New oxypnictide superconductors: PrOFe1-xCoxAs

Article abstract of DOI:10.1016/j.jssc.2009.11.026

Oxypnictides of the type PrOFe_1-xCo_xAs (x≤0.3) were synthesized for the first time by the sealed tube method. All the compounds were found to be monophasic and crystallize in the tetragonal ZrCuSiAs type structure (space group=P4/nmm) and the lattice parameters (a and c) decrease with increase in cobalt content. Moessbauer measurements of the compounds indicate low spin Fe²⁺ in tetrahedral coordination. Resistivity and magnetization studies reveal superconducting transitions in compounds with 'x'=0.05, 0.10 and 0.15, with maximum transition temperature (T_c) at ～14 K in the compound with 'x'=0.1. The variation of resistivity with temperature under different magnetic field has been studied to estimate the upper critical field (H_c2) (～50.2 T for the 'x'=0.1 composition). The Seebeck and Hall coefficient (R_H) suggests electron type charge carriers in these compound and the charge carrier density increases with increase in Co-doping.

Full text of DOI:10.1016/j.jssc.2009.11.026

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Journal of Solid State Chemistry 183 (2010) 338–343
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
New oxypnictide superconductors: PrOFe1ÀxCoxAs
Ã
Ã
J. Prakash a, S.J. Singh b, D. Das c, S. Patnaik b, , A.K. Ganguli a,
a Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
b School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
c UGC-DAE Consortium for Scientific Research, Kolkata 700098, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxypnictides of the type PrOFe1ÀxCoxAs (xr0.3) were synthesized for the first time by the sealed tube
method. All the compounds were found to be monophasic and crystallize in the tetragonal ZrCuSiAs
type structure (space group=P4/nmm) and the lattice parameters (a and c) decrease with increase in
cobalt content. Mo¨ssbauer measurements of the compounds indicate low spin Fe2+ in tetrahedral
coordination. Resistivity and magnetization studies reveal superconducting transitions in compounds
with ‘x’=0.05, 0.10 and 0.15, with maximum transition temperature (Tc) at ꢀ14 K in the compound with
‘x’=0.1. The variation of resistivity with temperature under different magnetic field has been studied to
estimate the upper critical field (Hc2) (ꢀ50.2 T for the ‘x’=0.1 composition). The Seebeck and Hall
coefficient (RH) suggests electron type charge carriers in these compound and the charge carrier density
increases with increase in Co-doping.
Received 29 September 2009
Received in revised form
18 November 2009
Accepted 23 November 2009
Available online 2 December 2009
Keywords:
Oxypnictide superconductors
Mo¨ssbauer effect
Hall effect
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
tetragonal to orthorhombic/monoclinic symmetry followed by a
magnetic transition leading to a spin density wave (SDW) with
antiferromagnetically coupled ‘Fe’ spins. There have been some
reports that the magnetic rare-earth spins like Pr3+ in PrOFeAs
order antiferromagnetically at much lower temperature (ꢀ14 K)
with the Pr moments aligned along the c-axis [15].
The discovery of superconductivity at 26 K in an unexpected
FeAs-based oxypnictide compound (La(O/F)FeAs) [1] has attracted
a great deal of attention among the scientific community. The
family of the FeAs-based superconductors can be divided into
three classes. The first class of these superconductors has the
general formula LnOFeAs (Ln=the rare-earth metals), which is
abbreviated as FeAs-1111 family. The second class has a general
formula AFe2As2 (A=K, Ca, K/Sr and Ca/Sr) and is denoted as A122
systems [2–3]. The third type MFeAs (M=alkali metals) has an
infinite layered structure and denoted as M111 system [4–5]. For
FeAs-1111 phase, the transition temperature (Tc) has been found
to increase with the substitution of smaller rare-earth metal ions
like Ce [6–7], Pr [8], Nd [9], Sm [10] and Gd [11] with the highest
Tc reported for Sm(O/F)FeAs at 55 K [12]. These compounds with
the general formula LnOFeAs (Ln=rare-earth metal) crystallize in
the tetragonal ZrCuSiAs type structure (space group=P4/nmm)
[13], which is a filled variant of the well-known PbFCl type
structure consisting of alternate layers of Ln–O and Fe–As layers
where the former acts as a charge reservoir and the latter acts as a
charge carrier [14]. The parent LnOFeAs compounds are semi-
metallic in nature and show an anomaly around 130–150 K
(depending on the rare-earth metal) in the resistivity and dc
magnetization measurements due the structural distortion from
The structural distortion and SDW in LnOFeAs compounds can
be suppressed with the evolution of superconductivity on doping
with electrons or holes. Electrons are injected in the FeAs layers in
the oxypnictides by replacing O2À ion with FÀ ion [1] or Ln3+ with
Th4+ [16] ions and also by introducing oxygen vacancies [17].
