Synthesis and characterization of bulk NaxCoO2 samples substituted by K and Rb is reported. Phase formation studies revealed a narrow stable region for Na-alkali metal-Co system. Whisker and platelike single crystalline structures have been found to form on the surface of the pellets in case of K doping. All samples were metallic and no characteristic anomaly in R-T curves was observed for Rb doped sample. Magnetoresistance measured has a pronounced positive response only for K-doped and pure NaxCoO2 phases, reaching 11% and 7% at 5 K temperature, respectively.Peer reviewe

Dou, S. X.

Jood, P.

Karppinen, Maarit

Peleckis, G.

Wang, X. L.

Yamauchi, H.

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Jood, P. & Peleckis, G. & Wang, X. L. & Dou, S. X. & Yamauchi, H. &
Karppinen, Maarit
Title: Phase formation and magnetotransport of alkali metal doped Na[sub
0.75]CoO[sub 2] thermoelectric oxide
Year: 2010
Version: Final published version
Please cite the original version:
Jood, P. & Peleckis, G. & Wang, X. L. & Dou, S. X. & Yamauchi, H. & Karppinen, Maarit.
2010. Phase formation and magnetotransport of alkali metal doped Na[sub
0.75]CoO[sub 2] thermoelectric oxide. Journal of Applied Physics. Volume 107, Issue 9.
09D716/1-3. ISSN 0021-8979 (printed). DOI: 10.1063/1.3365491.
Rights: © 2010 AIP Publishing. This is the accepted version of the following article: Jood, P. & Peleckis, G. & Wang,
X. L. & Dou, S. X. & Yamauchi, H. & Karppinen, Maarit. 2010. Phase formation and magnetotransport of
alkali metal doped Na[sub 0.75]CoO[sub 2] thermoelectric oxide. Journal of Applied Physics. Volume 107,
Issue 9. 09D716/1-3. ISSN 0021-8979 (printed). DOI: 10.1063/1.3365491, which has been published in final
form at http://scitation.aip.org/content/aip/journal/jap/107/9/10.1063/1.3365491.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
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Phase formation and magnetotransport of alkali metal doped Na 0.75 CoO 2
thermoelectric oxide
P. Jood, G. Peleckis, X. L. Wang, S. X. Dou, H. Yamauchi, and M. Karppinen 
 
Citation: Journal of Applied Physics 107, 09D716 (2010); doi: 10.1063/1.3365491 
View online: http://dx.doi.org/10.1063/1.3365491 
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/9?ver=pdfcov 
Published by the AIP Publishing 
 
