Thermoelectric properties of the Ru2Ni2Sb 12 ternary skutterudite

Article abstract of DOI:10.1016/j.jssc.2012.03.028

The synthesis of the Ru_2-xNi_2-xSb₁₂ compounds (0≤x≤0.2), their structural characterization and temperature dependencies of selected transport and thermal properties are reported. At x=0, Ru_2-xNi_2-xSb₁₂ displays cubic symmetry, space group Im3 with lattice parameter a=9.1767(1) A. From increasing electrical conductivity above 600 K the band gap (E_g～0.06 eV) was estimated using an Arrhenius plot. Different signs of the Seebeck coefficient (negative) and the Hall coefficient (positive) have been explained as a consequence of a multicarrier transport. The substitution on a cation site, i.e., formation of the Ru_2-xNi_2-xSb₁₂ ternary skutterudites proved to be effective way in suppressing of the thermal conductivity.

Full text of DOI:10.1016/j.jssc.2012.03.028

Journal of Solid State Chemistry 193 (2012) 2–7
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Thermoelectric properties of the Ru Ni Sb ternary skutterudite
2
2
12
a,n
,1
c,b
c,b,1
c,b
ˇ
, Cestm ı´ r Dra sˇ ar
Ji rˇ ı´ Navr a´ til
, Franti sˇ ek Laufek , Tom a´ sˇ Plech a´ cˇ ek
a
Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic v.v.i., Heyrovsky sq. 2, 12006 Prague, Czech Republic
Czech Geological Survey, Geologick a´ 6, 152 00 Praha 5, Czech Republic
b
c
Faculty of Chemical Technology, University of Pardubice, 53210 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Available online 27 March 2012
The synthesis of the Ru2 Ni
ꢀx 2ꢀx
Sb12 compounds (0rxr0.2), their structural characterization and
temperature dependencies of selected transport and thermal properties are reported. At x¼0,
Keywords:
˚
Ru2ꢀxNi2ꢀxSb12 displays cubic symmetry, space group Im3 with lattice parameter a¼9.1767(1) A.
Ternary skutterudite
Ru
2
Ni
2
Sb12
From increasing electrical conductivity above 600 K the band gap (E
g
ꢁ0.06 eV) was estimated using an
Thermoelectric properties
Crystal structure
Arrhenius plot. Different signs of the Seebeck coefficient (negative) and the Hall coefficient (positive)
have been explained as a consequence of a multicarrier transport. The substitution on a cation site, i.e.,
formation of the Ru2ꢀxNi2ꢀxSb12 ternary skutterudites proved to be effective way in suppressing of the
thermal conductivity.
&
2012 Elsevier Inc. All rights reserved.
1
. Introduction
pair of elements from 8th to 10th groups (e.g., Fe0.5Ni0.5Sb
3
[7],
Ru0.5Pd0.5Sb [8]).
3
There is a lot of experimental studies reported in literature in last
Similar substitution on a cation M-site by a pair of elements
from 8th to 10th groups but from different rows of the periodic
table led also to synthesis thermodynamically stable ternary
two decades demonstrating strong potential of materials with
skutterudite structure (general formula MX
where M¼Co, Ir or
Rh; X¼P, As or Sb) for advanced thermoelectric applications e.g.,
1,2]. Some of the binary skutterudites have good electrical proper-
3
2 2 2 2
skutterudite compounds—Fe Pd Sb12 and Ru Ni Sb12, respec-
[
tively. The existence of both these compounds is briefly men-
tioned in Fleurial’s list [9] of possible ternary skutterudites. More
detailed structure study and basic thermoelectric properties of
ties, however, they do not demonstrate sufficient thermoelectric
performance, because of a relatively high thermal conductivity. The
most effective approach in lowering of the high thermal conductiv-
ity of skutterudite materials relies on filling empty voids in their
crystal structure with heavy atoms, mostly with one e.g., [3] or more
rare earth atoms e.g., [4].
Besides traditional approach of lowering of the thermal con-
ductivity by a formation of solid solutions between two or more
binary skutterudites, there is also another way to lower lattice
part of thermal conductivity of the skutterudite materials, i.e., a
formation of ternary skutterudites by an isoelectronic substitu-
tion. These ternary skutterudites are materials isoelectronic to the
Fe
2
Pd
2
Sb12 were described by Navr a´ til et al. in [10].
In this work, we report on preparation and basic structural
aspects of Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) compounds. The effect
of cation substitution on transport and thermoelectric properties
is also reported.
2. Experimental
2.1. Synthesis and thermal analysis
3
binary skutterudites MX , and can be prepared, e.g., either by an
isoelectronic substitution at anionic site X by a pair of elements
from 14th to 16th groups (e.g., CoGe1.5Se1.5 [5], CoSn1.5Se1.5 [6]),
or by an isoelectronic substitution at the cationic site M by a
The Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) ternary compounds were
synthesized from elemental powders by high-temperature solid-
state reactions. Ni powder was first heated at 600 1C for 2 h in
2
H atmosphere to reduce possible oxides. The stoichiometric
amounts of Ru (99.9%), Ni (99.9%) and Sb (99.999%) were sealed
into evacuated silica glass tubes and heated at 1050 1C for 48 h in
a furnace. The material was then ground under acetone using
agate mortar and pestle and heated at 500 1C for 168 h. The
resultant material was once again ground under acetone and
heated again at 500 1C for 240 h. After the heat treatment, the
furnace was turned off and allowed to cool slowly to room
n
Corresponding author at University of Pardubice, Institute of Macromolecular
Chemistry of the Academy of Sciences, Joint Laboratory of Solid State Chemistry,
5
3210 Pardubice, Czech Republic. Fax: þ420 466036011.
E-mail address: Jiri.Navratil@upce.cz (J. Navr a´ til).
1
Present address: Joint Laboratory of Solid State Chemistry of Institute of
Macromolecular Chemistry of the Academy of Sciences of the Czech Republic v.v.i.
and the University of Pardubice, 53210 Pardubice, Czech Republic
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2012.03.028

