On the oxidation of EuFe4Sb12 and EuRu 4Sb12

Article abstract of DOI:10.1021/ic200619c

Rare-earth-filled transition-metal pnictides having the skutterudite-type structure have been proposed for use as high-temperature thermoelectric materials to recover waste heat from vehicle exhaust, among other applications. A previous investigation by this research group of one of the most studied skutterudites, CeFe₄Sb₁₂, found that, when exposed to air, this material oxidized at temperatures that are considerably below the proposed maximum operating temperature. Here, by the combined use of TGA, powder XRD, and XANES, it has been found that the substitution of Ce³⁺ and Fe²⁺ for larger rare-earth and transition-metal elements (Eu ²⁺ and Ru²⁺) results in a significantly higher oxidation temperature compared to that of CeFe₄Sb₁₂. This increase can be related to the increased orbital overlap provided by these larger atoms (Eu²⁺ and Ru²⁺ vs Ce³⁺ and Fe²⁺), enabling the development of stronger bonds. These results show how selective substitution of the constituent elements can significantly improve the thermal stability of materials.

Full text of DOI:10.1021/ic200619c

ARTICLE
pubs.acs.org/IC
On the Oxidation of EuFe4Sb12 and EuRu4Sb12
Jessica M. Peddle, Michael W. Gaultois, and Andrew P. Grosvenor*
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9
ABSTRACT: Rare-earth-filled transition-metal pnictides having the skutterudite-type struc-
ture have been proposed for use as high-temperature thermoelectric materials to recover waste
heat from vehicle exhaust, among other applications. A previous investigation by this research
group of one of the most studied skutterudites, CeFe4Sb12, found that, when exposed to air, this
material oxidized at temperatures that are considerably below the proposed maximum
operating temperature. Here, by the combined use of TGA, powder XRD, and XANES, it
has been found that the substitution of Ce3þ and Fe2þ for larger rare-earth and transition-metal
elements (Eu2þ and Ru2þ) results in a significantly higher oxidation temperature compared to
that of CeFe4Sb12. This increase can be related to the increased orbital overlap provided by
these larger atoms (Eu2þ and Ru2þ vs Ce3þ and Fe2þ), enabling the development of stronger bonds. These results show how
selective substitution of the constituent elements can significantly improve the thermal stability of materials.
RE is Eu. In this case, X-ray absorption near-edge spectroscopy
(XANES) has shown Eu to be nearly divalent, which suggests
the presence of ∼2 holes per formula unit.16ꢀ18 Rare-earth-filled
skutterudites have low thermal conductivity but exhibit
significant electrical conductivity.6,8 These materials have his-
torically been referred to as “phonon-glass, electron-crystals”,
with the reduced thermal conductivity being attributed to
phonon scattering resulting from the RE atoms “rattling” in the
icosahedral cage of Pn atoms.6,8,19 Recent studies, however, have
instead suggested that the RE-filled skutterudites might be better
described as “phonon- and electron-crystals” with the reduced
thermal conductivity resulting from a guestꢀhost coupling
interaction between the RE atoms and the M and Pn atoms.20
Along with the properties listed above, these materials are also
generally observed to be metals or narrow band gap
semiconductors.8
A recent investigation of CeFe4Sb12 found that this material
completely oxidizes when exposed to air at temperatures above
300 °C.21 Although fabrication methods are such that thermo-
electric devices are protected from the atmosphere, to increase
the lifetime of these materials, it seems reasonable to attempt to
enhance their resistance to high-temperature oxidation.1,22 Such
a resistance may be obtained through elemental substitution. It
has been shown previously that CeyRu4ꢀxIrxSb12 (x < 1.5)
decomposes at much higher temperatures than CeFe4Sb12 under
inert conditions; however, no investigation of the ability of these
materials to resist oxidation has been completed.7
1. INTRODUCTION
To increase the fuel efficiency of vehicles without reducing the
total power available for use, technologies are being (or have been)
developed to convert waste energy to electricity.1ꢀ3 Energy lost
during braking can be recaptured and waste heat produced from
vehicle exhaust can be converted to useful power.4,5 Thermo-
electric materials have been proposed for the conversion of waste
exhaust heat to electricity.2ꢀ4,6 For such applications, the max-
imum temperature that the device may be exposed to is relatively
high (∼500 °C) and, as such, it is important to develop materials
that are stable under these harsh operating conditions.1 Of the
thermoelectric materials proposed for this application, p- and
n-type rare-earth (RE) filled skutterudites (e.g., REM4Pn12; RE =
