Thermoelectric power of RFeAsO (R = Ce, Pr, Nd, Sm and Gd)

Article abstract of DOI:10.1016/j.physc.2009.04.013

Thermoelectric powers of a series of compounds RFeAsO (R = Ce, Pr, Nd, Sm and Gd) have been reported for temperatures ranging from 77 K up to room temperature. The behavior of S(T) in this temperature range can be divided into three regions. Every region

Full text of DOI:10.1016/j.physc.2009.04.013

Physica C 469 (2009) 789–794
Contents lists available at ScienceDirect
Physica C
journal homepage: www.elsevier.com/locate/physc
Thermoelectric power of RFeAsO (R = Ce, Pr, Nd, Sm and Gd)
b
c
b
b
Asok Poddar a, , Sanjoy Mukherjee , Tanmay Samanta , Rajat S. Saha , Rajarshi Mukherjee ,
*
Papri Dasgupta a, Chandan Mazumdar a, R. Ranganathan a
a Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, West Bengal, India
b Department of Physics, The University of Burdwan, Golapbag, Burdwan 713 104, West Bengal, India
c Rishra High School, 15 Tilakram Dan Ghat Lane, Rishra, Hooghly, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermoelectric powers of a series of compounds RFeAsO (R = Ce, Pr, Nd, Sm and Gd) have been reported
for temperatures ranging from 77 K up to room temperature. The behavior of S(T) in this temperature
range can be divided into three regions. Every region has been fitted with mathematical functions of T.
The physical significance of separate terms in the mathematical functions has been discussed. Some kind
of universality has been observed between different members of the series.
Received 4 February 2009
Accepted 27 April 2009
Available online 6 May 2009
PACS:
75.30.Fv
74.25.Fy
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Spin density wave
Transport properties
Iron–arsenide
1. Introduction
the physical properties of the un-doped parent compounds are also
necessary to understand the superconductivity in the doped
The recent discovery of superconductivity at 28 K in fluorine
doped LaFeAsO [1], and the subsequent reports of superconducting
transition temperatures even up to 50 K in some of the related
rare-earth (R) materials [2–7], have stimulated a great interest in
these rare earth oxypnictides. These compounds crystallize in the
tetragonal phase (space group P4/nmm) at room temperature,
and their structure is composed of two kinds of layers stacked
along c-axis. One layer is composed of tetrahedra, centered by
‘‘O” with the R at vertices as a block, and in the other layer Fe is
coordinated by As tetrahedron. While the R–O chemical bond is io-
nic, Fe–As has a predominantly covalent nature. The conductive
carriers are confined two-dimensionally in the Fe–As layer causing
strong interactions among the electrons in RFeAsO series of com-
pounds [1,8]. Doping of O2ꢀ by Fꢀ provides an extra positive charge
in the insulating layer (charge reservoir layer) and a negative
charge in the conduction layer. Later, it has also been observed that
superconductivity can even be introduced by hole doping [9] of the
non-superconducting parent compounds RFeAsO as well. The
superconductivity in such doped RFeAsO compounds appears to
be unconventional [8,10]. An extensive work has been done on
these doped compounds to have a clear understanding of the re-
lated physics as well as the effect of doping. However, studies of
compounds.
One interesting observation in the un-doped parent compounds
RFeAsO is the presence of long-range spin density wave. The spin
density wave (SDW) is an antiferromagnetic (AFM) ground state
for which the density of the conduction electron spins is spatially
modulated, and is a many-particle phenomenon of an itinerant
magnetism. An antiferromagnetic phase transition occurs at a low-
er temperature accompanied by a tetragonal to orthorhombic
structural distortion on cooling down below 200 K. The magnetic
structure of the oxypnictides within the a–b plane consists of
chains of parallel Fe spins. These chains are coupled antiferromag-
netically in the orthogonal direction, with an ordered moment less
than 1 lB. This again suggests that such systems are itinerant with
magnetism arising from a nesting-induced spin density wave
(SDW) [11].
LaFeAsO shows an anomaly near T = 150 K both in the resistivity
and in the magnetic susceptibility [12–14]. This is attributed to a
spin-density-wave (SDW) instability. It has also been reported,
that the system undergoes a structural phase transition from
tetragonal to orthorhombic structure on cooling below 155 K.