J. Navr a´ til et al. / Journal of Solid State Chemistry 193 (2012) 2–7
3
Fig. 2. The heating DTA curves of the Ru2ꢀxNi2þxSb12 compounds. (For inter-
pretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
Fig. 1. Powder X-ray diffraction patterns of the prepared Ru2ꢀxNi2þxSb12 com-
pounds (CuK radiation). (For interpretation of the references to color in this figure
a
legend, the reader is referred to the web version of this article.)
Table 1
2 2
Data collection and Rietveld analysis for Ru Nu Sb12. R
agreement factors defined according to [16].
temperature. Finally, obtained powder samples were, after verifi-
cation of the completion of the solid-state reaction with powder
X-ray diffraction, hot-pressed at 500 1C (HP-samples) and 80 MPa
for 1 h. The powder X-ray diffraction patterns of the prepared
samples are shown in Fig. 1.
The differential thermal analysis (DTA) was carried out using
the instrument R.M.I.–DTA 003 (Electronic Measuring Instru-
ments) at a non-isothermal regime in the temperature range
Data collection
Radiation type, source
Generator settings
X-ray, CuK
40 kV, 40 mA
a
Data collection temperature room temperature
Range in 2
y
(1)
10–110
0.02
Step size (1)
Crystal data
Space group
Im3 (No. 204)
3
4
00–1070 K. Small quartz ampoule with powdered sample (about
ꢀ
3
Unit cell content
Ru0.5Ni0.5 Sb
3
, Z¼8
0 mg of weight) was evacuated down to 10 Pa and heated
Unit cell parameters ( A˚ )
a¼9.1767(1)
ꢀ
1
with rate of 10 K min . The calibration was made with the help
of In, Al, Zn, Pb and Sn in order to eliminate the differences
between the temperature of the thermocouples in the furnace and
Rietveld analysis
No. of reflections
No. of structural parameters
No of. profile parameters
RBragg
102
2
2 3
in the vicinity of the sample. Pure Al O was used as a standard.
6
The heating DTA curves of the studied samples obtained under
above given conditions are presented in Fig. 2.
0.063
0.084
0.010
R
R
p
wp
Weighting scheme
1/y
o
2.2. Crystal structure refinement
The powder X-ray diffraction patterns of Ru2ꢀxNi2þxSb12
consecutive breakpoints in the diffraction pattern. Finally, 14 para-
meters were allowed to vary. The Ru and Ni atoms randomly
occupy the 8c position of the Im3 space group; no indications
of structural ordering were observed. The final Rietveld plot is
depicted in Fig. 3, Tables 2 and 3 show refined structural para-
(
x¼0, 0.1, 0.2) ternary compounds were collected in the Bragg–
Brentano geometry on a Bruker D8 Advance diffractometer. CuK
radiation was used. The details of data collection and basic
crystallographic facts for Ru Ni Sb12 are given in Table 1.
a
2
2
The crystal structures of Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) phases
were refined by the Rietveld method for X-ray powder diffraction
data using the FullProf program [11]. All synthesized phases were
2 2
meters for Ru Ni Sb12 phase and calculated lattice parameters for
Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) compounds.
found to be isostructural with the cubic skutterudite structure CoSb
Im3 symmetry). As follows from Fig. 1, a small amount of unreacted
antimony (ca 3 wt%) was detected in Ru Ni Sb12 and Ru1.9Ni2.1Sb12
samples. The refined parameters contain those describing peak
shape and width, peak asymmetry (2 parameters), unit cell para-
meter and fractional coordinates. The pseudo-Voigt function
was employed to model the line shape of diffraction profiles.
The background was determined by linear interpolation between
3
2.3. Thermoelectric properties measurements
(
2
2
Electrical conductivity was measured with four-probe method
using Lock-in Amplifier (EG&G model 5209) on the rectangular
3
parallelepiped of dimensions about 15 ꢂ 3.5 ꢂ 2 mm . The measure-
ments were performed on two probes, one in the temperature
region from about 100–350 K and the other from 300 to 800 K. On
the same samples Hall effect measurements were carried out over