Relatively fewer examples of hole doping (substitution of Ln3+
ions by Sr2+) are known in the literature in 1111-system. The
highest transition temperature achieved by hole doping is 26 K
[18] as compared to 55 K in electron-doped superconductors. We
have earlier shown that superconductivity can be realized by
doping KF and NaF in LaOFeAs [19–20]. We have also reported the
enhancement in transition temperature (Tc) and upper critical
field (Hc2) in Sb-doped La(O/F)FeAs [21]. It is interesting to note
that superconductivity can also be induced by cobalt doping at
the iron site [22–24] in LnOFeAs and AFe2As2, which is a
surprising fact since the transition temperature is lowered in
the well-known cuprate superconductors on substitution of
magnetic ions at the Cu sites. The structural distortion and SDW
in LnOFeAs and AFe2As2 type compounds can be suppressed by
the substitution of cobalt ions at the iron sites. The maximum Tc
in Co-doped LnOFeAs compound was observed at ꢀ15 K [24] in
SmO(Fe/Co)As superconductor, which is quite low as compared
to Tc observed in Sm(O/F)FeAs (max Tc=55 K)[12] compound. On
the other hand the maximum Tc was observed at 22 K [25] in
Ã
Corresponding authors. Fax: +91 11 26854715.
E-mail addresses: spatnaik@mail.jnu.ac.in (S. Patnaik),
ashok@chemistry.iitd.ernet.in, ashok@chemistry.iitd.ac.in (A.K. Ganguli).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.11.026

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339
cobalt-substituted Ba(Fe/Co)2As2 (FeAs-122). The substitution of
cobalt at iron sites in AFe2As2 compounds leads to decrease in
the c-lattice parameter while the a-lattice parameter remains
nearly constant [26]. A similar trend was observed in case of
cobalt-doped LnOFeAs [22,24].
In this paper, we have investigated the effect of cobalt doping
at the iron sites of semimetallic PrOFeAs on the structural and
transport properties. The transition temperature (Tc) varies with
Co-content with a maximum Tc of ꢀ14.2 K in PrOFe0.9Co0.1As. We
present temperature-dependent thermoelectric power (TEP) and
Mo¨ssbauer spectroscopic data complemented by Hall coefficient
data on the above oxypnictides. The upper critical field has also
been estimated of these novel superconductors.
In an effort to minimize errors, we have used very thin,
undeformed, pieces of the sintered disk for the Hall measure-
ments. No knowledge of the geometric dimensions of the sample
is needed in this procedure to calculate the thermopower, the
only requirement being an accurate measure of the temperature
gradient and voltage between the same points of the sample.