La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Yb; M = Fe, Ru, Os; Pn = pnicogen
= P, As, Sb) have received considerable attention because of
their relatively high thermoelectric figure of merit at high
temperatures.3,6,8ꢀ11 (The figure of merit, ZT, describes the
performance of a thermoelectric material.6)
The cubic structure adopted by the RE-filled skutterudites
(Figure 1) is based on that of the parent mineral phase, CoAs3.6,8,12
This structure contains tilted corner-sharing metal-centered octahe-
dra and nearly square Pn4 rings. The presence of Pn4-rings in the
structure leads to the formation of two distorted icosahedral cages
per unit cell.8 These cages can be occupied by RE atoms when the
transition-metal sites are primarily occupied by group 8 elements
(e.g., Fe, Ru, Os) with the charges usually represented as
RE3þM2þ4Pn1-12 and the M atoms being low-spin (d6).6,8,13
The ability of skutterudite-type materials to act as a thermo-
electric can be related to their unique electronic structure and
crystal structure. These materials are normally considered to be
hole-doped (p-type), but substituting Co2þ or Ir2þ into the
transition-metal site can lead to the development of n-type
materials (e.g., REFe4ꢀxCoxSb12, CeyRu4ꢀxIrxSb12), changing
the primary charge carrier from holes to electrons.6ꢀ8,14,15 The
p-type RE-containing skutterudites are usually thought to con-
tain one hole per formula unit (i.e., REM4Pn12) except when the
In this study, EuFe4Sb12 and EuRu4Sb12 have been examined
to determine how the substitution of larger rare-earth and
transition-metal atoms, compared to CeFe4Sb12, affects the
temperature at which point these materials oxidize. Thermo-
gravimetric analysis (TGA), powder X-ray diffraction (XRD),
and XANES have been used to study the oxidation of these
Received: March 25, 2011
Published: June 08, 2011
r
2011 American Chemical Society
6263
dx.doi.org/10.1021/ic200619c Inorg. Chem. 2011, 50, 6263–6268
|

Inorganic Chemistry
ARTICLE
Figure 1. Representation of the structure adopted by the RE-filled
skutterudites, REM4Pn12 (Im3) showing the formation of nearly square
Pn4 rings as well as the tilted MPn6 octahedra (a) and the distorted
icosahedral cage of Pn atoms, which can surround a RE atom (b). (The
a-axis is directed out of the page.)
Figure 2. TGA profiles from EuFe4Sb12 and EuRu4Sb12 heated in air. A
substantial increase in mass, resulting from oxidation, is observed at
∼360 °C for EuFe4Sb12 and at ∼460 °C for EuRu4Sb12.
materials and to identify the oxidation products. The observa-
tions discussed herein provide important insight into how
elemental substitution can influence the resistance of these
materials to oxidation.
specific temperatures, as the spectra are sensitive to the local coordina-
tion environment and do not require the presence of long-range order.
XANES spectra were collected using the Pacific Northwest Consor-
tium/X-ray Science Division-Collaborative Access Team (PNC/XSD-
CAT, sector 20) bending magnet beamline (20BM) located at the
Advanced Photon Source (APS), Argonne National Laboratory. Spectra
from finely ground powders sandwiched between Kapton tape were
collected using a Ge detector (fluorescence spectra) and N2/He-filled
ionization chambers (transmission spectra). Owing to the similarity in
energy, the Eu L3-edge spectra were calibrated by setting the peak
maximum of the first derivative of the Fe K-edge spectrum from Fe metal
foil to 7112.0 eV.24 The Ru L3-edge spectra were calibrated using Ru
metal with an edge energy of 2838.0 eV.24 The interpretation of spectra
from the oxidized skutterudites was aided by comparison to spectra from
standard materials containing the element of interest having different
oxidation states and to phases either equivalent to or similar to those
identified by powder XRD. The standard materials investigated were
Eu2O3 (Alfa Aesar), Fe2O3 (Alfa Aesar), RuSb2, RuO2 (Alfa Aesar),
RuCl3 (Alfa Aesar), Sb2O3 (Alfa Aesar), and Sb2O5 (Alfa Aesar). All
spectra were analyzed using the Athena software program.25
2. EXPERIMENTAL SECTION
2.1. Synthesis. EuFe4Sb12 and EuRu4Sb12 were synthesized via
direct reaction of the elements (Eu, HEFA Rare-Earths Canada, 99.99%;
Fe, Alfa Aesar, >99.9%; Ru, Smart-Elements, 99.97%; Sb, Alfa Aesar,
99.5%) in evacuated fused-silica ampules. An optimized heating profile
was used to synthesize these materials.23 The reactants were heated to
600 °C over 10 h and annealed at this temperature before the
temperature was ramped to 1050 °C over 12 h to allow for proper
mixing of the reactants, as this temperature is greater than the melting
point of both Eu and Sb. After annealing these materials for >40 h at
1050 °C, they were quench-cooled in air. Once cool, the products were
ground finely, sealed in new ampules, and annealed at 700 °C for >30 h.
