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Nanostructuring in b-Zn4Sb3 with
variable starting Zn compositions
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Phys. Status Solidi A 208, No. 7, 1652–1657 (2011) / DOI 10.1002/pssa.201026613
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Protima Rauwel* , Ole Martin Løvvik , Erwan Rauwel , Eric S. Toberer , G. Jeffrey Snyder ,
1
and Johan Taftø
1
Department of Physics, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway
SINTEF Materials and Chemistry, Forskningsveien, 1, 0314 Oslo, Norway
3
Department of Chemistry and inGaP, University of Oslo, 0315 Oslo, Norway
4
Materials Science, California Institute of Technology, 1200 East California Blvd., Pasadena, California 91125, USA
2
Received 7 October 2010, revised 28 February 2011, accepted 7 March 2011
Published online 1 April 2011
Keywords density functional theory, nanostructures, thermoelectric materials, transmission electron microscopy, Zn4Sb3
* Corresponding
author: e-mail protima.rauwel@fys.uio.no, Phone: þ47-22840694, Fax: þ47-22 05 06 51
Thermoelectric Zn4dSb3 samples with varying Zn concentrations were synthesized. The morphology, along with planar
and extended defects was studied using transmission electron
microscopy (TEM). For all the Zn concentrations, nanostructuring was observed. Depending on the Zn concentration,
two different types of nanostructures were observed: nanovoids
and nanograins. Three samples with Zn:Sb ratios of 1.28, 1.30,
and 1.36 were studied. The presence of the secondary phases
was discussed in terms of the phase diagram. Density functional
theory (DFT) calculations further elucidated the formation of
secondary phases as a function of different Zn:Sb ratio.
ZnSb nanoparticles in b-Zn4 Sb3 .
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1 Introduction Our search for ‘‘green’’ energy has led
to an extended focus on thermoelectric (TE) materials. In
recent years b-Zn4Sb3 has found its place as a promising TE
material [1]. A great deal of interest has been shown by the
thermoelectric community with regards to its performance
within the 473 to the 673 K temperature window owing to
reports on its thermoelectric figure of merit (ZT) attaining
values over 1 [2]. The essential trio of low thermal
conductivity, high electrical conductivity and high Seebeck
coefficient is indeed fulfilled by this compound. Interstitial
Zn in this case plays a very important part in creating disorder
in the material and therefore in lowering the mean free path
or velocity of the phonons. The framework structure is
Zn3.6Sb3, with the additional Zn atoms distributed amongst
three crystallographically unique sites.
The potential for nanostructuring to reduce the lattice
thermal conductivity has widely been accepted and has
repeatedly been used to enhance thermoelectric efficiency
[3, 4]. Nanostructuring reduces the thermal conductivity of a
material by creating scattering centers for longer wavelength
acoustic phonons otherwise difficult to scatter by Umklapp
processes or point defects. Defects in materials also
contribute to diffused scattering of phonons. Moreover,
reducing the grain size induces more grain boundaries
leading to more scattering centers. The improvement of ZT in
the nanostructures has already been predicted and experimental evidence of nanostructuring is available [5].
Extended and planar defects such as dislocations and
stacking faults on the other hand may reduce the thermal
conductivity but are also detrimental to the electrical
conductivity [6]. These along with point defects strongly
affect the transport properties of semiconductors [7]. Static
imperfections in the lattice can be an important source of
phonon scattering, especially at low temperatures when
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Phys. Status Solidi A 208, No. 7 (2011)
phonon–phonon scattering becomes very weak.
Imperfections can take the form of impurities, mass
fluctuations, point defects, dislocations, stacking fault,
lattice disorder (glass or amorphous structures) and crystal
grain boundaries [8]. At high temperatures large concentrations of lattice imperfections are required in order to
produce significant effects above that of phonon–phonon
scattering [9]. The introduction of irregularities in the
lattice by alloying often brings about sufficient disorder to
produce phonon scattering at high temperatures and is vital
to the improvement of the performance of thermoelectric
materials [10].
As mentioned above, b-Zn4Sb3 possesses a high density
of Zn interstitials leading to significant scattering of short
wavelength phonons [11]. Here we describe how additional
structural disorder viz. nanostructures can be introduced
through appropriate synthetic conditions and characterized
by transmission electron microscopy (TEM). In this work we
study the morphology and microstructure of b-Zn4Sb3
around stoechiometric synthesis compositions. We have
also performed density functional theory (DFT) calculations
and consequently discuss the observed nanoparticle phases
with reference to these calculations and to the phase diagram
in this paper.
