature drop to below 1120 1C. According to the
experiment results, the present method is proved to be more
efficient than the previous method.
2. Experimental
Polycrystalline samples were prepared by the solid-state reac-
tion using fine powders of REAs, Fe, Fe2O3, FeAs and REF3 with high
purity (Z99%). REAs was obtained by reacting RE pieces and
As powders at 500 1C for 15 h and then at 900 1C for 12 h. The
compound REF3 was prepared by mixing an aqueous solution
containing acetates RE(CH3COO)3 with an aqueous NH4F solution
in an ultrasonic bath at room temperature. The acetate
n Corresponding author at: Key Laboratory of Advanced Technology of Materials,
Ministry of Education of China, Superconductivity R&D Center (SRDC), Mail Stop
165#,Southwest Jiaotong University, Chengdu 610031, China.
Tel.: +86 28 87600786; fax: +86 28 87600785.
E-mail address: yzhao@swjtu.edu.cn (Y. Zhao).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.10.041

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Y.J. Cui et al. / Journal of Physics and Chemistry of Solids 72 (2011) 449–452
Fig. 1. X-ray diffraction patterns of the SmF3 and GdF3 samples with high purity.
RE(CH3COO)3 solution was prepared by dissolving the correspond-
ing RE2O3 in diluted acetic acid. NH4F was used with 10 wt% excess
relative to its stoichiometric quantity. Usually the resulting
precipitate was very fine and had a gel-like appearance. After
precipitation, the product was therefore put into a drying oven
overnight at 70 1C to evaporate the water. At these conditions the
byproduct, ammonium acetate, decomposes and sublimates. After
this step, the samples were annealed at 300 1C for 10 h. High purity
and nano-scaled grains REF3 (agglomerate structure) could be
detected by the X-ray diffraction patterns (see Fig. 1) and SEM
images (shown in Fig. 2), respectively.
Powders of the raw materials with the stoichiometric atomic
ratio of REFeAsO1ꢀxFx (RE¼Sm, Gd) where x¼0.05, 0.1, 0.15, 0.2, .25
were well mixed in an inert atmosphere (inside the glove box) and
then pressed into pellets. The pellets were wrapped with Ta foil and
sealed in an evacuate quartz tube. SmFeAsO1ꢀxFx and GdFeAsO1ꢀxFx
were then annealed for 48 h at 1100 1C and 1120 1C in Ar atmo-
sphere, respectively. The crystal structure was studied by powder
X-ray diffraction (XRD) using an X’Pert MRD diffractometer with
Cu Ka radiation. All observed reflections were indexed. Lattice
constants were determined from LeBail refinements. Microstructure
and composition of the sample were analyzed using a scanning
electron microscope (SEM, Quanta 200, FEI) equipped with an
energy dispersive spectrometer (EDS) spectrometer. DC magnetiza-
tion was measured with a SQUID magnetometer (MPMS, Quantum
Design) under dc mode and resistivity measurement was performed
with a physical property measurement system (PPMS, Quantum
Design).
Fig. 2. SEM images of the nano-scaled grains of SmF3 and GdF3 (agglomerate
structure).
3. Results and discussion
The XRD patterns of the present REFeAsO1ꢀxFx (RE¼Sm and Gd)
samples with x¼0.1 and 0.15 are shown in Fig. 3. The main peaks
can be well indexed based on the tetragonal ZrCuSiAs-type
structure with the space group P4/nmm. The microstructures of
these samples are all presented to be layered structures as the
previous reports [17,18]. No impurities and only a tiny amount of
GdOF have been detected in the samples of SmFeAsO0.9F0.1 and
GdFeAsO0.9F0.1. Small amount of REAs and REOF emerge when
xZ0.15 and the content of the impurities increased with increase
in doping level. In the previous reports on series of REFeAsO1ꢀxFx
samples [3–8], the impurities emerge at the doping level of
x¼0.075 and a large amount of REOF impurity phase would be
observed when the F doping level reaches 0.15. Compared with
these result, REOF in our samples is less which might suggest an
efficient fluorine doping effect in our present process.
The variation of the lattice parameters as a function of x for these
two series of samples has been displayed in Fig. 4. Both of a and c
shrunk systematically with the doping level in the range
0rxr0.15. For SmFeAsO1ꢀxFx, a and c then remain nearly
constant when xZ0.15, indicating a saturation of the F doping
content. The chemical phase boundary might exists in the range of
0.1oxo0.15, which is smaller than the previous report [13–16].
For GdFeAsO1ꢀxFx, the lattice parameters of the samples are
˚
definitely smaller than the previous report, a¼b¼4.001 A and
˚
c¼8.650 A, for GdFeAsO0.83F0.17 [4]. A slighter suppression of the
lattice parameters could still be observed in samples with x40.15,
suggesting the continuous fluorine doping in the GdFeAsO. The
present low temperature process would dramatically depress the
lattice parameters than the high one, which suggests the present
process is more effective to introduce F into the SmFeAsO system.