This final annealing temperature was used because the skutterudites
generally decompose at temperatures above this value under inert
environments.23 RuSb2 was synthesized to act as a standard by reacting
stoichiometric amounts of Ru and Sb at 1050 °C in an evacuated fused-
silica ampule. (For all synthesized materials, the initial reaction at
1050 °C was performed in acetone-pyrolyzed ampules to restrict the
interaction between the elements and the silica ampules.) The purity of
the synthesized materials was confirmed by powder X-ray diffraction
(XRD) using a Rigaku Rotaflex RU-200 rotating anode powder XRD.
Duplicates of each skutterudite material were synthesized, with some
being observed to be phase pure while others were found to contain
minor (<10%) contributions of FeSb2 or RuSb2 and/or Sb metal.
2.2. TGA and Powder XRD of Oxidized Materials. The air
oxidation of EuFe4Sb12 and EuRu4Sb12 was investigated using a TA
Instruments Q5000 TGA. Finely powdered samples of EuFe4Sb12 and
EuRu4Sb12 were heated in air (flow rate = 25 mL/min) in a Pt pan from
room temperature to 800 at 10 °C/min with the change in mass being
constantly measured. The oxidation of RuSb2 was also investigated using
identical parameters to those listed above. Portions of each powdered
Skutterudite were heated in air for two hours at temperatures ranging
from 300 to 800 °C to further study the oxidation of these materials.
(Oxidation temperatures were chosen based on examination of the TGA
data.) Powder XRD was performed to identify the oxide phases formed
using the instrument described above.
3. RESULTS AND DISCUSSION
3.1. TGA. If skutterudite materials are to be used in thermo-
electric devices to convert waste heat to electricity under
terrestrial conditions, it is important to understand how they
react with the atmosphere at various temperatures. TGA was
performed to study the resistance of these materials to oxidation
at high temperatures, and plots representing the percent mass
change versus temperature for EuFe4Sb12 and EuRu4Sb12 are
presented in Figure 2. The mass of the materials studied was
found to increase as the temperature was raised (i.e., oxidation
occurs). When exposed to the atmosphere, significant oxida-
tion of EuFe4Sb12 and EuRu4Sb12 begins above ∼360 and
∼460 °C, respectively. Both temperatures are considerably
higher than the temperature at which CeFe4Sb12 begins to
oxidize (∼300 °C).21
The TGA profiles presented in Figure 2 show multiple
changes in slope between ∼360 and ∼580 °C, which is likely
because of the formation of different oxides at various tempera-
tures (vide infra). The rate of change in mass for each sample
slows above ∼530 °C, which suggests that these skutterudites
2.3. XANES. A previous investigation of the oxidation of CeFe4Sb12
found that many of the products formed are amorphous, requiring
techniques other than powder XRD to identify them.21 Eu and Ru L3-
edge XANES was used to investigate the materials oxidized in air at
6264
dx.doi.org/10.1021/ic200619c |Inorg. Chem. 2011, 50, 6263–6268

Inorganic Chemistry
ARTICLE
Figure 3. Powder XRD patterns from Eu4Fe4Sb12 exposed to air at
room temperature (a), 400 °C for 2 h (b), and 500 °C for 2 h (c). The
different crystalline phases that were identified have been labeled using
symbols. Oxides involving all of the constituent elements were not
identified, suggesting the presence of some amorphous species.
have completely oxidized. (The slight dip observed at ∼570 °C
in the profile from EuRu4Sb12 may suggest sublimation of some
of the oxidation products at high temperature, which is an
unfortunate effect observed by many thermoelectric materials
and can influence device reliability.1,7) The difference in mass
change observed in EuFe4Sb12 versus EuRu4Sb12 is a result of the
greater atomic mass of Ru.
