Low-temperature transport properties of the filled and unfilled IrSb3
skutterudite system
Terry M. Tritta)
Materials Physics Branch, Naval Research Laboratory, Washington DC 20375
G. S. Nolas and G. A. Slack
Department of Physics, Rensslaer Polytechnic Institute, Troy, New York 12100
A. C. Ehrlich and D. J. Gillespie
Materials Physics Branch, Naval Research Laboratory, Washington DC 20375
Josh L. Cohn
Department of Physics, University of Miami, Coral Gables, Florida 33124
~Received 30 November 1995; accepted for publication 5 March 1996!
We have measured the electrical resistivity, r, thermoelectric power, a, and thermal conductivity, k,
of the skutterudite material IrSb3 in a temperature range from 300 down to 4 K. It is found that the
electrical resistivity and thermopower decrease monotonically as the temperature is reduced to
50–60 K. Below approximately 60 K the resistivity rises in a semiconducting manner. It appears the
thermopower exhibits a large phonon drag peak at around 20 K and then falls towards zero. The
thermal conductivity increases rapidly as the temperature is decreased with a maximum at around 20
K, corresponding to the peak in the thermopower. We will discuss these results and compare them
to higher temperature data from G. A. Slack and V. G. Tsoukala @~IrSb3! J. Appl. Phys. 76, 1635
~1994!#. We have also measured some of the so-called ‘‘filled skutterudites,’’ Ir4LaGe3Sb9 ,
Ir4NdGe3Sb9 and Ir4SaGe3Sb9 . The thermoelectric properties of these materials are considerably
different than those of the unfilled sample. The thermopower is considerably lower and the
resistivity is a factor of 2–4 times higher than the unfilled sample at room temperature. The thermal
conductivity is markedly reduced by the filling, as much as a factor of 20 reduction for some of the
systems. © 1996 American Institute of Physics. @S0021-8979~96!07311-2#
I. INTRODUCTION
Over the past few years there has been renewed interest
in the field of thermoelectrics driven by the need for new and
better materials for electronic refrigeration. There have been
reports of new materials and new concepts for materials with
higher performance than existing materials, although none of
these have been realized at this time.2,3 Approximately 30
years ago the field of thermoelectrics showed great promise
by utilizing alloys based on the Bi2Te3 and Bi1-x Sbx systems
as thermoelectric materials to perform a variety of solid state
refrigeration needs. These materials have been extensively
studied and optimized for their use in thermoelectric applications and are the state of the art materials for near room
temperature use. Given the harmful impact that standard
CFC refrigeration gases have on the environment and the
need for small-scale localized cooling in computers and electronics the field of thermoelectrics is in need of higher performance room-temperature materials than exist presently. In
addition, as the field of cryoelectronics grows the need for
lower temperature ~100–200 K! and higher performance
thermoelectric materials will also be necessary.
Thermoelectric energy conversion utilizes the Peltier
heat generated when an electric current is passed through a
thermoelectric material to provide a temperature difference
between the heat sink and the object being cooled or heated.
The advantages of thermoelectric solid state energy convera!
Electronic mail: [email protected]
8412
J. Appl. Phys. 79 (11), 1 June 1996
sion are that it is compact, environmentally safe, quiet ~no
moving parts!, and provides localized heating or cooling. It
also provides long term stability with lifetimes on the order
of 20 years or more. Some applications of thermoelectrics
include cooling of charge coupled devices ~CCDs!, infrared
detectors, low noise amplifiers, and computer chips. They
also are very stable and can be used for temperature stabilization of laser diodes and/or electronic components.
The essence of a good thermoelectric material is determined from the material’s figure of merit,
Z5
a 2s
,
k
~1!
where a is the Seebeck coefficient or thermopower, s the
electrical conductivity, and k the total thermal conductivity
~k5k L 1 k e , the sum of the lattice and electronic contributions!.