2 Experimental
2.1 Synthesis The samples were prepared with a
starting composition of 56.2, 56.5 and 57.2 at.% Zn and are
called sample A, B, C, respectively. The elements were
directly reacted in a fused-silica ampoule sealed under
vacuum (105 Torr). The melt was heated for 8 h at 800 8C
before being water quenched. The resulting ingot was ball
milled for 4 h, hot pressed at 350 8C for 1 h. Fine
homogenous powders were obtained by mechanical alloying
under argon in a SPEX Mixer/Miller 8000 series mill for a
total of 3 h. The powder was hot pressed under argon at
350 8C for 1 h. Finally, it was annealed without stress for 2 h
at 350 8C.
2.2 Characterization The sample was characterized
by X-ray diffraction to determine the crystal structure on a
Philips XPERT MPD diffractometer operated at 45 kV and
40 mA. Scanning electron microscopy (SEM) images were
recorded on a Quanta 200FEG, FEI. TEM studies were
carried out on a JEM2010F disposing a point to point
resolution of 1.9 Å.
2.3 Density functional theory Theoretical calculations based on DFT were performed to elucidate the atomic
and energetic properties of the Zn4Sb3 phase. The Vienna ab
initio Simulation Package (VASP) [12, 13], employing the
projector augmented wave method [14] and the PBE
generalized gradient approximation was used. A plane wave
cut-off of 300 eV was used for the relaxations, while 500 eV
was used for the total energy calculations. The k-point
density was at least 0.25 Å1. Together with a criterion for
self-consistence of 105 eV change of total energies we
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obtained relative energies with less than 1 meV error
originating from numerical sources. Similarly, the forces
were converged to less than 0.05 eV/Å. The relaxations
allowed simultaneous changing of atomic positions, unit cell
shape and cell size.
3 Results Table 1 recapitulates the various secondary
phases as observed from X-ray diffraction studies. When an
excess of Zn was used to synthesize Zn4Sb3 as is the case for
sample C one observes a Zn metal impurity phase. For the
sample prepared with a starting Zn:Sb ratio of 1.30 (sample
B) no secondary phases were observed. For the sample with
Zn:Sb starting ratio of 1.28 (sample A) a ZnSb secondary
phase was detected.
The Seebeck coefficient, thermal and electrical conductivity of all the three samples have been studied and the
present study at hand will focus on the structural properties at
the nano- and microscales [15]. Our discussion will be as
follows: first the ideal case of Zn:Sb of 1.30 corresponding to
a composition of Zn3.9Sb3will be presented (sample B)
followed by the result of decreasing the Zn:Sb ratio (sample
A). Finally, the effect of excess of Zn (sample C) on the
microstructure during the synthesis will be shown and
discussed.
TEM and SEM analyses performed on sample B with a
Zn:Sb ratio of 1.30 did not allow the detection of a secondary
phase and subsequently confirms the XRD results. After
checking for probable secondary phases using TEM,
morphological characterization was carried out. Figure 1a
is a low magnification TEM image of sample B with different
shades of gray. The white spots on the image correspond to
voids in the material with an average size of around 8 nm.
The SEM image presented in Fig. 1b shows dark spots
corresponding to larger voids at the microscale present in the
sample. EDX (Fig. 1c) performed on one of the voids in the
SEM image (Fig. 1b) gave no other element other than Zn
and Sb. The elemental quantification performed estimated a
Zn:Sb ratio of around 1.30. These voids were detected not
only at the grain boundary but also within the grain. The
higher magnification TEM image highlights the faceting of
these voids in Fig. 1d. One reason for the formation of these
voids could be an incomplete densification of the material
when hot pressed after ball milling. Another explanation for
the formation of these nanovoids could be related to the reabsorption of the Zn precipitates into the Zn4Sb3 matrix
leaving behind an agglomeration of Zn vacancies. In fact, Zn
precipitates into the b-Zn4Sb3 matrix when cooling down
from melt with Zn:Sb ratios starting from 1.30 (see DFT
section). The size of the precipitates is determined by the rate
Table 1 Secondary phases as obtained from X-ray diffraction for
the three samples observed [15].