3.2. Powder XRD. From the TGA results presented above, it can
be easily seen that both EuFe4Sb12 and EuRu4Sb12 oxidize at
temperatures much higher than CeFe4Sb12 does.21 However, it is
important to identify the oxidation products formed as this allows
for an analysis of the relative bond strengths between the elements
within the original material and an examination of whether all
bonds within the structure are broken at similar temperatures. In
this study, most of the oxidation products could be identified by
analysis of the oxidized materials by powder XRD (see Figures 3
and 4).
3.2.1. EuFe4Sb12. Powder XRD patterns from EuFe4Sb12 after
being exposed to air at room temperature (∼25 °C) and at 400
and 500 °C for 2 h are presented in Figure 3. The powder XRD
pattern presented in Figure 3a represents nearly pure-phase
EuFe4Sb12. After exposure to air at 400 °C (Figure 3b), Sb2O4
and Sb2O3 were identified by powder XRD along with a small
quantity of EuFe4Sb12. As no Eu- or Fe-bearing oxide phases
were identified, it is likely that these materials are amorphous.
(A similar observation was made for CeFe4Sb12 after being
exposed to air at temperatures slightly above 300 °C.21) After
exposure to air at 500 °C (Figure 3c), oxides containing all of the
constituent elements were identified by powder XRD (EuSbO4,
FeSbO4, and Sb2O4).
3.2.2. EuRu4Sb12. The powder XRD patterns from EuRu4Sb12
after exposure to air at room temperature and at 400, 500, and
600 °C for 2 h are presented in Figure 4. As mentioned in
the Experimental Section, synthesized samples of EuRu4Sb12
were often found to contain low concentrations of RuSb2
(see Figure 4a). After exposure of EuRu4Sb12 to air at 400 °C,
Figure 4. Powder XRD patterns from Eu4Ru4Sb12 exposed to air at
room temperature (a), 400 °C for 2 h (b), 500 °C for 2 h (c), and 600 °C
for 2 h (d). The crystalline phases that were identified in the powder
patterns have been labeled with symbols. To confirm the formation of
Ru metal during the oxidation of EuRu4Sb12, the oxidation of RuSb2 was
investigated by TGA; the powder XRD pattern from RuSb2 after the
TGA experiment was complete is presented in part e.
only EuRu4Sb12 and RuSb2 were identified in the corresponding
powder XRD pattern (Figure 4b), confirming its increased
resistance to oxidation compared to EuFe4Sb12 and CeFe4Sb12.21
When EuRu4Sb12 was exposed to air at 500 °C, only Sb2O4,
Sb2O3, and Ru metal were identified in the corresponding
powder XRD pattern along with a minor quantity of EuRu4Sb12
(Figure 4c): no other ordered materials containing either Ru or
Eu were observed. When EuRu4Sb12 was exposed to air at
600 °C, only Sb2O4 and Ru metal were unambiguously identified
in the corresponding powder XRD pattern (Figure 4d). Weak
intensity peaks that might suggest the presence of a very low
concentration of EuSbO4 (not labeled in Figure 4d) were also
observed in this powder diffraction pattern.
The observation of Ru metal in the powder XRD patterns
presented in Figure 4c,d was unexpected; however, such an
identification was also made by examination of the powder XRD
pattern collected from RuSb2 after investigating its oxidation in
air by TGA (Figure 4e). (RuSb2 was observed by TGA to oxidize
above 500 °C.) The formation of Ru metal has also been
observed during the oxidation of RuAl-based alloys and is a
result of the greater affinity of the other elements present to
oxidize compared to Ru, which is a noble metal.26
6265
dx.doi.org/10.1021/ic200619c |Inorg. Chem. 2011, 50, 6263–6268

Inorganic Chemistry
ARTICLE
Figure 5a) and confirms the predominantly divalent nature of Eu.