The Seebeck coefficient is related to the Peltier effect
~P! by
P5 a T5
QP
,
I
~2!
where P is the Peltier coefficient, Q P is the rate of heating or
cooling and I is the electrical current. Excellent reviews of
thermoelectric properties of materials and thermoelectric refrigeration are given in Refs. 4–6. The efficiency and coefficient of performance of a device is directly related to the
figure of merit of the material. Semiconductors have long
0021-8979/96/79(11)/8412/7/$10.00
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been proven to be the material of choice for thermoelectric
applications. It is found that the most promising materials
have a carrier concentration of approximately 1019
carriers/cm3. The power factor, a2s, is typically optimized
through doping to give the largest Z. In addition attempts are
made to lower the lattice thermal conductivity without decreasing the power factor proportionally and thus further increasing the figure of merit. The standard industrial material
for operation near room temperature is the ~Bi,Sb!2~Te,Se!3
system.4–7 For this material, Z'3.431023 K21 and thus the
dimensionless figure of merit ZT'1 at T5300 K. This
ZT51 has been an upper limit for more than 20 years but
with no theoretical or thermodynamic reason why it cannot
be larger if an appropriate material can be found. If a material with ZT'2 could be found feasible applications would
possibly increase by an order of magnitude.
One of a group of materials receiving increased attention
in thermoelectrics is the promising and controversial skutterudite system which includes IrSb3 and CoSb3 .2,8,9 Initial
studies by various groups have indicated that this material
shows some promise as a potential thermoelectric material.
These materials have a somewhat complex structure with 32
atoms per cubic unit cell. A complex unit cell structure with
a large number of atoms is typically associated with a low k,
but these materials have a rather high room-temperature thermal conductivity for a thermoelectric material, k'16–20
W/m K. The carriers have high mobilities ~m'1300 cm2/V s!
which is comparable to Bi2Te3 , ~m'500–1200 cm2/V s!.
There are large voids in the structure of these materials
which can possibly be filled with an additional atom. This
provides an opportunity for manipulating the thermal conductivity by increasing the phonon scattering with the filling
atoms. In general, these materials meet much of the basic
criteria for a material to exhibit a high ZT as pointed out by
Slack:10 large complex unit cell with heavy constituent atoms, high mobility carriers, and also a small band gap.1,10,11
From Slack’s estimation of the minimum thermal conductivity the total thermal conductivity in these skutterudite materials could be lowered by a factor of 40.1,11 Thus the possibility exists for greatly enhancing the figure of merit for
these materials.
The skutterudites get their name from a naturally occurring mineral, skutterudite or CoAs3 , found in Skutterud, Norway. The skutterudite structure, AB 3 ~where A is the transition metal element Ir, Co, or Rh, and B is the pnicogen
element such as P, As, and Sb!. The structure is cubic ~with
space group Im3! and the unit cell contains 8AB 3 groups. It
contains 32 atoms per unit cell as well as two voids. A good
picture of the structure is shown in Refs. 2 and 12. A recent
article by Morelli et al.12 gives an excellent summary of the
known transport properties of some these materials and concludes that our knowledge of them is inadequate, especially
their low-temperature behavior.
Very recently there has been research on the lowtemperature properties of CoSb3 by Morelli et al.12 and Mandrus et al.13 Both of these workers compare their results to
recent band structure calculations of Singh and Pickett
~S&P!14 who predict a semiconducting behavior for these
materials and a highly nonparabolic valence band. Mandrus
J. Appl. Phys., Vol. 79, No. 11, 1 June 1996
et al. find a semiconducting energy gap of E G '580 K ~50
meV! in the CoSb3 in agreement with S&P. Morelli et al.