Y.J. Cui et al. / Journal of Physics and Chemistry of Solids 72 (2011) 449–452
451
a trace of superconducting transition around 9 K has been observed
in the sample with x¼0.05. A superconducting transition with
zero-resistivity at 22, 37.5, 40.1 and 40.2 K for samples with x¼0.1,
0.15, 0.2 and 0.25 has also been observed. The value of Tc for
GdFeAsO0.85F0.15 determined from the M–T curves is even 10 K
higher than that in GdFeAsO0.83F0.17 reported in Ref. [4]. The
changes of Tc shows closely related to the variation of the lattice
parameters. When the lattice parameter shrinks systematically or
remain constant with the F doping level, the increase Tc changes in
the same behavior.
The relatively lower chemical phase boundary, dramatically
suppression of lattice parameters and the higher Tc in the so-
prepared samples indicate that the present synthesizing process
for REFeAsO1ꢀxFx is more efficient to introduce F into the REFeAsO
compounds than the previous results [3,13–16]. According to our
research, the use of nano-scaled REF3 as the source of F in the present
process would improve the activity of the fluoride, which could make
reactions between the fluoride and other raw materials much easier.
Further more, the melting and boiling point of REF3 is definitely
higher thanthe FeF3 used in the traditional high temperature process.
In this case, the high boiling point of the fluoride together with the
relatively low sintering temperature diminished the volatilization of
the fluorine. Accordingly, nearly all of the fluorine would react with
other raw materials and consequently, less fluorine would react with
the quartz tube. The slighter reactions between the fluorine and the
quartz tube together with the relatively low sintering temperature
reduced the crack possibilities of the quartz tube, inducing the
improvement of the experiment efficiency. To our experience, the
REF3 might also act as flux during the sintering process since the
fluorides are usually used as flux in the molten salt method. For
further exploration, we have investigated the superconductivity of
Fig. 3. XRD patterns of the present REFeAsO1ꢀxFx (RE¼Sm and Gd) samples with
x¼0.1 and 0.15.
extra-fluorine doped SmFeAsO0.8F0.2+
(d¼0, 0.2, 0.4) samples. Extra
d
SmF3 has been mixed into the raw materials during the preparation
of these samples. The Tc increased with increase in extra F in the
raw materials and it has been enhanced to around 55 K for
SmFeAsO0.8F0.6. This transition temperature at 55 K has reached
the highest value for F-doped SmFeAsO sample prepared under
high pressure [19]. On the other hand, the crystal size increased
with the increase in extra SmF3 content. Ofcourse, this extra SmF3
content must be maintained in a certain level. Further increase in
SmF3 would lead to the melt of the tantalum wrap and would destroy
superconductivity of the SmFeAsO1ꢀxFx samples. Above results
indicate that the REF3 quite probably acts as flux during the
sintering process, which would be helpful to achieve super-
conducting samples with higher Tc at low preparing temperature.
The advantages of the usage of nano-scaled REF3 mentioned
above indicated that the present low temperature process might
become a potential method to prepare the F doped iron-based
superconducting polycrystalline or single crystal samples.
Fig. 4. Variation of the lattice parameters a and c as a function of x in REFeAsO1ꢀxFx.
4. Conclusions
Fig. 5. Tc versus x in SmFeAsO1ꢀxFx and GdFeAsO1ꢀxFx.
In summary, superconducting REFeAsO1-xFx (RE¼Sm and Gd)
samples can be successfully prepared at a relatively low tempera-
ture of 1100 and 1120 1C using nano-scaled REF3 as the source of
fluorine. In the present method, nano-scaled REF3 is more effective
to introduce F into REFeAsO system with relatively lower chemical
phase boundary, dramatically suppression of lattice parameters
and the higher Tc. The use of the nano-scaled REF3 compound has
enhanced the reaction activities of the raw materials, weakened the
volatilization of the fluorine and consequently diminished the
possibilities of crack of the quartz tube, reduced the sintering
temperature. All of these advantages proved that the present
low temperature method might become a potential and more
effective technique to synthesize F-doped REFeAsO samples.
Fig. 5 shows the superconducting transition temperature
dependent on the F doping level x. The diamagnetic transition
for all of the samples seems to be sharper and clearer than the
report [8], indicating a great homogeneity of the samples. The Tc
onset reaches 39.5 K for SmFeAsO0.95F0.05 and 47.95 K for
SmFeAsO0.9F0.1, respectively. The Tc value in our present research
is definitely higher than the previous reports [13–16] among which
the best result shows Tc¼35.3 K at x¼0.05 [8]. The Tc has been
finally improved to around 53–54 K for the samples with the
doping level at x¼0.15, 0.2 and 0.25. For GdFeAsO1ꢀxFx samples,

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Y.J. Cui et al. / Journal of Physics and Chemistry of Solids 72 (2011) 449–452
Further explorations on the flux effect of REF3 might provide a more
efficient method to prepare the single crystals of the fluorine-
doped iron-based superconductors.
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The authors are grateful for the financial support of the National
Natural Science Foundation of China under Grant nos. 50588201,
50872116, 11004162, PCSIRT of the Ministry of Education of China
(IRT0751), the National High-Tech Program of China (Program 863
under Grant no. 2007AA03Z203), Research Fund for the Doctoral
Program of Higher Education of China (SRFDP200806130023)
the Fundamental Research Funds for the Central Universities
SWJTU09ZT24 and Australian Research Council under Grant nos.
DP0559872 and DP0881739. Yajing Cui is grateful for the financial
support of scientific research starting project for young teachers of
Southwest Jiaotong University (2009Q009) and the Fundamental
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