Zn:Sb
1.280
(sample A)
1.300
(sample B)
1.360
(sample C)
secondary phase
ZnSb
none
Zn
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Figure 1 (online color at: www.pss-a.com) (a) TEM image of a
Zn4Sb3 sample having Zn:Sb ratio of 1.30 (sample B). (b) SEM
image of sample B. (c) EDX spectra performed in a void shown in (b).
(d) Higher magnification TEM image of the voids.
of quenching [16]. Moreover, on account of the retrograde
solubility of Zn in the b phase, at some intermediate
temperature the Zn is reintegrated into the matrix leaving
behind voids [17].
In the case of sample A having a Zn:Sb ratio of 1.28,
XRD detected a ZnSb second phase (Table 1). TEM analyses
not only allowed the confirmation of this impurity phase but
also the study of its morphology. The ZnSb secondary
phase is present in nonnegligible quantities in sample A as a
whole. Figure 2a shows the ZnSb nanoparticles whose
average size is around 10 nm. A HRTEM image of a 16 nm
ZnSb nanoparticle is shown in Fig. 2b. In the inset, the power
spectrum confirms the ZnSb phase. The particle in this image
is oriented along the h101i zone axis of the Pbca space
group with lattice parameters a ¼ 6.20 Å, b ¼ 7.74 Å, and
c ¼ 8.10 Å. SEM images recorded on sample C also confirm
the presence of the second phase (lighter zones) (Fig. 2c).
The EDX spectra (not shown here) confirmed the presence of
only Zn and Sb with a ratio of 1:1. SEM study on sample C
also pointed out to its rather high porosity (pores are black).
The phase boundary between ZnSb and b-Zn4Sb3 was
studied using HRTEM. Figure 2d exhibits a clean phase
boundary without any amorphous phases. It was also
characterized as being semi-coherent as a large number of
misfit dislocations compensating for the lattice mismatch
were observed. The b-Zn4Sb3 grain boundaries (not shown
here) are also free of amorphous phases. The presence of
crystal grain boundaries provides another possibility for
phonon damping.
As for the b-Zn4Sb3 sample C with a Zn:Sb ratio of 1.36,
the X-ray spectra detected a second phase and further
confirmed the secondary phase as a Zn impurity. The latter
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
P. Rauwel et al.: Nanostructuring in b-Zn4Sb3
Figure 2 (a) TEM images of ZnSb nanoparticles in sample C with a
Zn:Sb ratio of 1.28, inset is the corresponding SAED of the nanoparticles. (b) High resolution image of a ZnSb nanoparticle. Inset
shows the power spectrum of the nanoparticle. (c) SEM image
sample C. (d) Phase boundary between a ZnSb grain and a b-Zn4Sb3
grain.
crystallizes in the form of nanoparticles shown in Fig. 3a
which is in accord with a recently published work [18].
Figure 3a is a TEM image taken from a crushed sample
dispersed in ethanol and supported by a carbon grid. In this
Figure 3 (online color at: www.pss-a.com) Sample A with Zn:Sb
ratio of 1.36. (a) TEM image of sample A crushed. (b) Higher
magnification image of (a). (c) Stacking faults. The a lattice parameter of the Zn4Sb3 phase is indicated (d) extended defects, dislocations. Black arrows point out to a few of the Zn metal nanoparticles.
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Phys. Status Solidi A 208, No. 7 (2011)
zone, nanosized Zn grains are protruding from the Zn4Sb3
micrometric grain. Figure 3b is a high magnification image
consisting of isolated Zn nanoparticles. The TEM image
highlights the faceting of the nanoparticles. The average size
of the Zn metal particles is around 8 nm.
In small grains, no defects were observed. Moreover, no
impurities on the grain boundary nor at the triple point were
observed. However in the ion milled sample large grains
(200 mm) along with planar and extended defects were
observed. In Fig. 3c two edge dislocations are observed.
These two edge dislocations are numbered (1) and (2) in
Fig. 3c. Edge dislocation (1) is characterized by an extra
(010) plane inserted at ½ (120) whereas edge dislocation
(2) is the result of an extra (100) plane inserted at ½ (1–20).
The glide planes of both dislocations are at 608 to each other.
Extended defects were also observed in the sample.
Figure 3d is one such example of dislocations in the
Zn4Sb3 sample containing an excess of the Zn element.