4
J. Navr a´ til et al. / Journal of Solid State Chemistry 193 (2012) 2–7
Fig. 4. (a) Polyhedral representation of Ru
corner-sharing octahedra are emphasized. (b) The four-membered ring in
Ru Ni Sb12. (For interpretation of the references to color in this figure legend,
2 2 6
Ni Sb12 crystal structure. The [Ru/NiSb ]
Fig. 3. Observed (circles), calculated (solid line) and difference Rietveld profiles
2
2
2 2 2 2
for Ru Ni Sb12. The upper reflection bars correspond to Ru Ni Sb12 and the lower
the reader is referred to the web version of this article.)
bars to a 3 mass percent Sb impurity. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
the original skutterudite structure is preserved. The skutterudite
structure can be viewed as a derivative of the perovskite struc-
ture, characterized by elimination of the A toms and by tilting of
Table 2
Refined atomic coordinates for the Ru
2
Ni
2
Sb12
.
Atom
Site
x
y
z
Occ.
iso (A )
˚
2
6
the BX octahedra [12,13]. The tilt angle (j) can be calculated
from the unit–cell parameter a and the M–X distance according to
B
Ru
Ni
Sb
8c
1/4
1/4
0
1/4
1/4
1/4
0.5
0.5
1
0.94(9)
0.94(9)
0.67(3)
a relationship: [14]
8c
1/4
24g
0.3379(2)
0.1556(2)
3a
cosð
j
Þ ¼ 8 ꢀ0:5
ð1Þ
d
The calculated value of octahedral tilt for Ru
2
Ni
2
Sb12 phase is
Table 3
3
3.61, which can be compared to the value of 33.71 found
Pd Ni Sb12 [10]. This value of the octahedral tilt falls between
values calculated from binary Sb-bearing skutterudites CoSb
(32.41) [7] and IrSb (34.31) [7]. Considering the covalent radii
Lattice parameters and unit–cell volume for Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2)
2
2
studied samples.
3
Weighted composition
˚
a (A)
˚
3
V (A )
3
of Ni, Co, Pd, Rh and Ir, these numbers of tilt angles support the
general trend observed in skutterudites: for a given anion, the tilt
Ru Ni Sb12
2
2
9.1767(1)
9.1762(2)
9.1757(2)
772.79(1)
772.66(3)
772.53(3)
Ru1.9Ni2.1Sb12
Ru1.8Ni2.2Sb12
angle (
As is typical for the skutterudite structure, the Sb atoms in
Ni Sb12 form the four-membered rings [Sb ] of rectangular
shape (Fig. 4(b)). The short (2.856 A) and long (2.975) Sb–Sb
distances alternate within these [Sb ] rings (Fig. 4(b)). The ratio
j) increases with the increasing size of the cation [12].
Ru
2
2
4
˚
the temperature range 100–400 K, using an alternating current of
the frequency of 1020 Hz and stationary magnetic field of an
induction B¼0.7 T. The ohmic current contacts were made by
means of sputtered Au layer and Ag-conductive paste. The mechan-
ical contacts for measuring the Hall voltage were used.
4
between these two Sb–Sb distances is 1.04, which falls between
values observed for unfilled binary skutterudites (1.03–1.05) [7].
3.2. Thermoelectric and thermal properties
The Seebeck coefficient was determined by means of static dc
method on rectangular shaped samples. The temperature gradient
between two points was measured by two shielded K-type
thermocouples that were pressed against the sample surface. A
potential difference dU corresponding to the gradient dT was
measured across the same legs of both attached thermocouples.
The absolute Seebeck coefficient was determined from the slope
of dU/dT dependence using 20 values of dT not exceeding 3 K. The
thermal diffusivity was measured on round hot-pressed sample
with help of LFA 457 (Netzsch). The thermal conductivity was
then calculated using Pyroceram 9606as a heat capacity standard.
DTA measurement, which was carried out on the studied
samples (see Fig. 2),revealed a strong endo-thermic peak at about
950–960 K, which is undoubtedly connected with decomposition
of the compounds. This is the reason why all experiments were
carried out up to maximal temperature of 800 K. Sb, NiSb
2
and
very likely RbSb were identified as breakdown products after
2
DTA-treatment. One smaller endo-thermic peak visible in DTA
curve at about 900 K corresponds to the melting of the residual
unreacted Sb (see Fig. 1).
The temperature dependencies of the electrical conductivity
s
of Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) samples are presented in
Fig. 5. From this figure it is evident that electrical conductivities of
3
. Results and discussion
all studied samples initially decrease, reach a minimum, and then
increase with the increasing temperature. The increase of
T4600 K is related to the transition of electrons across a band
s at
3.1. Crystal structure of Ru2ꢀxNi2þxSb12 samples
gap. This is a typical feature for heavily doped narrow band-
The Ru2ꢀxNi2þxSb12 (x¼0, 0.1, 0.2) phases show the skutter-
gap semiconductors. Fig. 6 shows
temperature. An exponential fit
dependencies in the area of increasing conduction provides value
s
s
as a function of reciprocal
ꢃ exp(E /2 kT) of these
udite-type structure. Their crystal structures can be described as
¼
s
0
g
an infinite array of [(Ni/Ru)Sb
6
] octahedra sharing corners with
þ
þ þ
six neighboring octahedra (Fig. 4(a)). The tilt system a
a
a
of
of activation energy E connected with the above mentioned
g