3. Results and discussion
Fig. 1(i) shows the powder X-ray diffraction (PXRD) patterns of
the compounds with nominal composition, PrOFe1ÀxCoxAs
(0oxo0.3). All the observed reflections could be satisfactorily
indexed based on the tetragonal ZrCuSiAs (space group=P4/nmm)
type structure and hence testify the monophasic nature of these
compounds. The refined c-lattice parameter shrinks significantly
while the a-lattice parameter decreases slightly with an increase
in Co-substitution (Fig. 1(ii)). Similar behavior has also been
observed earlier in the compounds with other rare-earths (Ln=La,
Ce and Sm) [22–23] reflecting the substitution of Co2+ in place
2. Experimental
Polycrystalline samples with nominal compositions of
PrOFe1ÀxCoxAs were synthesized by sealed tube method using
high-purity Pr, Pr6O11, Co3O4, FeAs and As. FeAs was obtained by
heating Fe and As chips in evacuated sealed silica tubes at 800 1C
for 24 h. The reactants were weighed in stoichiometric amounts
and sealed in evacuated silica (10À4 Torr) tubes and heated at
950 1C for 24 h. The resulting powder was ground and compacted
into disks. The disks were wrapped in Ta foil and sealed in
evacuated silica ampoules and sintered at 1100 1C for 48 h and
then cooled to room temperature. All the chemical manipulations
were performed in a nitrogen-filled glove box. Powder X-ray
diffraction (PXRD) studies were carried out on a Bruker D8
of Fe2+
.
The 57Fe Mo¨ssbauer spectra for PrOFeAs and PrOFe0.9Co0.1As
compounds as shown in Fig. 2. The corresponding hyperfine
parameters are given in Table 1. The observed isomer shifts (d)
can be attributed to low spin Fe2+ state in tetrahedral
x = 0.3
Advance diffractometer with Ni-filtered CuK
a radiation using a
scan speed of 1 s and scan step of 0.051. The lattice parameters
were obtained from a least squares fit to the observed d values
using the PowderCell software [27].
x = 0.2
Mo¨ssbauer spectra were recorded in transmission geometry
using a constant acceleration-type Mo¨ssbauer spectrometer with
a 10-mCi 57Co source in Rh matrix for 57Fe spectra. A Xe-filled
proportional counter was used as the detector. The data were
acquired in the MCS mode in a multichannel analyzer containing
1024 channels. The velocity scale was calibrated using a 57Co
source and a metallic iron foil absorber. Mo¨ssbauer spectra of all
the samples were recorded at 298 and 20 K. The low temperature
measurements were carried out using a Janis, closed cycle
refrigerator fitted with a special anti-vibration stand. The spectra
were fitted by a least squares fitting program with Lorentzian
profiles. All the isomer shift values were calculated with respect
x = 0.15
x = 0.1
x = 0.05
x = 0
20
30
40
50
60
2θ (degree)
to the center of the a-Fe spectrum.
4.000
3.998
3.996
3.994
3.992
3.990
3.988
3.986
3.984
8.62
Four probe resistivity, magnetic and Hall-effect measurements
were carried out using a Cryogenic 8 T Cryogen-free magnet in
conjunction with a variable temperature insert (VTI). Contacts
were made using 44 gauge copper wires with air drying
conducting silver paste. For magnetic measurements, the mag-
netic field (0–5 T) was applied perpendicular 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 (2.3 MHz)
penetration depth technique [28]. A change in magnetic state of
the sample results in a change in the inductance of the coil and is
reflected as a shift in the oscillator frequency, which is measured
by an Agilent 53131A counter. The thermoelectric power
measurement was carried out by bridge geometry in the
temperature range 13–300 K. The samples were directly attached
to two Au/Fe+Chromyl thermocouple using silver paint. The
8.61
8.60
8.59
8.58
8.57
8.56
8.55
8.54
8.53
0.0
0.1
0.2
0.3
Co content (x)
voltage difference,
was measured by a Keithley 2182 Nanovoltmeter using copper
wires.
DV, between hot and cold ends of the sample
Fig. 1. (i) Powder X-ray diffraction patterns (PXRD) of PrOFe1ÀxCoxAs sintered at
1100 1C and (ii) variation of lattice parameters by short bars (ꢀ0.1%) with doping
for PrOFe1ÀxCoxAs.

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undergoing magnetic relaxation and undergoes magnetic
ordering though it did not go to the blocked state till 20 K.