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130.233.216.27 On: Tue, 13 Oct 2015 07:10:24
Phase formation and magnetotransport of alkali metal doped Na0.75CoO2
thermoelectric oxide
P. Jood,1 G. Peleckis,1,a X. L. Wang,1 S. X. Dou,1 H. Yamauchi,2 and M. Karppinen2
1Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South
Wales 2519, Australia
2Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015, Finland
Presented 19 January 2010; received 30 October 2009; accepted 30 November 2009;
published online 21 April 2010
Synthesis and characterization of bulk NaxCoO2 samples substituted by K and Rb is reported. Phase
formation studies revealed a narrow stable region for Na-alkali metal-Co system. Whisker and
platelike single crystalline structures have been found to form on the surface of the pellets in case
of K doping. All samples were metallic and no characteristic anomaly in R-T curves was observed
for Rb doped sample. Magnetoresistance measured has a pronounced positive response only for
K-doped and pure NaxCoO2 phases, reaching 11% and 7% at 5 K temperature, respectively. © 2010
American Institute of Physics. doi:10.1063/1.3365491
In recent years constantly growing concern over decreas-
ing amount of primary energy sources has prompted wide-
spread research on development of alternative techniques
and materials for power generation. Waste heat from power
plants, factories, or automobiles equals to 70% of the total
primary energy, but it is difficult to use as its sources are
highly dispersed and small in size. The only effective method
of overcoming these problems by converting heat energy di-
rectly into electrical energy is thermoelectric TE power
generation. Good TE material should possess high conver-
sion efficiency, be built out of nonvolatile and nontoxic ele-
ments, highly abundant and most important chemically
stable at temperatures as high as 1000 K in air. Thus, oxide
based TE materials are most promising candidates for TE
applications due to their stability at high temperatures in air.
After Terasaki et al.1 have reported that NaxCoO2 com-
pound exhibits simultaneously high TE power and low resis-
tivity S100 V /K and 200  cm at 300 K, many
researchers put a lot of effort to develop new Co-based ox-
ides that would exhibit similar or even better TE properties.
Among many Co-based compounds three types of layered
cobalt oxides have attracted the highest attention: NaxCoO2,
Ca3Co4O9,2 Bi2Sr3Co2O9.3 In NaxCoO2 TE oxide the sodium
layers are sandwiched between CoO2 blocks and these are
stacked along the c axis. CoO2 layers are responsible for
electrical conductivity and Seebeck coefficient while the Na
layers are insulating and determine thermal conductivity of
the oxide. Fouassier et al.4 reported that a large amount of
vacancies can easily be introduced into the sodium layer,
giving a possibility for a high Na nonstoichiometry. It was
also shown that highly sodium deficient and water interca-
lated NaxCoO2yH2O is superconducting at low
temperatures.5 Moreover, Balsys et al.6 reported that due to
the peculiar layered structure, Na ions can freely migrate in
between the oxygen layers. Such migration of Na ions might
be responsible for structural phase transitions close to room
temperature which were described by Tojo et al.7 They have
found existence of three first order transitions at 288.7,
296.3, and 302.1 K. This result is supported by Motohashi et
al.,8 who showed anomalous behavior in electrical resistivity
curves for NaxCoO2 polycrystalline samples.
As being a transition metal oxide, NaxCoO2 is very at-
tractive for chemical substitution studies. Both cation sites,
Na and Co, can be substituted by various elements. Kawata
et al.9 showed enhanced TE properties for Ca-substituted
samples. On the other hand Sr does not go into Na site, but
forms an individual compound SrxCoO2.10 Other studies
showed that Mn, Ru, and Rh act as strong scatters to increase
electrical resistivity rapidly, while Cu, Zn, and Pd substitu-
tion does not alter the electric conduction.11,12 In this article
