J. Prakash et al. / Physica C 469 (2009) 82–85
83
in helium vapor and the temperature of the sample was measured
with an accuracy of 0.05 K using a calibrated Cernox sensor wired
to a Lakeshore 340 temperature controller. Standard four probe
technique was used for transport measurements. Contacts were
made using 44 gauge copper wires with air drying conducting sil-
ver paste. The external magnetic field (0–5 T) was applied perpen-
dicular to the probe current direction and the data were recorded
during the warming cycle with heating rate of 1 K/min. The
inductive part of the magnetic susceptibility was measured via
a tunnel diode based rf penetration depth technique [11]. The
sample was kept inside an inductor that formed a part of an LC
circuit of an ultrastable oscillator (ꢁ2.3 MHz). A change in the
magnetic state of the sample results in a change in the inductance
of the coil and is reflected as a shift in oscillator frequency which
is measured by an Agilent 53131A counter. To obtain the super-
conducting fraction, DC magnetization studies were carried out
using a Quantum Design MPMS SQUID magnetometer. Energy dis-
persive analysis by X-rays (EDX) was carried out on sintered pel-
lets using a Zeiss electron microscope in conjunction with a
BRUKER EDX system. The thermoelectric power data were ob-
tained in the bridge geometry across a 2 mm by 3 mm rectangular
disk.
a
b
3. Results and discussion
Fig. 1 shows the XRD pattern for the sample with nominal
composition of CeO0.9F0.1FeAs and CeO0.8F0.2FeAs. All the observed
reflections could be satisfactorily indexed based on the tetragonal
CeOFeAs (space group P4/nmm) phase for the x = 0.1 composition
(pure phase). Small amount of CeAs and Fe2As were observed as
secondary phases for the x = 0.2 composition. The refined lattice
parameters were found to be a = 3.991(1) Å and c = 8.613(3) Å
for x = 0.10 and a = 3.988(3) Å and c = 8.607(8) Å for the x = 0.2
phase. The lattice parameters are smaller than the parent com-
pound CeOFeAs (a = 3.996 Å and c = 8.648 Å) reported earlier [3].
Compared to the undoped phase (CeOFeAs), the reduction of the
lattice volume upon F-doping indicates a successful chemical sub-
stitution. In Fig. 1c we present the energy dispersive X-ray micro-
analysis (EDX) spectrum of one typical grain, which shows that
the main elements of the grains are Ce, Fe, As, O and F. The
spectrum also confirmed 1:1:1 atomic percentage ratio between
Ce, Fe and As. It is thus safe to conclude that the superconductiv-
ity observed here comes from the main phase CeO1ꢀxFxFeAs. There
is a small peak of Si possibly due to the formation of SiO vapor
from the quartz tube employed for the high temperature
synthesis.
20
30
40
2θ
50
60
c
The inset of Fig. 2b shows the variation of resistivity of
CeO0.8F0.2FeAs with temperature. The shape of the curve indicates
a good metallic behavior in the normal state. As shown in the main
1
2
3
4
5
6
7
8
9
10 11 12
Energy (keV)
panel of Fig. 2a and b, for CeO0.9F0.1FeAs (sample a) and CeO0.8F0.2
-
FeAs (sample b), the onset temperatures of the superconducting
transition are found to be 38.4 and 42.5 K respectively. The transi-
tion width is estimated to be ꢁ2 K for sample a and ꢁ7 K for sam-
ple b. The onset temperature for CeO0.8F0.2FeAs is higher than that
reported earlier [3,8], but we observe that the transition is rela-
tively broad. Note that we have used a similar criterion (as eluci-
dated schematically in the Fig. 2) for determination of transition
temperature as has been used elsewhere [9]. We also point out that
using the criteria as mentioned in Ref. [3] would yield a still higher
Fig. 1. Powder X-ray diffraction patterns (XRD) patterns of (a) CeO0.9F0.1FeAs and
(b) CeO0.8F0.2FeAs annealed at 1180 °C. The impurity phases are Fe2As (ꢂ) and CeAs
(O) and (c) A representative EDX spectra of CeO0.9F0.1FeAs.
netic Tc is lower than the resistive Tc and this difference increases
with the width of the resistive transition. Nevertheless, we empha-
size that there are clear indications that the optimized Tc for
CeO0.8F0.2FeAs could be higher than that reported in Ref. [3]. We
have also estimated the superconducting volume fraction (ꢁ75%)
from the DC magnetic susceptibility data (inset of Fig. 2a) of sam-
ple a.
The temperature dependence of resistivity under different mag-
netic fields for (a) CeO0.9F0.1FeAs and (b) CeO0.8F0.2FeAs are shown
in Fig. 3. It is clear that the onset temperature shifts gradually
Tc. The residual resistivity ratio (RRR = q300/q45) for CeO0.8F0.2FeAs
and CeO0.9F0.1FeAs is about 6.67 and 4.34 respectively. This indi-
cates good intergrain connectivity. The normalized v0 plotted in
the insets of Fig. 2a and b attests the onset of bulk diamagnetic
behavior at 38.2 K for sample a and 40.4 K for sample b respec-
tively. As expected, in these polycrystalline superconductors, mag-