find that the CoSb3 material exhibits large hole mobilities,
large lattice thermal conductivity to low temperatures, and
large phonon drag effects evident in the thermoelectric
power. In addition, very recently there was a relatively thorough investigation of the CoSb3 system by Sharp et al.15
They found that the compound, Co0.972x Fex Ir0.03Sb2.85As0.15
showed the highest ZT for a p-type skutterudite material
~ZT50.3@575 K! and Co0.97Ir0.03Sb2.852x TexAs0.15 showed
the highest ZT for an n-type skutterudite material ~ZT
50.6@700 K!. These values do not compete with the standard Bi2Te2.25Se0.75 materials ~n type! with ZT'1 at 500 K
that are currently in use. Sharp et al. conclude from their
data that the skutterudite family seems unlikely to yield a
superior thermoelectric material. However, Morelli and
Meisner16 recently reported that in a filled skutterudite material, Fe4CeSb12 , the thermal conductivity is reduced considerably over the unfilled systems, presumably due to the
large Ce atom, and that the other thermoelectric properties
are characteristic of a heavy fermion system. Because of the
metallic character of the electronic properties this may not be
a good choice of the filled skutterudites for eventual application.
We have chosen to investigate the filling of the voids in
the IrSb3 system with rare earth atoms, La, Nd, and Sm. The
IrSb3 system is chosen because, as we discussed previously,
it has the largest void radius as well as exhibiting promising
thermoelectric electronic properties.17 The focus of this current article will be primarily on first establishing the lowtemperature electronic transport properties of IrSb3 . We will
then present data on the impact of filling these voids with La,
Nd, and Sm on the electrical properties, resistivity, and thermoelectric power, and the temperature dependence of these
properties as well as the effect on the thermal conductivity.
II. EXPERIMENTAL PROCEDURE
The description of the preparation of these materials is
given in detail in another publication.17 Our samples have
been made by hot isostatic pressing ~Hipping! of powders.
These samples are polycrystalline with typical sizes of 12
mm35 mm33 mm and were cut into these sizes with a
high-speed diamond saw. We have performed standard fourprobe resistance measurements on all our samples. We used
0.005 in. Au–Fe ~0.07 at %! vs chromel thermocouples attached to the samples with GE 7031 varnish to make the DT
measurements. The sample voltage (V S ) as well as the thermocouple voltage (V TC ) were measured with a Keithley 182
nanovoltmeter. The Au–Fe vs chromel thermocouples provide adequate sensitivity even at the lower temperatures
~T'4 K!. The temperature of the sample is determined using
a Lake Shore calibrated Cernox sensor. The resistance of the
sample was obtained by a dc method of reversing the current
at each temperature and measuring the sample voltage and
then taking an average. Typical sample currents were 5–50
mA.
The thermopower was measured in two ways. First a
standard V S vs DT curve was taken at several temperatures
with the gradient on the sample being swept from 2DT to
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FIG. 1. The resistivity ~r! and thermopower ~a! of polycrystalline IrSb3
~82% dense! as a function of temperature.
1DT with DT'2%–5% of T. The slope was calculated and
the Au lead contribution subtracted thus giving the absolute
thermopower of the material. We used this method to check
our more typical method at several temperatures. In the more
typical method a DT of 2%–5% T was established across the
sample. Then under computer control the sample current was
cycled in the following sequence; ~1! 1I, ~2! I50, ~3! 2I,
~4! I50; and ~5! 1I, with sample current and voltage, absolute temperature, and thermocouple voltage being read and
recorded at each step. The sample resistance was calculated
from averaging steps 1, 3, and 5 and the thermopower was
calculated from an average of steps 2 and 4. Excellent agreement was found between the two methods. Then the temperature of the sample was slowly lowered in a variable temperature dewar from T5300 K to T54 K over a period of
14–16 h and the resistance and thermopower monitored as a
function of temperature.
The thermal conductivity k is the most difficult parameter to measure accurately in the determination of a material’s figure of merit. There are many corrections for problems
such as radiation effects and they can vary considerably in
importance with sample geometry. Good thermoelectric materials ~high Z! have a relatively low k, on the order of 1–5
W/m K, further complicating the measurement. The samples
in this work are rather large for thermal conductivity measurements. We have measured the thermal conductivity of
smaller pieces of these samples at General Motors and the
University of Miami using a standard steady-state technique
as described elsewhere.18,19
FIG. 2. The resistivity ~r! and thermopower ~a! of polycrystalline IrSb3
~82% dense! as a function of temperature for T,100 K.
decreases with temperature typical of linear diffusion thermopower down to T'40 K with a strong peak at T'25 K
and then decreasing toward zero as T goes to zero. This is
typical of a strong phonon drag thermopower. We compare
our room-temperature measurements on the 82% dense
sample to measurements made on a 98% dense sample reported by Slack and Tsoukala ~S&T!.1 They are as follows
with our results given first; thermopower ~176.7, 173!
mV/K, resistivity ~4.23, 4.4!31024 V cm, and thermal conductivity ~20, 17.5! W/m K. Thus, there is very good agreement between the results on this sample and that in Ref. 1.