Fig. 3(i) shows the temperature dependence of resistivity for
(a) PrOFeAs, (b) PrOFe0.95Co0.05As, (c) PrOFe0.9Co0.1As, (d)
PrOFe0.85Co0.15As, (e) PrOFe0.8Co0.2As and (f) PrOFe0.7Co0.3As. For
the parent compound, an anomaly characterized by a drop in
resistivity was observed at ꢀ155 K, which is clearly observed
from the temperature dependence of the derivative of the
resistivity as shown in Fig. 4, which is qualitatively similar to a
previous report [30]. This anomalous resistivity drop is associated
with a structural distortion from tetragonal to orthorhombic
symmetry. Below the occurrence of SDW, the resistivity drops
steeply and then becomes almost independent of temperature,
below ꢀ84 K. A down turn was observed below ꢀ10 K, which is
attributed to the decrease in scattering due to the ordering of the
Pr moments. Upon Co-doping, the resistivity shows a drastic
change and there was no observation of any anomaly due to
structural distortion or SDW formation. Superconducting
1.00
0.99
0.98
0.97
0.96
0.95
0.94
0.93
PrOFeAs
20 K
100 K
RT
-10
-5
0
5
10
15
Velocity (mm/s)
1.000
0.995
0.990
0.985
0.980
0.975
PrOFeAs
6
4
2
0
PrOFe0.95Co0.05As
PrOFe0.9Co0.1As
PrOFe0.85Co0.15As
PrOFe0.8Co0.2As
PrOFe0.7Co0.3As
4.7 K
5.9 K
PrOFe0.9Co0.1As
14.2K
at RT
I.S. = 0.30 mm/s
-12
-8
-4
0
4
8
12
Velocity (mm/s)
0
50
100
150
200
Fig. 2. 57Fe Mo¨ssbauer spectra of (a) PrOFeAs and (b) PrOFe0.9Co0.1As.
Temperature (K)
2.0
1.5
1.0
0.5
0.0
Table 1
Tc = 14.2 K
Hyperfine parameters for PrOFeAs and PrOFe0.9Co0.1As.
PrOFe0.9Co0.1As
Sl. no.
Compound
Temperature
(K)
Isomer shift
(mm/s)
FWHM
(mm/s)
0.0
1
2
3
4
PrOFeAs
298
100
20
0.32
0.18
0.23
0.30
0.40
4.7
Tc = 13.8
PrOFeAs
PrOFeAs
7.2
-0.5
-1.0
PrOFe0.9Co0.1As
298
0.46
coordination. The significant decrease in the value of isomer shift
for PrOFeAs below 100 K indicates the structural distortion from
0
6
12
18
d
T (K)
tetragonal to orthorhombic symmetry [29]. Both PrOFeAs and
PrOFe0.9Co0.1FeAs compounds show a clear sharp singlet at room
temperature, which suggests that both the compounds are not
magnetically ordered at room temperature. The sharp singlet also
confirms the absence of any iron bearing secondary phases like
FeAs and Fe2As (impurity phase would give either a sextet pattern
or a broadened single/double line absorption pattern) as observed
in earlier reported LnOFeAs compounds [1,19]. In Fig. 2(a), it can
be seen that as the temperature is lowered the intensity of the
singlet pattern for PrOFeAs progressively reduces and the line
width increases, which is a clear signature that the sample is
0
4
8
12
16
20
Temperature (K)
Fig. 3. (i) Temperature dependence of resistivity
(r) for (a) PrOFeAs, (b)
PrOFe0.95Co0.05As, (c) PrOFe0.9Co0.1As , (d) PrOFe0.85Co0.15As, (e) PrOFe0.8Co0.2As
and (f) PrOFe0.7Co0.3As and (ii) temperature dependence of resistivity near the
superconducting transition temperature. Inset shows the magnetic susceptibility
as a function of temperature.