we present synthesis and characterization of alkali metal
doped NaxRyCoO2 R=K,Rb polycrystalline samples. The
introduction of bigger alkali metal ions might decrease Na
migration in the layer and thus improve transport properties
of the material.
All samples were prepared applying “rapid heat-up”
technique.8 Phase purity was checked using x-ray powder
diffraction MAC Science, MXP18VAHF, Cu K1 radia-
tion. Phase formation was studied by means of thermogravi-
metric TG analysis Perkin Elmer, Pyris 1. For full syn-
thesis simulation, a 5–30 mg amount of sample was put into
the thermobalance. In order to avoid fast alkali metal evapo-
ration, the heating rate of 2 °C /min was adopted. The mor-
phology and microstructure of the samples was investigated
by scanning electron microscopy SEM Hitachi, S-4500.
Chemical composition of the samples was determined using
inductively coupled plasma atomic emission spectrometer
ICP-AES Seiko, SPS1500VR. Transport properties were
analyzed by physical property measurement system Quan-
tum Design in a temperature range of 5–350 K. For magne-
toresistance studies the applied field was varied from 0
to 7 T.
aAuthor to whom correspondence should be addressed. Electronic mail:
peleckis@uow.edu.au.
JOURNAL OF APPLIED PHYSICS 107, 09D716 2010
0021-8979/2010/1079/09D716/3/$30.00 © 2010 American Institute of Physics107, 09D716-1
 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
130.233.216.27 On: Tue, 13 Oct 2015 07:10:24
Figure 1 shows x-ray diffraction patterns of undoped and
K/Rb doped NaxCoO2 samples after sintering. The samples
are of single phase corresponding to that of hexagonal
-Na0.75CoO2 oxide. The 002 and 004 peaks at 2=16.2°
and 33.4° are much higher in intensity for the Rb doped
sample as opposed to other ones indicating a more pro-
nounced c axis orientation. Lattice parameters calculated
from the x-ray diffraction patterns revealed that introduction
of bigger alkali ion in the system caused elongation of the
lattice parameter c, i.e., cun-doped=10.912 Å and cRb-doped
=10.946 Å, respectively.
Figures 2a and 2b show SEM micrographs of
Na0.75Rb0.25CoO2 calcined powders and sintered pellets, re-
spectively. As we can see, the sample morphology has very
distinctive differences. For calcined powders the particles are
embedded in some sort of melt and the shape of the particles
is somewhat circular. In contrary, the particles in the sintered
sample Fig. 2b are well formed and separated, have clear
signs of layered structure, and are elongated in shape. Some
residue of melting or sublimation can be seen on the surface
of the crystallites. The average particle size is about 20 m.
Furthermore, it was found that for K-doped samples, single
crystallinelike formations have been observed Fig. 2c.
Two kinds of distinctive products were formed: whisker-type
and petal-like crystals as indicated in Fig. 2c. The forma-
tion of the product was found to be highly dependant on K
amount in the samples and sample sintering temperature, i.e.,
higher K content and higher temperatures caused larger
amount of petal-like crystals to be formed. Detailed phase
formation analysis and TE properties of whisker type crystals
is reported elsewhere.13 We were unable to measure electric
properties of petal like crystals as they were very brittle and
thin.
To establish and understand the proper synthesis proce-
dure we have performed TG simulation of synthesis proce-
dure. Figure 3 shows TG curve for the simulation of solid-
state reaction, when Rb is doped into NaxCoO2 sample. The
TG curve has distinctive difference as compared to published
data for Na-pure samples.8 At around 600–700 °C we have
stable triple cation system, i.e., Na–Rb–Co. However, we can
see a drop in weight at temperatures above 760 °C. This can
be attributed to the evaporation of Rb from the samples. At
temperature of 830 °C almost half of Rb content is already
lost and is completely vanished at 860 °C. This is in contrast
to K-doped samples, where stable Na–K–Co temperature re-