The structural distortion has been suggested to occur over a wide
range of temperature starting from 200 K, and sufficient distortion
takes place around 160 K. This distortion results in carrier localiza-
tion and local moment formation on the Fe atoms. These local
moments order antiferromagnetically near 145 K [15]. Local probe
* Corresponding author. Tel.: +91 33 23375345x2460; fax: +91 33 23374637.
E-mail address: asok.poddar@saha.ac.in (A. Poddar).
0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2009.04.013

790
A. Poddar et al. / Physica C 469 (2009) 789–794
measurements of the magnetic properties of LaFeAsO by 57Fe
Mossbauer spectroscopy and muon-spin relaxation, in absence of
external field suggest a static magnetic order below TN = 138 K
with a strongly reduced ordered moment at the Fe site [14].
Assuming that the full sample volume is contributing to the mag-
netic scattering an ordered moment of ꢁ0.35 lB is predicted from
the weak superlattice reflections in powder neutron diffraction
[15]. Neutron scattering study of CeFeAsO [16] suggests that CeFe-
AsO undergoes a structural lattice distortion from tetragonal to
orthorhombic structure near 155 K followed by a commensurate
AFM ordering on the Fe sublattice below ꢁ140 K. Neutron scatter-
ing studies of PrFeAsO suggest antiferromagnetic ordering below
127 K, with an ordered moment of 0.48(9) lB [17]. This iron mag-
netic ordering occurs below the structural transition, that takes
place around 153 K. The magnetic moments on the Pr sites are also
antiferromagnetically ordered below 14 K, similar to the parent
compounds of the other rare earth FeAs-based superconductors
such as CeFeAsO and NdFeAsO [17]. Polarized neutron diffraction
measurements in NdFeAsO suggest the antiferromagnetic ordering
below 141(6) K, with an ordered moment of 0.25(7) lB [11]. How-
ever, there are also reports of the absence of the SDW antiferro-
magnetic order of the type discovered in LaFeAsO, along with the
occurrence of the structural transition at 150 K [18]. Rietveld
refinement [19] of high resolution synchrotron powder diffraction
data on SmFeAsO collected at 300 K and 100 K suggests that, it
crystallizes in the tetragonal phase at 300 K and in the orthorhom-
bic phase at 100 K. Both resistive and magnetic measurements re-
veal that the tetragonal to orthorhombic phase transition is located
at T ꢁ 140 K. GdFeAsO is a less studied system in which role of oxy-
gen deficiency [20] in facilitating superconductivity has also been
proved.
Table 1
Lattice parameters of RFeAsO obtained from XRD measurements.
Sample
a (Å)
c (Å)
CeFeAsO
PrFeAsO
NdFeAsO
SmFeAsO
GdFeAsO
3.997
3.977
3.961
3.939
3.908
8.652
8.606
8.572
8.499
8.446
The thermoelectric power of each sample has been measured
using a differential technique where a temperature gradient is cre-
ated across the sample. The voltage developed (DE) between the
hot and cold ends is measured. In our apparatus, we have used
two heaters (one at the top and the other at the bottom of the sam-
ple) so that either end of the sample may be heated with respect to
the other. The temperature difference
DT between two ends of the
sample is kept in the range ꢁ 0.5 K throughout the measured
temperature range (77–300 K). The temperature as well as the
temperature difference across the sample were measured with
Chromel–Alumel thermocouples. The sample temperature is con-
trolled by a manganin heater, and monitored by a Si-diode sensor.
At a particular temperature the values of
and TEP (S) of the sample is calculated from the relation
T = (Scu ꢀ S)/(Schromel ꢀ Salumel).