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341
2.0
1.5
1.0
0.5
0.0
3
2
1
0
dHc2/dT|T = T
= - 5.11 T/Kc
0
3 T
-10
-20
-30
-40
0 T
12
13
T (K)
14
12
8
PrOFeAs
30 K
60 K
100 K
150 K
200 K
250 K
4
0
100
150
T (K)
200
8
9
10
11
12
13
14
15
0.0
0.9
1.8
2.7
3.6
4.5
Temperature (K)
μ0H (T)
Fig. 4. Temperature dependence of the electrical resistivity of PrOFe0.9Co0.1As
under different magnetic fields. The upper inset shows the temperature
dependence of estimated upper critical field (’) and irreversibility field (
lower inset indicates the temperature derivative of the resistivity (d /dT).
ꢁ) and
r
0
-7
transition is observed at 4.7 K for ‘x’=0.05 (Fig. 3(i)) and increases
with ‘x’ with highest Tc of 14.2 K for ‘x’=0.1 composition. However,
with higher Co-content, (PrOFe0.85Co0.15As, PrOFe0.8Co0.2As and
PrOFe0.7Co0.3As) the transition temperature (Tc) decreases. For
‘x’=0.15, the resistivity drops steeply around 5.9 K and shows zero
resistivity at ꢀ2.5 K as shown in Fig. 3(i). However, for
-14
-21
-28
-35
PrOFeAs
compositions with ‘x’=0.2 and 0.3, though the resistivity (
r)
drops steeply around 4 K, suggesting superconducting
a
PrOFeAs0.95Co0.05As
PrOFeAs0.9Co0.1As
transition, zero resistivity was not observed down to lowest
temperature (1.6 K). The normal state shows metallic behavior at
higher temperature (Fig. 3(i)), but semiconducting behavior is
always observed just above the transition temperature (Tc).
Fig. 3(ii) shows the zero field resistivity in the low temperature
region (o20 K) for PrOFe0.9Co0.1As. The resistivity decreases
monotonously with decreasing temperature and a rapid drop
was observed at 14.2 K showing onset of superconductivity. The
criteria used for determination of transition temperature are same
as that has been reported [31] and schematically shown in
Fig. 3(ii). The onset of diamagnetism below the transition
temperature (Tc) is observed from the plot of the real part of
susceptibility for sample PrOFe0.9Co0.1As (as shown in upper inset
of Fig. 3(ii)). A slightly lower Tc of ꢀ13.8 K is observed as
compared to the resistivity studies. In our earlier report of
CeOFe0.9Co0.1FeAs [23] a superconducting transition was observed
at 11.3 K, which indicates that the smaller praseodymium ion
(as compared to cerium) plays an important role in the
enhancement of Tc. It is expected that substitution of cobalt at
the Fe site in the FeAs layer induces disorder which might
increase with the cobalt concentration and this disorder may be
responsible for the suppression of superconductivity at higher
cobalt content.
T = 30 K
0
1
2
3
4
μ0H (T)
-2
-3
-4
-5
-6
0
-2
-4
-6
PrOFeAs
PrOFeAs0.95Co0.05As
PrOFeAs0.9Co0.1As
-8
-10
0
40
80
120
160
200
240
280
In all the known cobalt-doped superconductors LnOFe1ÀxCoxAs
(Ln=La, Ce, Pr and Sm), the highest Tc was observed around
‘x’=0.1. Increase in Co-content also leads to appreciable deviation
of the Fe–As–Fe bond angle from the ideal tetrahedral angle [22].
Thus the FeAs layer plays an important role in these super-
conductors. However, the superconducting transition tempera-
ture was found to be less dependent on the charge reservoir
(Ln–O) layer in these Co-doped superconductors as compared to
fluoride-doped superconductors where substitution of La with Sm
Temperature (K)
Fig. 5. (a) Hall resistivity (
resistivity ( xy) versus the magnetic field (
PrOFe0.9Co0.1As at 30 K (5% error bar estimated at 30 K) and (c) temperature
r
xy) versus the magnetic field (
m0H) for PrOFeAs, (b) Hall
r
m0H) for PrOFeAs, PrOFe0.95Co0.05As and
dependence of the Hall coefficient (RH) for PrOFeAs (’), PrOFe0.95Co0.05As (
ꢁ) and
PrOFe0.9Co0.1As (m).