gion is broader, i.e., 650–830 °C.13 The observed Rb evapo-
ration is supported by ICP-AES results, which showed that
Na0.75Rb0.25CoO2 precursor powders region between steps
2 and 3 after sintering after step 3 were in fact
Na0.73Rb0.02CoO2.
Electrical properties measured revealed that all samples
are metallic, with lowest electrical resistivity  at 300 K for
Rb substituted Na0.75CoO2, i.e., =0.9 m cm. The Rb
doped sample does not show characteristic anomaly in the
electrical resistivity curve at room temperature which is in
contrast to undoped8 Na0.75CoO2. Furthermore, we have in-
FIG. 1. X-ray diffraction patterns for sintered NaxRyCoO2 R=K,Rb
samples.
FIG. 2. Color online SEM pictures of a calcined, b sintered
Na0.75Rb0.25CoO2 samples, and c single crystalline products on the surface
of the pellets for K-doped samples.
09D716-2 Jood et al. J. Appl. Phys. 107, 09D716 2010
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130.233.216.27 On: Tue, 13 Oct 2015 07:10:24
vestigated magnetotransport properties of our samples at 5 K
Fig. 4b. The undoped sample shows positive magnetore-
sistance MR effect reaching 11% at 5 K. In the case of K
doping this MR response is suppressed to 7%. Lastly, for Rb
doped sample the MR effect is positive and negative, yet
quite small, i.e., 4%. The origin of such suppression of the
MR effect might be related to the structural reordering and
elongation of lattice parameter c. It is possible that due to the
longer distances between CoO2 layers, the magnetic ordering
in the material is lower than that for undoped compound.
In summary, K and Rb doped samples of Na0.75CoO2 TE
oxide were prepared via “rapid heat-up” synthesis technique.
Single crystalline structures were observed on the surface of
the pellets for K-doped samples. TG analysis showed deple-
tion of doped alkali ion during sintering procedure. All
samples were metallic with positive MR effect for undoped
and K-doped samples reaching 11% and 7% at 5 K, respec-
tively. In contrast, Rb doped sample showed weakly pro-
nounced MR features which might be caused by significant
structural/microstructural changes.
ACKNOWLEDGMENTS
G.P. thanks the Australian Research Council for support
under Discovery Grant No. DP0 879 714. P.J. thanks the
University of Wollongong for providing matching scholar-
ship for her Ph.D. studies.
1I. Terasaki, Y. Sasago, and K. Uchinokura, Phys. Rev. B 56, R12685
1997.
2A. C. Masset, C. Michel, A. Magnan, M. Hevein, O. Ooulemonde, F.
Studier, and B. Raveau, Phys. Rev. B 62, 166 2000.
3J. M. Tarascon, R. Rajesh, P. Darboux, M. S. Hedge, G. W. Hull, L. H.
Greene, M. Giroux, Y. Lesage, W. R. mcKinnon, J. V. Waszczak, and L. F.
Schneemeyer, Solid State Commun. 71, 663 1989.
4C. Fouassier, G. Matejka, J.-M. Reau, and P. Hagenmuller, J. Solid State
Chem. 6, 532 1973.
5K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian,
and T. Sasaki, Nature London 422, 53 2003.
6R. J. Balsys and R. L. Davis, Solid State Ionics 93, 279 1997.
7T. Tojo, H. Kawaji, T. Atake, Y. Yamamura, M. Hashida, and T. Tsuji,
Phys. Rev. B 65, 052105 2002.
8T. Motohashi, E. Naujalis, R. Ueda, K. Isawa, M. Karppinen, and H.
Yamauchi, Appl. Phys. Lett. 79, 1480 2001.
9T. Kawata, Y. Iguchi, T. Itoh, K. Takahara, and I. Terasaki, Phys. Rev. B
60, 10584 1999.
10R. Ishikawa, Y. Ono, Y. Miyazaki, and T. Kajitani, Jpn. J. Appl. Phys., Part
2 41, L337 2002.
11R. Kitawaki and I. Terasaki, J. Phys.: Condens. Matter 14, 12495 2002.
12S. Li, R. Funahashi, I. Matsubara, and S. Sodeoka, Mater. Res. Bull. 35,
2371 2000.
13G. Peleckis, M. Karppinen, and H. Yamauchi, Appl. Phys. Lett. 83, 5416
2003.
FIG. 3. TG curve of a raw material powder mixture Na:Rb:Co
=0.75:0.25:1 upon heating in O2 gas. ¬ Evaporation of H2O; − decar-
bonation process; and ® depletion of Rb. Inset shows the same data with
respect to time.
FIG. 4. MR for pure and alkali metal doped Na0.75CoO2 measured at 5 K.
09D716-3 Jood et al. J. Appl. Phys. 107, 09D716 2010
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130.233.216.27 On: Tue, 13 Oct 2015 07:10:24