J. Navr a´ til et al. / Journal of Solid State Chemistry 193 (2012) 2–7
5
Fig. 5. Temperature dependencies of electrical conductivity
s of Ru2ꢀxNi2þxSb12
samples. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Fig. 7. Temperature dependencies of the Seebeck coefficient S of Ru2ꢀxNi2þxSb12
samples. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Fig. 6. Electrical conductivity
s versus inverse temperature for Ru2ꢀxNi2þxSb12
samples. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Fig. 8. Temperature dependencies of the Hall coefficient R
H
of Ru2ꢀxNi2þxSb12
samples. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
transition. It is approximately the same for all samples, i.e., about
0
.06 eV. Using the same procedure we obtained for analogical
skutterudite compound Fe Pd Sb12 the value about 0.22 eV [10].
The other related compound, Fe Ni Sb12, has value of E about
.16 eV [9].
Another important finding comes from the comparison of
conductivity have lower Seebeck coefficients and vice versa.
Contrary to the n-type character of their Seebeck coefficient
all samples exhibit positive values of their Hall coefficient. This
is not surprising since a similar behavior was observed also for
2
2
2
2
g
0
Fe
2
Ni
2
Sb12 compound [9] and very complex behavior of the both
Pd Sb12 [10]. Different signs of
temperature dependencies of the Seebeck coefficient (Fig. 7) and
the Hall coefficient (Fig. 8) of the prepared samples. The negative
values of Seebeck coefficient for all samples in the whole
observed temperature region reflect prevailing n-type form of
electron transport. As expected, samples with higher electrical
properties was presented for Fe
2
2
the Seebeck and Hall coefficients are usually the sign of a multi-
carrier nature of transport consisting of at least one type of electrons
and one type of holes, which participate in the conduction process
and hence mixed conduction prevails in carrier transport.