(A decrease in oxidation state, and therefore an increase in
screening of the nuclear charge, leads to a lower absorption
energy.) Along with the high intensity peak located at ∼6975 eV,
the spectra from unoxidized EuFe4Sb12 and EuRu4Sb12 also
contain a low intensity, higher energy peak at ∼6983 eV (see
Figure 5). Such a peak could correspond to the presence of a
minor concentration of Eu3þ, as proposed previously (cf. the
Eu2O3 spectrum presented in the inset of Figure 5a).16ꢀ18
Alternatively, this higher energy peak has been described as
being a satellite peak of the main Eu2þ excitation resulting from a
final-state shakeup process.14
Shake-up satellite peaks, which are most often observed in
X-ray photoelectron spectra (XPS), are a result of the transition
of valence electrons to empty conduction states following the
excitation of a core electron.27 Examination of Eu 3d XPS spectra
from EuF3 and EuS suggests that the separation in energy
between the main, core-line XPS peak and shakeup peak (∼2
eV) is smaller than the width of the most intense peak observed
in the L3-edge XANES spectrum of the unoxidized skutterudites
(cf. Figure 5).28 Also, as no higher energy satellite-peak was
observed in the L3-edge spectra from Eu2O3 (inset of Figure 5a),
which would be expected if a shakeup peak were observable in
these spectra, the presence of a shakeup peak in the Eu L3-edge
XANES spectra from the skutterudites is unlikely. The
observations described above imply that, in agreement with
previous studies, Eu2þ and Eu3þ are present in EuFe4Sb12 and
EuRu4Sb12, with 2þ being the dominant oxidation state.16ꢀ18
3.3.1.2. Oxidation of Eu in EuFe4Sb12 and EuRu4Sb12. Along
with confirming the oxidation state of Eu in the unoxidized
materials, Eu L3-edge XANES was also used to identify the
oxidation state of the amorphous, Eu-containing oxidation pro-
ducts formedinthese materials after being exposedtoair atvarious
temperatures. In agreement with both the TGA and powder XRD
results, XANES spectra from EuFe4Sb12 and EuRu4Sb12 after
exposure to air at temperatures g400 °C (Figure 5) have line
Figure 5. Eu L3-edge XANES spectra from EuFe4Sb12 (a) and
EuRu4Sb12 (b) before and after being exposed to air at high tempera-
tures. To aid in the identification of the oxidation state of Eu in the
oxidized and unoxidized materials, a standard spectrum from Eu2O3 was
collected and is presented in the inset of part a.
3.3. XANES. Analysis of the oxidized skutterudites by powder
X-ray diffraction allowed for a confirmation of the TGA results
with most, but not all, of the oxidation products being deter-
mined. As some of the oxidation products were likely amorphous,
techniques other than XRD were required to identify them, and
Eu and Ru L3-edge XANES spectra were collected.
shapes that are consistent with the oxidation of Eu2þ to Eu3þ
.
Following exposure to air at 400 °C, the EuFe4Sb12 Eu L3-edge
spectrum is very similar to that of Eu2O3, but a low-energy
shoulder is also observed (see arrow in Figure 5a). The
observation of this shoulder implies the presence of a small
contribution from Eu2þ and agrees with the corresponding
powder XRD pattern (Figure 3b) in which a minor concentration
of EuFe4Sb12 was observed. (No Eu oxides were detected by
powder XRD (cf. Figure 3b), which indicates that the Eu3þ oxide
was amorphous.) After exposure to air at 500 °C, the Eu L3-edge
3.3.1. Eu L3-Edge. The Eu L3-edge XANES spectra result
principally from a 2p3/2 f 5d excitation and have been
investigated in the past to identify the oxidation state of Eu in
EuM4Sb12 (M = Fe, Ru, Os).4 Through this analysis, some
studies have concluded that Eu2þ and a minor concentration of
Eu3þ are present while other studies suggested only the
spectrum from EuFe4Sb12 showed the presence of only Eu3þ
,
which was confirmed by the detection of EuSbO4 by powder
XRD (Figure 3c).
presence of Eu .
2þ 14,16ꢀ18 Eu L3-edge spectra from EuFe4Sb12
and EuRu4Sb12 before and after exposure to air at high-
temperatures are presented in Figure 5. To aid in the determi-
nation of the Eu oxidation state of the oxidized materials, a
standard spectrum from Eu2O3 was collected and is presented
in the inset of Figure 5a.
3.3.1.1. Oxidation State of Eu in Eu(Fe,Ru)4Sb12. To under-
stand how Eu in EuFe4Sb12 and EuRu4Sb12 oxidizes, it is
important to first confirm its oxidation state in the unoxidized
materials, which can be accomplished using Eu L3-edge XANES.
