Low temperature synthesis and pressure induced
insulator-metal transition of the newly found NdFeAsO0.75
Qingping Ding, Yangguang Shi, Hongbo Huang, Shaolong Tang, Shaoguang Yang*
National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China
From the discovery of superconductivity with transition temperature (Tc)
of 26 K in LaO1-xFxFeAs system,1 iron-based layered compound ReFeAsO
attracted much attention in the scientific community. More and more efforts
have been devoted in the study of this kind of materials. The transition
temperature has soon been enhanced a lot,2-7 now the reported highest Tc is
56.3 K.8 But much work remains in this field, for example, synthesis of such
iron-based compounds in an easy way remains a serious problem. Quebe P.
et al reported a synthesis method at low temperature with KCl/NaCl as
mineralizer,9 but it is difficult to avoid unwanted element contamination
with this method. The synthesis temperature in all other reported methods
was higher than 1150 oC. 1-8,10,11 This makes it difficult in the preparation of
such kind of materials, especially for the strength of the quartz tube for
sealing the vacuum at high temperature. So synthesis of the newly found
tetragonal phase iron-based compounds at relatively low temperature is very
important currently. Here we report an illustration of synthesis of iron-based
compound NdFeAsO0.75 at a temperature of 900
oC. This synthesis
temperature is the lowest among all the reported methods without the help of
any mineralizer. In the materials research region, high pressure is often used
as an effective parameter to tailor the property of materials. In iron-based
superconductors, it has been confirmed that pressure can either enhance or
suppress the Tc by both theories and experiments.12-15 By applying high
pressure to our NdFeAsO0.75 sample prepared at 900 oC, pressure induced
insulator to metal phase transformation in NdFeAsO0.75 is reported. To our
knowledge, this is the first experiment of insulator-metal transition in
iron-based layered compound ReFeAsO.
In our present work, the polycrystalline sample was prepared by the
conventional solid state reaction. NdAs, Fe, Fe2O3 were used as starting
materials and weighed according to the chemical stoichiometry of
NdFeAsO0.75. NdAs was obtained by reacting Nd chips with As pieces at
600 oC for 5 hours and then 900 oC for 10 hours. The raw materials were
thoroughly grinded and pressed into pellets. Then pellets were sealed in an
evacuated quartz tube, and annealed at 900 oC for 36 hours. The prepared
sample was further treated under a pressure of 6 GPa and temperature of
1300 oC for 2 hours.
The structure of the samples prepared at 900 oC and after high pressure
were characterized by a powder X-ray diffraction (XRD) method with Cu
Kα radiation (λ=1.5418 Å) in the 2θ range of 20−80 degree with the step of
0.02 degree at room temperature. Scanning electronic microscopy (SEM)
was used to characterize the morphologies of the samples. The SEM analysis
was performed on a Philips XL30 microscope operated at 20.0 KV. To
realize the temperature dependence of resistivity, standard 4-probe dc
resistivity measurements were preformed from 300 K down to 2 K in a
Physical Property Measurement System (PPMS) of Quantum Design
Figure 1. (a) X-ray diffraction patterns of NdFeAsO0.75 before and after high
pressure treatment, most peaks could be well indexed on the basis of
tetragonal ZrCuSiAs-type structure with the space group P4/nmm. AP:
ambient pressure, without high pressure treatment; HP: after high pressure
treatment. (b) Magnification of peak (102) in figure 1a. Peak (102) shifted
from 30.629 to 30.663 degree, which means that the distance between two
neighboring (102) was compressed after the high pressure treatment.
The XRD patterns for the prepared samples are shown in figure 1a, most
peaks could be well indexed on the basis of tetragonal ZrCuSiAs-type
structure with the space group P4/nmm. Almost pure phase was achieved for
the samples before high pressure treatment. Two weak peaks (maked with *)
assigned to Nd2O3 can be observed in the sample after high pressure
treatment, which shows that very small impurity was formed in the high
pressure process. Part of the highest XRD peak (102) was magnified as
shown in figure 1b. From this figure, it can be found that the (102) peak
moved from 30.629 to 30.663 degree. It means that the distance between two
neighboring (102) was compressed after the high pressure treatment.
Figure 2. SEM images of NdFeAsO0.75 before (a) and after (b) high pressure
treatment. The sample after high pressure treatment shows better
compactness and layered structure feature.
SEM images of NdFeAsO0.75 samples are presented in figure 2. Figure 2a
and figure 2b are typical images of the sample before and after high pressure
treatment. Compared with the sample prepared at 900 oC, the sample after
high pressure treatment shows better compactness and layered structure
feature. These two images were recorded under the same condition, but the
definition of figure 2b is better than that of figure 2a. This means the sample
after high pressure treatment shows better electrical transport ability than the
Figure 3. R-T measurement results of NdFeAsO0.75 before (a) and after (b)
high pressure treatment. The resistivity has decreased 3-4 orders in the
measured temperature range, the sample has been changed from
insulator-like phase to metal-like phase.
Figure 3a shows the temperature dependence of the resistivity of the 900
oC synthesis sample. From this figure, it can be found that the resistivity
decreases with the temperature increase and the dc resistivity value is rather
high. Figure 3b shows the dc resistivity dependence of the temperature of the
sample treated after high pressure. The resistivity increases with the
temperature, and the resistivity is much smaller than before high pressure
treatment at the same temperature. Compared the resistivity results of the
two samples, a very clear difference between them can be found. The
resisitity has decreased 500 times (at 300 K) to 21000 times (at 2K), and the
sample has been changed from insulator-like phase to metal-like phase after
the high pressure treatment.
From the XRD measurements, it can be concluded that both samples
were almost pure tetragonal phase NdFeAsO0.75. The electrical transport
property mainly comes from the tetragonal phase NdFeAsO0.75. Although
very small amount of Nd2O3 was observed in the high pressure treated
sample, this little Nd2O3 should not influence the tendency of the main phase
of the sample. As we know, by modifying the chemical composition or the
lattice parameters the electron correlation strength can be controlled while
essentially keeping the original lattice structure unchanged.16 By control of
the transfer interaction or the one-electron bandwidth, control of electron
correlation strength usually can be achieved. Pressure is often used as an
effective parameter to tailor the one-electron bandwidth. Generally speaking,
applying a pressure decreases the interatomic distance and thus increases the
transfer interaction. This may be the reason of great decrease of the electric
resistivity and the insulator-metal transition.
To summarize, low temperature synthesis method has been successfully
performed in the synthesis of tetragonal phase NdFeAsO0.75. The synthesis
method may be applied for other rare earth substitution iron based tetragonal
phase materials, ReFeAsO1-xFx or ReFeAsO1-x (RE = La, Ce, Pr, Nd, Sm,
Gd) compounds. XRD measurements illustrate that almost pure phase
tetragonal phase NdFeAsO0.75 for both two samples before and after high
pressure treatment. Temperature dependence of resistivity shows that its
electrical transport property has changed from insulator-like phase to
metal-like phase. Although further theoretical research such as first principle
calculation is needed to quantify the great decrease of resistivity and this
insulator-metal transition. The present results add a new and important
feature into the transport property of iron-based layered compound
ReFeAsO. More phase transitions are expected via applying high
temperature high pressure process treatment.
Aknowledgement. This work was supported by the NSFC (10774068),
NCET(07-0430) and 973 Program(2006CB921800).
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