D
E and
D
T were recorded
D
E/
D
3. Results and discussion
Fig. 1 shows thermopower S as a function of temperature for the
parent compound CeFeAsO. In the higher temperature region
(200–300 K), S is negative, and its value increases linearly as a
function of temperature. This negative value suggests that the elec-
trons dominate in the electrical conduction. It may be mentioned
that negative Hall coefficient has also been observed in SmFeAsO
[22]. A linear dependence of S on T is a signature of metallic behav-
ior. Thus in contrast to the Cuprates, parent compound is not a
Mott–Hubbard insulator but a metal. This can be explained within
the framework of the nearly free electron (NFE) model [22]. In the
absence of magnetic ordering, thermoelectric power consists of
We have measured systematically the thermopower of a num-
ber of parent compounds RFeAsO (R = Ce, Pr, Nd, Sm and Gd) over
a range (77–300 K) of temperature in which AFM ordering and
structural distortion have been reported for these compounds.
Among various transport properties, thermopower is a simple
and sensitive one for analyzing charge carrier dynamics. Thermo-
power can predict the nature of the charge carriers, and is also sen-
sitive to local moments. Moreover, as the thermoelectric response
is driven by the temperature gradient rather than by the electric
field, it is insensitive to grain boundaries [21].
two components, viz., ‘‘diffusion component” (Sd) and ‘‘phonon-
drag component (Sg). Sd is associated with only the electron gas
and its dispersion whereas Sg is related with the phonon dispersion
and the electron–phonon interaction. NFE model [22] gives an
2. Experimental
The samples were prepared by the solid-state reaction route
using the elements R (Ce, Pr, Nd, Sm and Gd), Fe, Fe2O3 and As.
Firstly, RAs were prepared by taking stoichiometric amounts of R
(99.99%) and As (99.99%) chips in 1:1 ratio, pressed into pellets
and sealed in evacuated quartz tube. With repeated heat treat-
ment, attaining a maximum temperature of 900 °C, and grinding
inside a glove box filled with inert Ar gas, single phase RAs were
obtained. RAs were then mixed thoroughly with Fe2O3 powder
(purity 99.995%) and Fe powder in stoichiometry and pressed into
pellets. The pellets were wrapped with Ta foil and sealed in an
evacuated quartz tube. They were then annealed at 1150–
1200 °C for 40–45 h to obtain the final samples of RFeAsO
(R = Ce, Pr, Nd and Sm). In case of R = Gd, stoichiometric amounts
of GdAs, Gd2O3, FeAs and Fe were used. FeAs was prepared using
Fe powder (99.99%) and As chips (99.99%) mixed in 1:1 ratio by
the procedure similar to that of RAs. The phase purity of each sam-
ple at room temperature was checked by powder X-ray diffraction
10
S = S0 +S1T+S2T2+S4T4
S0 = 13.25+ 0.05, S1 = -0.49600,
S2 = 0.00650, S4 = -1.989 × 10-7
5
0
S = S0 +S T+S T2+S4T4
S0 = 22761.92+ 20.06, S1 = -37.255,
S2 = 0.17000, S4 = -1.050 × 10-6
-5
-10
-15
-20
CeFeAsO
S = -45.53 + 0.132 T
100
150
200
T(K)
250
300
(XRD) method with Cu Ka radiation. XRD patterns of all the com-
pounds suggest tetragonal ZrCuSiAs-type structure at room tem-
perature with space group P4/nmm. The lattice parameters of the
samples are summarized in Table 1.
Fig. 1. Thermoelectric power S as a function of temperature for CeFeAsO.

A. Poddar et al. / Physica C 469 (2009) 789–794
791
expression of the characteristic diffusion thermopower of a metal
as
S = S +S1T+S2T2+S T4
S0 = -010.22+ 0.01, S14= -0.33700,
0
-5
ꢀ
ꢁ
S2 = 0.00650, S4 = -2.024 × 10-7
p
2k2BT NðEFÞ 1 d
s
dEF
SðTÞ ¼
þ
ð1Þ
3e
n
s
E¼EF
S = S0 +S T+S T2+S4T4
where kB is the Boltzmann’s constant, EF is the Fermi energy, N(E) is
the density of states, is the relaxation time and e is the charge of
s
S0 = 16141.29+ 20.05, S1 = -26.0,
S2 = 0.11640, S4 = -6.92 5 × 10-7
-10
-15
-20
the carriers. The second term contributes mainly below the transi-
tion where it is related with the changes in mobility of the carriers
with doping and temperature. In the absence of any phase transi-
tion, the contribution from the second term can be neglected. The
first term scales as 1/EF, and we have
S = -39.56+ 0.0996 T
NdFeAsO
p
3eEF
2k2BT
SðTÞ ¼
ð2Þ
Thus S(T) is a linear function of T for such a metal, and the slope can
give an estimate for the Fermi energy EF. For CeFeAsO the estimated
EF is 0.19 eV.