The Eu L3-edge XANES spectra from both materials before
exposure to air at high temperature (Figure 5a,b) show an intense
peak centered at ∼6975 eV. This peak is significantly lower in
energy than the main peak observed for Eu2O3 (see inset of
The Eu L3-edge spectra from EuRu4Sb12 oxidized in air are
presented in Figure 5b. The Eu L3-edge spectrum from
EuRu4Sb12 exposed to air at 400 °C for 2 h has two intense
peaks corresponding to Eu2þ and Eu3þ. Such an observation
implies that, although significant weight gain is not observed by
TGA until temperatures above ∼460 °C are reached (see
Figure 2), some oxidation of EuRu4Sb12 does occur. (Note that
this discrepancy may also be a result of the material being held at
400 °C for 2 h, which is significantly longer than the time that it
was held at this temperature during the TGA experiment.) After
exposing EuRu4Sb12 to air at 500 °C, the Eu L3-edge spectrum
contained only a single peak, which corresponds to Eu3þ
(Figure 5b).
6266
dx.doi.org/10.1021/ic200619c |Inorg. Chem. 2011, 50, 6263–6268

Inorganic Chemistry
ARTICLE
increasing ionic radius, the extent to which the valence orbitals
from Eu2þ/3þ and Sb1- and Ru2þ and Sb1- overlap increases,
compared to the equivalent CeꢀSb and FeꢀSb bonds in
CeFe4Sb12.29 This greater orbital overlap results in the devel-
opment of stronger bonds that require more energy to break,
relative to CeFe4Sb12, and as might be expected, the tempera-
ture at which point oxidation occurs increases. [Comparing the
energies of bonds for equivalent interactions involving the
elements present in the skutterudites investigated also allows
for such a suggestion (e.g., the RuꢀO and FeꢀO bond energies
are 528 ( 42 and 407 ( 1 kJ/mol, respectively).30] Along with
investigating the oxidation of these materials, examination of
the Eu L3-edge XANES spectra from EuFe4Sb12 and
EuRu4Sb12 before oxidation confirmed that, in agreement with
previous studies, the oxidation state of Eu is primarily 2þ, with
a small concentration of 3þ also being present.16ꢀ18
The combined analysis of the materials investigated before
and after oxidation by XANES and XRD has allowed for the
observation that, at the initiation of oxidation, all bonds in the
original materials are broken. This can be concluded on the
basis of the fact that none of the oxidation products identified by
XRD have isolated Fe/RuꢀSb or EuꢀSb bonds and that the
amorphous materials detected by XANES have charges that are
very different from the original materials. This implies that,
compared to CeFe4Sb12, the substitution of Eu2þ/3þ for Ce3þ
and/or Ru2þ for Fe2þ enables a strengthening of the bonds
throughout the entire structure, allowing for an increased
resistance to oxidation. The oxidation temperature was ob-
served to increase more significantly when Ru replaced Fe then
when Eu replaced Ce. This difference can be linked to the fact
that the EuꢀSb bond lengths in these materials are long
(∼3.97 Å) compared to a nominal EuꢀSb bond (g3.3 Å in
EuSb2 and Eu2Sb3).16,31,32
The usefulness of combining TGA analysis with XANES and
powder XRD has been shown here, as the XANES spectra were
able to detect that some oxidation of EuRu4Sb12 occurred before
a significant weight increase was detected by TGA. Examination
of EuRu4Sb12 by powder XRD was also able to track the
formation of Ru metal during oxidation. Although the substitu-
tion of Ru increases the temperature at which the skutterudites
oxidize, the possibility of Ru2þ being reduced to Ru0 may exclude
the usefulness of this element in thermoelectric materials, as the
formation of Ru metal during operation could inhibit device
performance.
Understanding the temperature range over which a material
could oxidize is important to determining its applications and the
level of protection needed from the environment to ensure
device reliability. As has been shown here, substitution of the
RE and M elements can greatly affect the temperature at which
materials adopting the skutterudite-type structure oxidize. The
challenge now will be to determine the proper level of substitu-
tion needed to increase the thermal stability of these materials
without adversely affecting their thermoelectric properties. For
example, EuFe4Sb12 oxidizes at a much higher temperature than
CeFe4Sb12 does, but its ability to perform as a thermoelectric
material is poorer.8,33
Figure 6. The Ru L3-edge XANES spectra from EuRu4Sb12 before and
after being exposed to air at high temperatures are presented in (a). To aid
the examination of how the Ru oxidation state changes upon exposure to
air at high temperature, spectra from standard materials having known
oxidation states were collected and are presented in (b). The Ru L3-edge
spectra show an increase in energy and intensity with increasing
oxidation state.