The present data have not been corrected for the density
difference.
In Fig. 2 is the same data on an expanded scale to show
the low-temperature region ~T,150 K!. It became more difficult to establish a temperature gradient on this sample
around the region of the peak, since the thermal conductivity
was becoming much larger. We measured r, a, and l of a
small sliver on the outside edge of this sample. The thermal
conductivity results are shown in Fig. 3. There is a large
peak in the thermal conductivity at approximately the same
temperature as the ‘‘phonon drag’’ peak in the thermopower,
thus confirming a large phonon contribution to both param-
III. RESULTS AND DISCUSSION
The temperature dependence of the resistivity and thermopower of a standard unfilled IrSb3 with a density of 82%
of theoretical is shown in Fig. 1. Due to the fact that the
filled skutterudites prepared for this study densified to approximately 80% of theoretical density, this IrSb3 sample
was intentionally prepared to a similar density for experimental comparison.17 The resistance is monotonically decreasing with decreasing temperature down to T'60 K,
where there is a broad minimum and then a change in sign of
dR/dT below this. The thermopower a also monotonically
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J. Appl. Phys., Vol. 79, No. 11, 1 June 1996
FIG. 3. The total thermal conductivity kT of polycrystalline IrSb3 ~82%
dense! and Ir4LaGe3Sb9 ~also 82% dense! as a function of temperature.
These data have not been corrected for radiative losses.
Tritt et al.
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FIG. 4. The resistivity ~r! and thermopower ~a! of a La filled skutterudite,
Ir4LaGe3Sb9 as a function of temperature.
eters at these low temperatures. The thermal conductivity of
IrSb3 has a strong temperature dependence and is very similar to that measured by Morelli et al.12 for the CoSb3 . That
material also shows a phonon drag peak in the thermopower
close to the peak in the thermal conductivity. There was very
good agreement with the resistivity measurements but the
thermopower measurements showed some differences between the larger sample and the small sliver. In the smaller
sample the large phonon drag peak was not evident and the
room-temperature thermopower was less, 62 mV/K as compared to 76 mV/K, than those in Fig. 1. It may be that the
sample was somewhat nonuniform. It is difficult to obtain
iridium very pure and thus some of the impurities in the
iridium could be affecting our results. We are presently performing experiments to check this possibility.
Note the large upturn in resistance at T560 K in Fig. 2,
where the resistance now shows a semiconducting type behavior which would correspond to a narrow band gap of
E G '10 meV. Singh and Pickett14 actually predict that IrSb3
would be essentially a zero gap semiconductor. Mandrus
et al.13 performed measurements on the low-temperature
properties of CoSb3 and found excellent agreement with the
predicted band gap of 50 meV by S&P. Our results on the
IrSb3 are consistent with a very small gap or a heavily doped
large-gap material with shallow acceptors.