100
150
200
250
300
T(K)
The structural distortion in these compounds starts around
200 K, where S(T) deviates from its linear behavior. Then it reaches
a minimum (Tmin) around 170 K. Similar feature of minima in S(T)
curve has also been reported recently in (Nd/Sm)FeAsO polycrys-
Fig. 4. Thermoelectric power S as a function of temperature for NdFeAsO.
S = S0 +S1T+S2T2+S4T4
10
S0 = 12.10+ 0.04, S1 = -0.42560,
S2 = 0.00650, S4 = -2.314 × 10-7
5
0
S = S +S1T+S2T2+S T4
S0 = -042.34+ 0.06, S14= -0.07300,
-5
0
S2 = 0.0065, S4 = -2.285 × 10-7
S = S0 +S1T+S T2+S4T4
S0 = 1240.34+20.06, S1 = -20.184,
-5
-10
SmFeAsO
S2 = 0.09120, S4 = -5.550 × 10-7
-10
-15
-20
-25
-30
-35
S = S0 +S T+S T2+S4T4
S0 = 36721.87+ 20.16, S1 = -58.045,
S2 = 0.25515, S4 = -1.456 × 10-6
-15
S = -38.66 + 0.0989 T
-20
LaFeAsO
reported
S = -72.03 + 0.188 T
100
150
200
250
300
T(K)
100
150
200
250
300
Fig. 5. Thermoelectric power S as a function of temperature for SmFeAsO.
T(K)
Fig. 2. Thermoelectric power S as a function of temperature for LaFeAsO [24].
5
S = S +S1T+S2T2+S4T4
S0 = -06.54+ 0.03, S1 = -0.28032,
S = S +S1T+S2T2+S T4
5
0
S2 = 0.00650, S4 = -2.673 × 10-7
S0 = -017.93+ 0.04, S14 = -0.21500,
S2 = 0.0065, S4 = -2.168 × 10-7
0
S = S0 +S1T+S2T2+S T4
S0 = 722.10+ 0.05, S41 = -12.569,
-5
-5
S2 = 0.06050, S4 = -4.177 × 10-7
S = S0 +S1T+S T2+S4T4
S0 = 1624.04+20.07, S1 = -24.870,
-10
-10
S2 = 0.10550, S4 = -5.647 × 10-7
GdFeAsO
-15
PrFeAsO
S = -27.92 + 0.066 T
250 300
-20
-15
S = -55.5+ 0.142 T
-25
100
150
200
T(K)
-30
Fig. 6. Thermoelectric power S as a function of temperature for GdFeAsO.
100
150
200
250
300
T(K)
talline samples [23]. The existence of this Tmin might be considered
as a signature of the development of antiferromagnetic correlation
Fig. 3. Thermoelectric power S as a function of temperature for PrFeAsO.

792
A. Poddar et al. / Physica C 469 (2009) 789–794
Table 2
Tmin and values of different coefficients obtained by fitting thermopower curves of RFeAsO for different temperature ranges using Eq. (4).
System
Parameters obtained from literature
AFM ordering below 140 K
Temperature range and T-dependence of S(T)
Tmin and values of different coefficients
Tmin = 172 K
CeFeAsO
77–134 K
;
Structural transition at 155 K
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = 13.25 0.05,
S1 = ꢀ0.49600,
S2 = 0.00650,
S4 = ꢀ1.989 ꢂ 10ꢀ7
135–172 K
S0 = 2276.92 0.06,
S1 = ꢀ37.255,
;
S(T) = S0 + S1T + S2T2 + S4T4a
S2 = 0.17000,
S4 = ꢀ1.050 ꢂ 10ꢀ6
S = ꢀ45.53 + 0.132T
200–300 K ?