3.3.2. Ru L3-Edge XANES from EuRu4Sb12. To identify the
oxidation states of Ru in EuRu4Sb12 before and after exposure to
air at various temperatures, Ru L3-edge XANES spectra were
collected and compared to materials having well-known oxida-
tion states (Figure 6). As these spectra result from the excitation
of 2p electrons to unoccupied 4d states, both the energy and
intensity of the spectra yield information on the Ru oxidation
state. The Ru L3-edge spectrum from unoxidized EuRu4Sb12 is
similar in energy and intensity to that of RuSb2. After exposure to
air at high-temperature, the Ru L3-edge spectra became more
intense and the peak maximum shifted to slightly higher energies
(∼0.2 eV). The increase in the peak intensity implies an increase
in the average Ru oxidation state upon exposure to air at
temperatures above 400 °C (cf. Figure 6a,b). As only Ru metal
was detected by powder XRD in samples exposed to air at
g500 °C (Figure 3), the higher oxidation state Ru-bearing
materials detected by XANES are considered to be amorphous.
4. CONCLUSIONS
’ AUTHOR INFORMATION
The oxidation of EuFe4Sb12 and EuRu4Sb12 has been
investigated by TGA, powder XRD, and XANES and it was
found that the temperature at which these materials begin to
oxidize is significantly higher than for CeFe4Sb12.21 With
Corresponding Author
*E-mail: andrew.grosvenor@usask.ca. Phone: (306) 966-4660.
Fax: (306) 966-4730.
6267
dx.doi.org/10.1021/ic200619c |Inorg. Chem. 2011, 50, 6263–6268

Inorganic Chemistry
ARTICLE
’ ACKNOWLEDGMENT
(24) Thompson, A.; Attwood, D.; Gullikson, E.; Howells, M.; Kim,
K.-J.; Kirz, J.; Kortright, J.; Lindau, I.; Peanetta, P.; Robinson, A.;
Scofield, J.; Underwood, J.; Vaughan, D.; Williams, G.; Winick, H. X-
ray Data Booklet; Lawrence Berkeley National Laboratory, University of
California: Berkeley, CA, 2001.
(25) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12,
537–541.
(26) Cao, F.; Nandy, T. K.; Stobbe, D.; Pollock, T. M. Intermetallics
2007, 15, 34–43.
(27) Michels, G.; Junk., S.; Schlabitz, W.; Holland-Moritz, E.; Abd-
Elmeguid, M. M.; D€unner, J.; Mewis, A. J. Phys.: Condens. Matter 1994,
6, 1769–1778.
(28) Vercaemst, R.; Poelman, D.; Fiermans, L.; Van Meirhaeghe,
R. L.; Laflꢁere, W. H.; Cardon, F. J. Electron Spectrosc. Relat. Phenom. 1995,
74, 45–56.
(29) Shannon, R. D. Acta Cryst. A 1976, 32, 751–767.
(30) Luo, Y.-R. In CRC Handbook of Chemistry and Physics, 91st ed.;
Haynes, W. M., Ed.; CRC Press: Boca Raton, FL, 2011; pp 9-65ꢀ9-98.
(31) Hulliger, F.; Schmelczer, R. J. Solid State Chem. 1978, 26,
389–396.
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada through a
discovery grant awarded to A.P.G. The University of Saskatch-
ewan is also thanked for supporting this work through a new-
faculty start-up grant awarded to A.P.G. Dr. Robert Scott
(Department of Chemistry, University of Saskatchewan) is
thanked for providing access to the TGA. Dr. Robert Gordon
is thanked for help in carrying out XANES experiments at PNC/
XSD-CAT, sector 20, at the APS. PNC/XSD facilities at the
Advanced Photon Source and research at these facilities are
supported by the US Department of Energy—Basic Energy
Sciences, a Major Resources Support grant from NSERC, the
University of Washington, Simon Fraser University, and the
Advanced Photon Source. Use of the Advanced Photon Source is
also supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Contract DE-
AC02-06CH11357.