6
J. Navr a´ til et al. / Journal of Solid State Chemistry 193 (2012) 2–7
In such a two-carrier system, the Seebeck coefficient is
expressed as
S
e
n
me þS pmh
h
S ¼
ð2Þ
n
me þpm
h
where e is the charge of the carrier, p and n are the partial carrier
concentrations, and are the electron and hole mobility and
and S are the partial electron and hole Seebeck coefficients.
m
e
m
h
S
e
h
Therefore, the Seebeck coefficient is determined by both concen-
tration and mobility of carriers. In the case of the Hall coefficient,
it is the sum of quadratic terms in the carrier mobility, as given by
following expression:
2
2
h
ꢀ
n
me þp
m
R
H
¼
ð3Þ
2
eðnme þpmh
Þ
Thus, the Hall coefficient can have an opposite sign with
respect to the Seebeck coefficient. In the case of skutterudites
the hole mobility is much higher than that of electrons e.g., [15]
H
and thus R value can be positive, even though the concentration
of electrons exceeds the concentration of holes. This fact follows
from Eq. (2) and from the negative sign of measured Seebeck
coefficient. Since we deal with more than one group of carriers,
we cannot deduce the concentration of carriers from the Hall
coefficient directly. Hall mobility, or more precisely in this case
H
product of R .s,of the studied samples reach values of a few tens
2
ꢀ1 ꢀ1
Fig. 10. Thermal conductivity
L e
l and its lattice l and electronic l components
of cm V
s
(see Fig. 9). Their temperature dependencies
versus temperature for Ru Ni Sb12 sample. (For interpretation of the references to
2
2
resemble similar dependence published for Ru
2
2
Pd Sb12 com-
color in this figure legend, the reader is referred to the web version of this article.)
pound [8]. Due to the above mentioned coexistence of at least
two types of carriers one cannot make any conclusions on
dominant scattering mechanism from the dependencies just due
to the above mentioned multicarrier nature of the transport
phenomena.
Thermal conductivity of Ru
2 2
Ni Sb12 (see Fig. 10) is in the
measured temperature region (300–650 K) almost constant with
ꢀ
1
ꢀ1
value about 3.5 W m
reported value for Ru0.5Pd0.5Sb
the same value as for related skutterudite compound Fe
10]. Assuming validity of the Wiedemann–Franz law (with the
K
, i.e., about four times lower then
compound [8] and approximately
Pd
3
2
2
Sb12
[
Fig. 11. Temperature dependencies of calculated ZT values of Ru2ꢀxNi2þxSb12
samples. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
ꢀ
8
ꢀ1
Lorenz number L
0
¼2.45 ꢂ 10 W K ) we calculated electronic
part of thermal conductivity and after subtraction of the values
from total thermal conductivity we obtained its lattice com-
ponent. Its value decreases with increasing temperature from
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
about 2.5 W m
K
(300 K) down to ꢁ1.5 W m
K
(650 K).
Values of the total thermal conductivity of the other two studied
compounds slightly decrease with increasing Ni content, i.e.,
H
Fig. 9. Product R .s vs. temperature for Ru2ꢀxNi2þxSb12 samples. (For interpreta-
tion of the references to color in this figure legend, the reader is referred to the
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
3.2 W m
K
at 300 K for Ru1.9Ni2.1Sb12 and 2.9 W m
K
web version of this article.)
at 300 K for Ru1.8Ni2.2Sb12
.

J. Navr a´ til et al. / Journal of Solid State Chemistry 193 (2012) 2–7
7
Thermoelectric properties of the samples are significantly
degraded due to the participation of comparable quantities of
electrons and holes on electronic transport. The fact seems to be a
reason of low ZT values (Fig. 11) calculated for the studied
samples. They achieve maximal value ZTꢁ0.08 at 630 K for
Ru1.8Ni2.2Sb12 sample. The suppression of the concentration of
either electrons or holes could lead to an improvement of the
thermoelectric properties.
the Czech Geological Survey (project no. 332300) is greatly
appreciated.
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0
.1, 0.2) samples were prepared and their skutterudite-type struc-
(
ture with space group Im3 was confirmed. No indications of
structural ordering were found. Anomalous behavior, i.e., different
signs of Seebeck and Hall coefficient was attributed to the multi-
carrier nature of the electronic transport in the studied compounds.
The compounds exhibit a low value of lattice part of thermal
[
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ꢀ
1
ꢀ1
conductivity, i.e., about 1.5 W m
K
at 650 K. Suppression of
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5
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[
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Ontario, 2002.
Acknowledgments
[
[
[
[
13] P. Vaqueiro, G.G. Sobany, M. Stindl, J. Solid State Chem. 181 (2008) 768–776.
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15] T. Cailat, A. Borshchevsky, J.-P. Fleurial, J. Appl. Phys. 80 (1996) 4442–4449.
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Financial support from the Grant Agency of the Czech Republic
GA CR), project no. P108/10/1315, from the Ministry of Education
of the Czech Republic project MSM0021627501 and from
(