LaFeAsO [24]
AFM ordering below 138 K
Structural transition at 156 K
77–135 K
Tmin = 169 K
;
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = ꢀ42.34 0.06,
S1 = ꢀ0.07300,
S2 = 0.00650,
S4 = ꢀ2.285 ꢂ 10ꢀ7
138–169 K
S0 = 3672.87 0.16,
S1 = ꢀ58.045,
;
S(T) = S0 + S1T + S2T2 + S4T4a
S2 = 0.25515,
S4 = ꢀ1.456 ꢂ 10ꢀ6
S = ꢀ72.03 + 0.188T
200–304 K ?
PrFeAsO
AFM ordering below 127 K
Structural transition at 153 K
77–134 K
Tmin = 178 K
;
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = ꢀ17.93 0.04,
S1 = ꢀ0.21500,
S2 = 0.00650,
S4 = ꢀ2.168 ꢂ 10ꢀ7
137–173 K
S0 = 1624.04 0.07,
S1 = ꢀ24.870,
;
S(T) = S0 + S1T + S2T2 + S4T4a
S2 = 0.10550,
S4 = ꢀ5.647 ꢂ 10ꢀ7
S = ꢀ55.5 + 0.142T
200–307 K ?
NdFeAsO
AFM ordering below 141 K
Structural transition at 150 K
77–134 K
Tmin = 177 K
;
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = ꢀ10.22 0.01,
S1 = ꢀ0.33700,
S2 = 0.00650,
S4 = ꢀ2.024 ꢂ 10ꢀ7
135–177 K
;
S0 = 1614.29 0.05, S1 = ꢀ26.0,
S2 = 0.11640,
S(T) = S0 + S1T + S2T2 + S4T4a
200–307 K ?
S4 = ꢀ6.925 ꢂ 10ꢀ7
S = ꢀ39.56 + 0.0996T
SmFeAsO
Resistivity anomaly around T = 135–140 K
77–133 K
Tmin = 174 K
;
Drop in electron density below 150 K
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = 12.10 0.04,
S1 = ꢀ0.42560,
S2 = 0.00650,
S4 = ꢀ2.314 ꢂ 10ꢀ7
134–173 K
S0 = 1240.34 0.06,
S1 = ꢀ20.184,
;
S(T) = S0 + S1T + S2T2 + S4T4a
S2 = 0.09120,
S4 = ꢀ5.550 ꢂ 10ꢀ7
S = ꢀ38.66 + 0.0989T
200–303 K?
GdFeAsO
77–123 K
Tmin = 177 K
;
S(T) = S0 + S1T + S2T2 + S4T4a
S0 = ꢀ6.54 0.03,
S1 = ꢀ0.28032,
S2 = 0.00650,
S4 = ꢀ2.673 ꢂ 10ꢀ7
S0 = 722.10 0.05,
S1 = ꢀ12.569,
126–172 K
;
S(T) = S0 + S1T + S2T2 + S4T4a
S2 = 0.06050,
S4 = ꢀ4.177 ꢂ 10ꢀ7
S = ꢀ27.92 + 0.066T
206–304 K ?
a
S0, S1, S2 and S4 are expressed in l , l , l l
V Kꢀ1 V Kꢀ2 V Kꢀ3 and V Kꢀ4, respectively.

A. Poddar et al. / Physica C 469 (2009) 789–794
793
[23]. In the region below this minimum S increases with lowering
of temperature and finally crossing the zero-line it shows a peak
(Tpeak) around 100 K.
scattering is the mechanism that mainly limits the carrier mobility
above the SDW transition [22].
For SmFeAsO, an anomaly in molar susceptibility has also been
observed in this temperature region 135–140 K [26]. These anom-
alies suggest an intimate coupling between the structural phase
transition and the electronic as well as magnetic properties of
This can be compared with the previous measurement of S(T) in
LaFeAsO (Fig. 2) [24], in which they observed S to be negative over
the entire temperature range. Besides this, |S| has a similar overall
behavior in the temperature range (77–300 K) of measurement.