(32) Chapuis, G.; Hulliger, F.; Schmelczer, R. J. Solid State Chem.
1980, 31, 59–67.
(33) Kuznetsov, V. L.; Rowe, D. M. J. Phys.: Condens. Matter 2000,
12, 7915–7921.
’ REFERENCES
(1) Yang, J.; Caillat, T. MRS Bull. 2006, 31, 224–229.
(2) Vining, C. B. Nat. Mater. 2009, 8, 83–85.
(3) Fleurial, J.-P. JOM 2009, 61, 79–85.
(4) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105–114.
(5) Demird€oven, N; Deutch, J. Science 2004, 305, 974–976.
(6) Nolas, G. S.; Merelli, D. T.; Tritt, T. M. Annu. Rev. Mater. Sci.
1999, 29, 89–116.
(7) Jewell, A. D.; Paik, J.-A.; Caillat, T. Mater. Res. Soc. Symp. Proc.
2006, 886, 369–374.
(8) Sales, B. C. In Handbook on the Physics and Chemistry of Rare
Earths; Gschneidner, K. A., Jr., Bunzli, J.-C. G., Pecharsky, V. K., Eds.;
Elsevier: Amsterdam, 2003; Vol. 33, pp 1ꢀ34.
(9) Ravi, V.; Firdosy, S.; Caillat, T.; Brandon, E.; van der Walde, K.;
Maricic, L.; Sayir, A. J. Electron. Mater. 2009, 38, 1433–1442.
(10) Fleurial, J.-P.; Borshchevsky, A.; Caillat, T.; Ewell, R. Proc.
Intersoc. Energy Convers. Eng. Conf. 1997, 32, 1080–1085.
(11) Caillat, T.; Fleurial, J.-P.; Borshchevsky, A. Int. Conf. Thermo-
electr. 1996, 15, 100–106.
(12) Jeitschko, W.; Braun, D. Acta Crystallogr. B 1977, 33, 3401–3406.
(13) Grosvenor, A. P.; Cavell, R. G.; Mar, A. Chem. Mater. 2006, 18,
1650–1657.
(14) Grytsiv, A.; Rogl, P.; Berger, St.; Paul, Ch.; Bauer, E.; Godart,
C.; Ni, B.; Abd-Elmeguid, M. M.; Saccone, A.; Ferro, R.; Kaczorowski, D.
Phys. Rev. B 2002, 66, 094411/1–11.
(15) Sales, B. C.; Jin, R.; Mandrus, D.; Khalifah, P. Phys. Rev. B
2006, 73, 224435/1–8.
ꢀ
(16) Bauer, E. D.; Slebarski, A.; Frederick, N. A.; Yuhasz, W. M.;
Maple, M. B.; Cao, D.; Bridges, F.; Giester, G.; Rogl, P. J. Phys.: Condens.
Matter. 2004, 16, 5095–5107.
(17) Krishnamurthy, V. V.; Keavney, D. J.; Haskel, D.; Lang, J. C.;
Srajer, G.; Sales, B. C.; Mandrus, D. G.; Robertson, J. L. Phys. Rev. B
2009, 79, 014426/1–8.
(18) Krishnamurthy, V. V.; Lang, J. C.; Haskel, D.; Keavney, D. J.;
Srajer, G.; Roberston, J. L.; Sales, B. C.; Mandrus, D. G.; Singh, D. J.; Bilc,
D. I. Phys. Rev. Lett. 2007, 98, 126403/1–4.
(19) Chakoumakos, B. C.; Sales, B. C.; Mandrus, D.; Keppens, V.
Acta. Crystallogr. B 1999, 55, 341–347.
(20) Koza, M. M.; Johnson, M. R.; Viennois, R.; Mutka, H.; Girard,
L.; Ravot, D. Nat. Mater. 2008, 7, 805–810.
(21) Sklad, A. C.; Gaultois, M. W.; Grosvenor, A. P. J. Alloys Compd.
2010, 505, L6–L9.
(22) Stark, I.; Stordeur, M. Int. Conf. Thermoelectr. 1999, 18,
465–472.
(23) Sales, B. C.; Mandrus, D.; Chakoumakos, B. C.; Keppens, V.;
Thompson, J. R. Phys. Rev. B 1997, 56, 15081–15089.
6268
dx.doi.org/10.1021/ic200619c |Inorg. Chem. 2011, 50, 6263–6268