In Fig. 4 we show a and r of one of the filled skutterudites, Ir4LaGe3Sb9 , where La occupies all of the voids of the
IrSb3 structure. The Ge was added in an attempt at charge
compensation for the La. It is apparent from the values of a
and r that these properties of the filled skutterudite were not
optimized. Obviously, more work needs to done on the
charge compensation of the rare-earth ion’s donated electrons and this is under way. Notice that the resistivity has a
more metallic temperature dependence even to 4 K and the
room-temperature resistivity is approximately two times that
of the unfilled sample. The room-temperature thermopower
is approximately a factor of 10–12 smaller than the unfilled
sample. There still exists a low-temperature peak in the thermopower at about the same temperature as the IrSb3 . This
peak in the thermopower corresponds to the peak in the thermal conductivity shown in Fig. 3. In addition the therJ. Appl. Phys., Vol. 79, No. 11, 1 June 1996
mopower undergoes a change in sign at T'180 K indicating
a change in dominant carrier from p type to n type suggesting multi-band conduction. We hope to confirm this with
Hall measurements that are planned for the future. However,
initial Hall probe data of Ir4LaGe3Sb9 taken at 300 and 77 K
agree with this observation.17 We also measured a and r of a
small sliver of the Ir4LaGe3Sb9 sample. The measurements
on the small samples of IrSb3 and Ir4LaGe3Sb9 were conducted at the University of Miami. There was excellent
agreement of the data from the two Ir4LaGe3Sb9 samples.
The thermal conductivity of the La filled skutterudite is
reduced by approximately an order of magnitude in comparison with the unfilled material, IrSb3 . The respective values at
room temperature are 20 W/m K for IrSb3 and 3.7 W/m K
for the La filled system. This shows that the thermal conductivity can be substantially altered by this filling of the voids,
but obviously much more work needs to be done to optimize
the electrical properties through charge compensation. One
must remember that the electrical properties and the thermal
conductivity ~through kE ! are not independent and that they
are related through the Wiedeman–Franz law, k E 5L 0 s T,
where L 0 is the Lorentz number ~2.4531028 W V/K2!, s is
the electrical conductivity ~s51/r!, and T is the temperature.
Also, changing the scattering only of the phonons without
affecting the electron scattering would be very difficult.
Therefore changing the phonon scattering in order to manipulate the thermal conductivity will also affect the electronic properties. The advantage in these skutterudites is that
the lattice thermal conductivity is approximately 90% of the
total thermal conductivity so it provides an opportunity to
find a compromise between these effects.
We have also measured the room-temperature resistivity
and thermopower of a Nd, ~Ir4NdGe3Sb9! and Sm
~Ir4SmGe3Sb9! filled sample. The results are shown, respectively, in Figs. 5 and 6. The Nd filled sample has a roomtemperature thermopower of 11 mV/K and a resistivity of
1731024 V cm. The thermal conductivity for both samples
has been measured by one of the co-authors ~Morelli! at
General Motors and is presented in another article.17 In fact,
the temperature dependence for all four systems is shown in
Fig. 2 of that article. The thermal conductivity at 300 K is
even lower than that for the La filled sample at 300 K with
k51.2 W/m K. The thermal conductivity of the Sm filled
sample was comparable in magnitude ~k'1.6 W/m K! and in
temperature dependence with that of the Nd filled sample. As
shown in Figs. 4 and 5, the temperature dependence of the
resistivity for the La and Nd filled samples shows a
metalliclike dependence, that is dR/dT.0 for all T ~4 K,T
,300 K! and magnitudes at T5300 K of 9 and 16 ~31024
V cm!, respectively. The Sm filled sample shows comparable magnitude ~1231024 V cm! and temperature dependence, as seen in Fig. 6, but contrasts at low temperatures
with dR/dT,0 below approximately 40 K.
The thermopower data for the three materials is shown
in Figs. 4–6. All these filled materials exhibit electron and
hole conduction as evidenced by the change in sign of the
thermopower at T'190 K ~La filled!, T'290 K ~Nd filled!,
and T'115 K ~Sm filled!. All three have a p-type thermopower at room temperature as does the unfilled IrSb3 .
Tritt et al.
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FIG. 5. The resistivity ~r! and thermopower ~a! of a Nd filled skutterudite,
Ir4NdGe3Sb9 , as a function of temperature.
FIG. 6. The resistivity ~r! and thermopower ~a! of a Sm filled skutterudite,
Ir4SmGe3Sb9 , as a function of temperature.
However, the conduction is electron dominated for the La
and Nd systems at low temperatures in contrast to the unfilled IrSb3 which has hole dominated conduction. The Sm
filled sample exhibits hole conduction down to approximately 115 K and then changes sign to electronlike from 115
to 60 K and then slightly holelike again. The magnitude of
the thermopower is very small below 115 K ~uau,0.5 mV/K!.