The thermopower as a function of temperature shows similar over-
all behavior for all our measured samples (RFeAsO; R = Ce, Pr, Nd,
Sm and Gd). The details are given in Figs. 1–6 and Table 2.
Below Tmin the absolute value of S decreases. A similar decrease
in |S| along with a decrease in the carrier density ‘n’ is observed be-
low about 155 K in LaFeAsO [24]. They suggest, that the observed
decrease in both |S| and n through the transition indicates, that
the second term in Eq. (1) is dominant in this temperature regime.
In fact, the charge carrier scattering mechanism is changed signif-
icantly as the material passes through the phase transition region,
and suggests reduction of electron–phonon interactions in the
orthorhombic phase. Finally, we will see that the electron–electron
scattering plays the dominant role below Tmin. Further evidence of
such reduction has been obtained from the analysis of carrier
mobility and thermal conductivity. This may be considered as an
evidence of strong electron–phonon coupling in such systems,
which are metals with conduction dominated by electrons and
with no local magnetic moment at high temperatures [24].
After crossing the zero line S becomes positive. This can be ex-
plained by the appearance of hole-like conduction in addition to
electron like conduction. This supports the multi-band nature of
the oxypinctides. For two bands, one electron- and the other
hole-like, S becomes:
the system. For LaFeAsO [14], a sharp decrease of
v near the struc-
tural transition (around 160 K) has been suggested as a signature
of the enhancement of antiferromagnetic correlations. This is re-
flected in S(T) behavior by the S4 term. This term may be associated
[27] with the spin-wave fluctuation in the higher temperature re-
gion associated with antiferromagnetic correlations and shows the
strong impact of structural transition on magnetism. This S4 also
reduces significantly below 130 K.
4. Conclusions
In summary, the qualitative behavior of S(T) is the same for all
studied RFeAsO (R = Ce, Pr, Nd, Sm and Gd). In the temperature
range (200–300 K) all the samples show metallic behavior with
S(T) = S0 + S1T. S1 is in the vicinity of 0.1 l
V Kꢀ2 suggesting an EF
of about 0.25 eV. All the samples show a minimum in S(T) around
Tmin = 170–180 K and a change in slope around 130 K. The region
below Tmin is described by S(T) = S0 + S1T + S2T2 + S4T.4 Strong elec-
tron–electron scattering plays an important role in the systems,
and we observe a reduction in that scattering below 130 K, that is
consistent with SDW picture. Spin fluctuations associated with anti-
ferromagnetic correlations give S4 term in the expression of S(T).
Acknowledgement
ð
rHjSHj ꢀ rejSejÞ
S ¼
ð3Þ
e(h) and Se(h) are the contributions of electrons (holes) to
The authors would like to thank Mr. A.K. Paul for his technical
help during sample preparation and measurements.
ð
rH
þ
re
Þ
where
r
the electrical conductivity and Seebeck coefficient, respectively.
At around 130 K, S(T) suffers a distinct change in slope. This may
be associated with the transition to SDW state. In the region (130–
Tmin K), as well as in the temperature range (77–130 K), the behav-
ior of S(T) can be well described by the expression
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SðTÞ ¼ S0 þ S1T þ S2T2 þ S4T4
ð4Þ
In the temperature range (130–Tmin K), S1, the coefficient of the lin-
ear term becomes large and negative. For our measured samples
(R = Ce, Pr, Nd, Sm and Gd), it is minimum for Gd (ꢀ12.569
and maximum for Ce (ꢀ37.255
V Kꢀ2). Being negative its absolute
value again decreases below 130 K.
l )
V Kꢀ2
l
Electron–electron scattering in metals shows a T2 dependence
of S(T) [25]. From (77–Tmin K), we observe a T2 dependence of
S(T). In the region (130–Tmin K) the value of S2 is minimum for Gd
(0.0605
low 130 K the coefficient S2 reduces drastically, and its value re-
mains same (0.0065
V Kꢀ3) for all the samples. This indicates
l l
V Kꢀ3), and maximum for Ce (0.17 V Kꢀ3). However, be-
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l
that below Tmin electron–electron scattering plays a significant role
in these systems, and the scattering reduces drastically below
130 K. This reduction in electron–electron scattering suggests the
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