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J. Prakash et al. / Journal of Solid State Chemistry 183 (2010) 338–343
ions results in enhancement of Tc from 26 to 55 K. This suggests
subtle differences in the two families of LnOFeAs superconductors
(fluoride- and cobalt-doped).
To obtain the upper critical field (Hc2), we have studied the
temperature dependence of the resistivity under different
magnetic fields (Fig. 4) for PrOFe0.9Co0.1As. The onset temperature
shifts weakly with magnetic field, but the zero resistivity
temperature shifts more rapidly to lower values. By taking a
suppressed on cobalt doping for xZ0.05, consistent with
resistivity data. The magnitude of S for the superconducting
sample is smaller than that for the non-superconducting sample
PrOFeAs [33].
4. Conclusions
We have successfully synthesized monophasic samples of
PrOFe1ÀxCoxAs superconductors with maximum Tc at 14.2 K for
‘x’=0.1 composition. Cobalt substitution at the iron site in
PrOFeAs suppresses the structural distortion and magnetic
instability (SDW) with the evolution of superconductivity. Higher
cobalt doping leads to suppression of the superconducting
transition temperature. Mo¨ssbauer spectroscopy data indicates
divalent state of Fe both in undoped and cobalt-doped samples.
The upper critical field at 0 K is estimated to be about 50.2 T. The
negative values of the thermopower and Hall coefficient (RH) over
a wide temperature range suggests that the electron type charge
carriers dominate the conduction in this system. The strong
temperature dependence of RH is normally associated with a
multiband effect (two band model with different charge carriers).
Hall measurements also indicate that conduction electrons are
added with increasing Co-doping.
criterion of 90% and 10% of normal state resistivity (rn), we
calculated the upper critical field Hc2 and the irreversibility field
H*(T), respectively. The H–T phase diagram for this sample is
shown in inset of Fig. 4. By using the Werthamer–Helfand–
Hohenberg (WHH) formula [32], the zero field upper critical field
Hc2(0) can be calculated [23] to be 50.2 T for PrOFe0.9Co0.1As.
These values are slightly higher than the reported Hc2 value of
CeO(Fe/Co)As [23].
The variation of transverse resistivity (
for PrOFeAs compound is shown in Fig. 5(a). Transverse resistivity
xy) remains negative at all temperatures above the critical
rxy) with magnetic field
(
r
temperature for compound PrOFeAs. We have also showed the
transverse resistivity at 30 K for PrOFeAs, PrOFe0.95Co0.05As and
PrOFe0.9Co0.1As in Fig. 5(b). This indicates that the electrical
transport is dominated by the electron-like charge carriers and
not holes. For a clean sample, the nonlinear Hall effect is a good
sign of a multiband superconductor, and the effect is weaker in
dirtier samples. From the above data, the Hall coefficient RH=rxy
m
/
Acknowledgments
0H was determined for compounds with ‘x’=0, 0.05 and 0.1
(Fig. 5 (c)). The charge carrier density calculated from the
equation n=1/RHe is about 0.074 Â 1021 cmÀ3, 0.27 Â 1021 cmÀ3
and 1.16 Â 1021 cmÀ3 for PrOFeAs, PrOFe0.95Co0.05As and
PrOFe0.9Co0.1As, respectively, at 30 K. It shows that charge
carriers increase with Co-doping.
Fig. 6 shows the temperature dependence of thermopower (S)
in PrOFe0.9Co0.1As from room temperature to 13 K. The
thermopower (S) is negative in the entire temperature range,
which shows that the electron-like charge carriers dominate as
seen from Hall effect. For the undoped parent compound, it is
reported [33] that as the thermopower starts to increase
abnormally around Tanom the resistivity starts to decrease. This
remarkable change in the thermopower is probably caused by the
change in the electronic state when the system undergoes the
structural phase transition and SDW transition. This anomaly is
A.K.G. and S.P. thank DST, Government of India for the financial
support. J.P. and S.J.S. thank CSIR, Government of India, for
fellowships. The authors thank B. Bhusan and Satya Prakash for
help with the low temperature Mo¨ssbauer measurements and
data fitting.
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