Transmission electron microscopy specimens were
prepared in two ways for this study. The first method
consisted of crushing the bulk sample and mixing the powder
in ethanol with the help of an ultrasonic bath. A drop of the
suspension was then placed on a holey carbon grid for
observation. The second method required mechanical
thinning of the sample followed by ion milling to obtain
electron transparency. TEM observations on both samples
gave the same morphology. Nanostructuring was observed
irrespective of the TEM sample preparation technique. Ion
beam thinning induces local heating and is a source of
artifacts in TEM observations as reported by Lensch-Falk
et al. [19]. They observe nanoparticles of Ag2Te precipitates
in thermoelectric PbTe due to ion milling and have attributed
it to the high mobility of Ag due to local heating. This is an
important point for the b-Zn4Sb3 sample with a Zn:Sb ¼ 1.36
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containing Zn nanoparticles as Zn has a very high mobility in
this material and can form Zn clusters [20]. In our case, since
we have used both techniques, we can safely eliminate the
contribution of artifacts. In our study, nanoparticle
morphology was observed and size distribution histograms
were plotted for the crushed samples while observation of
defects and grain boundaries were carried out on ion-milled
samples.
Scanning electron microscopy images acquired point out
to the presence of Zn in Fig. 4a. The dark spots correspond to
the Zn metal. Various EDX spectra were acquired and
quantified from various zones of Fig. 4a. Zone 1 is a Zn
precipitate free zone and the corresponding EDX spectra
after quantification indicates that it is indeed Zn4Sb3
(Fig. 4c). Zones 2 and 3 are the ones with Zn impurities
and their corresponding EDX spectra show the presence of
Zn along with negligible amounts of Sb (Fig. 4b and d). The
average size of the Zn precipitates at this scale is about 1 mm.
The ground state thermodynamic stability of the Zn4Sb3
phase was assessed by DFT calculations. The Zn content was
varied by changing the amount of interstitial Zn in the
atomistic models. No interstitial Zn corresponds to a Zn:Sb
ratio of 1.2; then only the crystallographic Zn1 site is
occupied. Higher Zn concentrations were achieved by filling
the interstitial sites Zn2, Zn3, and Zn4 introduced in the
experimental study by Snyder et al. [1]. Combinations of two
interstitial sites along with a Zn1 vacancy were also
investigated, and it was found that such defect clusters
represented the most stable configurations. The resulting
energetic of interstitial Zn formation (compared to Zn6Sb5
and pure Zn) is shown in Fig. 5. It is evident that interstitial
Zn is thermodynamically stable below a Zn:Sb ratio of
approximately 1.32, while it is unstable above this ratio.
There also seems to be a thermodynamic minimum around
Figure 4 (online color at: www.pss-a.com)
SEM study on sample A with Zn:Sb ratio of
1.36 (a) SEM image, (b) EDX from zone 3, (c)
EDX from zone 1 (d) and EDX from zone 2.
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Figure 5 (online color at: www.pss-a.com) The ground state thermodynamic stability of Zn4Sb3 with respect to decomposition to the
model without interstitial Zn (with a Zn:Sb ratio of 1.2) and pure Zn.
Several different possibilities of including the Zn were tested, using
combinations of the crystallographic interstitial sites Zn2, Zn3, and
Zn4 [1] as well as clusters of Zn vacancies and interstitial sites (see
text for details). The dashed curve is drawn as a guide to the eye only,
and has been fitted to the lowest energy (most stable) models. A
positive DH (above the dashed line) implies that the phase is unstable
with respect to pure Zn and Zn6Sb5.
the Zn:Sb ratio 1.27; however since the curve is rather flat at
the bottom and since there are many sources of error,
the safest conclusions that one can draw from the shape of the
curve in Fig. 5 is that the ratios between 1.23 and 1.30 are the
most stable from the theoretical point of view. This suggests
that when the Zn:Sb ratio at the starting point is larger than
1.30 one should expect a mixture of Zn and Zn4Sb3 from
the calculations. On the low side of the Zn:Sb ratio (<1.23),
there seems to be on the other hand a thermodynamic driving
force for the formation of additional interstitial Zn, moving
towards a Zn4Sb3 phase with a Zn:Sb ratio of 1.23. This must
be accompanied by the formation of ZnSb in order to balance
the reaction. It should be mentioned here that additional
calculations (not shown here) demonstrate that all the Zn4Sb3
phases are unstable with respect to decomposition into ZnSb
and Zn at 0 K, similar to what was previously predicted for
models Zn13Sb10 (Zn:Sb ¼ 1.3) and Zn6Sb5 (Zn:Sb ¼ 1.2) by
Mikhaylushkin et al. [21]. All these calculations neglect
temperature and entropy, which are most probably the
main reasons for stability of Zn4Sb3 in this region of the
phase diagram. Nevertheless, these results can be seen as a
rationalization for the formation of ZnSb phase at low ZnSb
ratios and for the formation of Zn at higher ratios. The
calculations predict that the Zn4Sb3 phase should exhibit a
range of Zn:Sb ratios, tentatively between 1.23 and 1.30. One
can expect significant changes in this region on including in
the calculations parameters such as temperature, entropy,
increase of unit cell size, etc.