It is therefore difficult to attribute the behavior to either carrier domination or phonon drag effects. As discussed earlier,
it appears the La filled sample exhibits a low-temperature
phonon drag peak at around 45 K, also evident in the thermal
conductivity as shown in Fig. 3. The Nd filled sample exhibited a more unusual temperature dependence of the thermopower with a broad peak between approximately 80 and
160 K, especially since the resistivity had the most
metalliclike temperature dependence. The temperature dependence of the thermopower is very similar to a system
~TaSe3! studied by one of the authors ~Tritt! earlier.20 TaSe3
is a semimetallic material with very small overlap of the
bands. The temperature dependence of the lattice part of the
thermal conductivity for the Nd and Sm samples is shown in
Fig. 2 of Ref. 17 and is much less than that of IrSb3 . Thus
the thermopower data ~Figs. 5 and 6! and thermal conductivity data ~Fig. 2, Ref. 17! suggest that there does not appear to
be the strong phonon drag effects in these materials that are
evident in the IrSb3 and Ir4LaGe3Sb9 systems ~Figs. 1, 3, and
4 of this article!.
All of the results for these four samples are summarized
in Table I. We have listed the room-temperature properties as
well as the 77 and 10 K properties. We have also estimated
the lattice thermal conductivity by calculating the electronic
part of the thermal conductivity kE from the Wiedemann–
Franz law and this is given in Table I for T5300 K. Even
though we are greatly reducing the thermal conductivity the
dimensionless figure of merit ZT ~T5300 K! is much too
low for these materials ~ZT,2.131022 for all samples!. As
stated previously, charge compensation is obviously an essential part of this problem. We need to keep the large thermopower and high electrical conductivity while reducing the
thermal conductivity and this has not yet been accomplished.
Other methods of charge compensation are currently being
investigated and what follows is a discussion concerning the
charge compensation aspect of this investigation. Of course,
another possibility is to fill the voids with a neutral atom that
will scatter the phonons while minimizing the scattering effect on the electrons.
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J. Appl. Phys., Vol. 79, No. 11, 1 June 1996
IV. CHARGE COMPENSATION
As was discussed earlier, the crystal structure of the
skutterudites has two large voids per cubic unit cell. These
voids can be populated by a number of different atoms or can
remain vacant. Rare-earth and alkaline earth atoms have
been placed in the voids by a number of different synthesis
techniques. Such atoms usually end up as positively charged
ions in the skutterudite structure, their electrons having been
donated to the conduction or valence bands of these semiconductors. For example, Zemni et al.21 have put La, Ce, Pr,
Nd, and Yb into CoP3 using a molten tin solvent. The voids
were partially filled ~20%–25%! and the lattice parameter
increased ~0.4%! due to the filling atoms. No electrical properties were measured, but we presume the samples were n
type with excess electrons.
The other filled skutterudites so far reported in the
literature16,22–31 such as Fe4LaP12 , Fe4LaAs12 , Fe4LaSb12 ,
Fe4CeSb12 , and Ru4LaP12 are metallic, not semiconducting.
The compounds27,28 Fe4CeP12 , Fe4LaAs12 , and Fe4UP12 , in
TABLE I. Summary of properties.
a
~mV/K!
r
~1024 V cm!
lT
~W/m K!
lL
~W/m K!
300
77
10
300
77
10
300
77
10
300
77
10
K
K
K
K
K
K
K
K
K
K
K
K
1IrSh3
w/La
w/Nd
w/Sm
176.7
135
144
4.23
3.2
3.8
20
46
60
18.2
45.4
59.9
16.4
23.5
20.1
9.04
6.5
5.6
3.7
2.7
3.0
2.9
2.4
2.95
10.9
28.0
22.2
16.9
12
11.1
1.2
1.1
0.42
0.57
0.9
0.4
17.2
20.2
10.3
11.8
6.6
6.5
2.1
2.3
0.84
1.5
2.0
0.8
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TABLE II. Covalent radii of some possible electron donors.