4 Discussion The three samples being investigated
have compositions corresponding to 56.2, 56.5, and
57.2 at.% Zn. The final heat treatment was at 350 8C under
the limit of creation of high temperature Zn4Sb3 phases. Our
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
P. Rauwel et al.: Nanostructuring in b-Zn4Sb3
study shows that the crystallization of phases in the samples
are heavily dependant on the starting Zn:Sb ratio. In fact, for
a low Zn:Sb ratio (Zn:Sb < 1.30), i.e., 56.2 at.% Zn, we
observe a phase separation and the presence of ZnSb
embedded in the Zn4Sb3 matrix. This is consistent with the
DFT calculations. Moreover, Zn4Sb3 remains stable for a
Zn:Sb ratio of 1.30 where no splitting of phases is visible.
However, an increase in the Zn:Sb ratio to 1.36 induces a
phase separation or in other words the nanocrystallization
of Zn particles. This observation is supported by DFT
calculations where one predicts a Zn metal phase separation
for Zn:Sb ratios exceeding 1.32. The study therefore
indicates that no phase separation should occur between
Zn:Sb ratios of 1.30:1.32. Pedersen et al. [22] have also
observed this phase separation on substituting Zn with Hg.
However in their study along with Zn and ZnSb precipitates
they also observe Hg precipitates at the micrometer scale due
to the limited solubility of Hg in the b-Zn4Sb3 structure.
With respect to the phase diagram [23], the presence of a
ZnSb secondary phase in sample A for a starting Zn:Sb ratio
of 1.28 puts it to the left hand side of the single phase field.
For the Zn:Sb ratio of 1.30 (sample B) where no second phase
was detected, places the material in the single phase field.
Lastly, with regards to sample C with a Zn:Sb ratio of 1.36
where Zn impurity is present in nonnegligible quantities, one
would position it on the right hand side of the single phase
field of the Zn–Sb phase diagram.
The morphology of the three samples as observed on the
SEM images was also different. Sample A possesses the
highest porosity among the 3. Sample compactness has
known to affect the thermopower of Zn4Sb3 [24]. The
nanosized secondary phases and voids present in the TEM
images also have microsized counterparts present in the
SEM images. The precipitation of the secondary phases
starts with small agglomerates of Zn or ZnSb atoms in the
early stages giving rise to nanometric size precipitates. Over
time the nanometric precipitates grow and coalesce to form
micrometric precipitates.
For the first time we have demonstrated that the phase
separation in Zn4Sb3 not only occurs at the microscale but
also at the nanoscale in the form of nanovoids, ZnSb, or Zn
nanoparticles as a function of the Zn:Sb starting ratio. The
thermoelectric performance of the Zn4Sb3 containing
nanostructures has already been published. For a Zn:Sb
starting ratio of 1.36 an excess of Zn not only induces a Zn
secondary phase but also extended defects in the sample.
Moreover, both the ZnSb and Zn secondary phases are highly
crystalline. The grain boundaries and phase boundaries
observed in this study are semi-coherent and highly
crystalline.
5 Conclusion In conclusion three different Zn concentrations were studied (56.2, 56.5, and 57.2 at.% Zn). TEM
studies showed that for different starting Zn concentrations
secondary phases and nanovoids appear during the synthesis
of Zn4Sb3. The presence of these secondary phases such as
ZnSb and Zn with varying Zn concentrations are also
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Phys. Status Solidi A 208, No. 7 (2011)
predicted by DFT calculations. The electron microscopy
studies also demonstrated that phase separation was not only
occurring at the micrometer scale but also at the nanometer
scale with the self-insertion of these nanoparticles or
nanovoids within the Zn4Sb3 matrix.
Acknowledgements Ole Bjørn Karlsen is thanked for
fruitful discussions. The Norwegian Research Council and Marie
Curie-PERG05-GA-2009-249243 are thanked for financial support.
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