Candidate
Element
Covalent
radius Å
No. of Covalent
Electrons
Zn
Cd
Hg
Ga
In
Ge
Sn
As
Sb
1.22
1.39
1.39
1.20
1.36
1.225
1.405
1.260
1.450
2
2
2
3
3
4
4
5
5
which the donated electron count per formula unit can be
equal to 72, are semiconductors. This behavior has been explained by King32 and Dudkin.33 In the metallic samples the
electron count is usually less than 72.
Our goal is to make filled skutterudites in which the
electron count is exactly 72.00. Thus if we place trivalent
rare-earth ions in the voids we need a framework structure
where the electron count is exactly 69.00. In Ir4Sb12 , the
count is 72.00; each Ir atom donates three electrons and each
Sb donates five electrons. Thus if we can replace some of the
Sb atoms, all tetrahedrally bonded, by other atoms that will
donate fewer electrons, will have tetrahedral coordination
and will have a covalent radius close to that of Sb. Some
candidates are give in Table II. The best candidates are Sn
and In, followed by Ge, Cd, and Ga. We have chosen to
replace three Sb atoms with three Ge atoms in Ir4Sb12 . The
electrical measurements on these samples indicate rather
high carrier concentrations. Whether this high carrier concentration is caused by a chemical imbalance in the desired
one La to three Ge ratio in Ir4LaGe3Sb9 or whether it is
caused by a decrease in the band gap is not known. Further
experiments on Sn, In, or Ga compensation are required to
answer this question.
Another possible compensation mechanism25 would be
to replace Ir with Os. Thus to just balance the three electrons
from the La we would want IrOs3LaSb12 . There is some
uncertainty of the valence of Os in such a structure. We
would like the Os to donate two electrons. However, in the
compound OsSb2 it donates four electrons when surrounded
by six Sb atoms. Similarly, we could try IrRu3LaSb12 , but
the same uncertainty as to the Ru donation of electrons exists.
Careful attention to such details will be required in order
to produce skutterudites with low lattice thermal conductivity and the optimum doping level for high Z materials.
V. CONCLUSION AND SUMMARY
We have measured electrical resistivity, thermopower,
and the thermal conductivity of a series of four materials; the
unfilled IrSb3 and three filled skutterudites, Ir4LaGe3Sb9 ,
Ir4NdGe3Sb9 and Ir4SmGe3Sb9 . We have found that the thermal conductivity can be greatly effected by this filling and
reduced by a factor of 20 in agreement with Slack’s minimum thermal conductivity theory which estimates a possible
reduction of a factor of 40. Unfortunately, we also find that
J. Appl. Phys., Vol. 79, No. 11, 1 June 1996
the electronic properties are affected in an undesirable manner in relation to a good thermoelectric material. Primarily,
the thermoelectric power is reduced to a totally unacceptable
magnitude for thermoelectric applications ~uau,10 mV/K!
over the entire temperature range T,300 K. We have found
that these materials exhibit interesting temperature dependence of these properties at low temperatures, T,300 K.
Much more work needs to be done to optimize the electrical
properties of these materials through charge compensation
for filling with the rare-earth atoms. Alternately the insertion
of neutral atoms may have a smaller effect on the electronic
properties. The skutterudite materials are susceptible to doping in such a way as to change the dominant carrier concentration but one must be able to maintain relatively high values for the thermopower ~uau>100 mV/K!. As presented in
this article and in Ref. 17, we have made considerable
progress in the area of minimizing the thermal conductivity
but much more research needs to be done if this material or,
as Sharp et al.15 conclude, any skutterudite material is to be
considered as viable for thermoelectric applications.
ACKNOWLEDGMENTS
The authors would like to thank Dr. Hylan B. Lyon, Jr.,
and Dr. J.-P. Fleurial for helpful discussions. They would
also like to thank D. T. Morelli for thermal conductivity
measurements on the Nd and Sm filled samples. This work
was supported, in part, by the U.S. Office of Naval Research,
Grant No. N00014-94-1-0341.
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