English

Phase formation and magnetotransport of alkali metal doped Na[sub 0.75]CoO[sub 2] thermoelectric oxide

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Jood, P. & Peleckis, G. & Wang, X. L. & Dou, S. X. & Yamauchi, H. &Karppinen, MaaritTitle: Phase formation and magnetotransport of alkali metal doped Na[sub0.75]CoO[sub 2] thermoelectric oxideYear: 2010Version: Final published versionPlease cite the original version:Jood, P. & Peleckis, G. & Wang, X. L. & Dou, S. X. & Yamauchi, H. & Karppinen, Maarit.2010. Phase formation and magnetotransport of alkali metal doped Na[sub0.75]CoO[sub 2] thermoelectric oxide. Journal of Applied Physics. Volume 107, Issue 9.09D716/1-3. ISSN 0021-8979 (printed). DOI: 10.1063/1.3365491.Rights: © 2010 AIP Publishing. This is the accepted version of the following article: Jood, P. & Peleckis, G. & Wang,X. L. & Dou, S. X. & Yamauchi, H. & Karppinen, Maarit. 2010. Phase formation and magnetotransport ofalkali metal doped Na[sub 0.75]CoO[sub 2] thermoelectric oxide. Journal of Applied Physics. Volume 107,Issue 9. 09D716/1-3. ISSN 0021-8979 (printed). DOI: 10.1063/1.3365491, which has been published in finalform at http://scitation.aip.org/content/aip/journal/jap/107/9/10.1063/1.3365491.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Phase formation and magnetotransport of alkali metal doped Na 0.75 CoO 2thermoelectric oxideP. Jood, G. Peleckis, X. L. Wang, S. X. Dou, H. Yamauchi, and M. Karppinen  Citation: Journal of Applied Physics 107, 09D716 (2010); doi: 10.1063/1.3365491 View online: http://dx.doi.org/10.1063/1.3365491 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/9?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Tuning the functional properties of PMN-PT single crystals via doping and thermoelectrical treatments J. Appl. Phys. 114, 224112 (2013); 10.1063/1.4847975  Influence of rare earth doping on thermoelectric properties of SrTiO3 ceramics J. Appl. Phys. 114, 223714 (2013); 10.1063/1.4847455  Effect of electron doping on thermoelectric properties for narrow-bandgap intermetallic compound RuGa2 J. Appl. Phys. 113, 023713 (2013); 10.1063/1.4775602  Cosubstitution effect on the magnetic, transport, and thermoelectric properties of the electron-doped perovskitemanganite CaMnO 3 J. Appl. Phys. 108, 103702 (2010); 10.1063/1.3505756  Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method J. Appl. Phys. 96, 931 (2004); 10.1063/1.1753070    [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.233.216.27 On: Tue, 13 Oct 2015 07:10:24Phase formation and magnetotransport of alkali metal doped Na0.75CoO2thermoelectric oxideP. Jood,1 G. Peleckis,1,a X. L. Wang,1 S. X. Dou,1 H. Yamauchi,2 and M. Karppinen21Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New SouthWales 2519, Australia2Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015, FinlandPresented 19 January 2010; received 30 October 2009; accepted 30 November 2009;published online 21 April 2010Synthesis and characterization of bulk NaxCoO2 samples substituted by K and Rb is reported. Phaseformation studies revealed a narrow stable region for Na-alkali metal-Co system. Whisker andplatelike single crystalline structures have been found to form on the surface of the pellets in caseof K doping. All samples were metallic and no characteristic anomaly in R-T curves was observedfor Rb doped sample. Magnetoresistance measured has a pronounced positive response only forK-doped and pure NaxCoO2 phases, reaching 11% and 7% at 5 K temperature, respectively. © 2010American Institute of Physics. doi:10.1063/1.3365491In recent years constantly growing concern over decreas-ing amount of primary energy sources has prompted wide-spread research on development of alternative techniquesand materials for power generation. Waste heat from powerplants, factories, or automobiles equals to 70% of the totalprimary energy, but it is difficult to use as its sources arehighly dispersed and small in size. The only effective methodof overcoming these problems by converting heat energy di-rectly into electrical energy is thermoelectric TE powergeneration. Good TE material should possess high conver-sion efficiency, be built out of nonvolatile and nontoxic ele-ments, highly abundant and most important chemicallystable at temperatures as high as 1000 K in air. Thus, oxidebased TE materials are most promising candidates for TEapplications due to their stability at high temperatures in air.After Terasaki et al.1 have reported that NaxCoO2 com-pound exhibits simultaneously high TE power and low resis-tivity S100 V /K and 200  cm at 300 K, manyresearchers put a lot of effort to develop new Co-based ox-ides that would exhibit similar or even better TE properties.Among many Co-based compounds three types of layeredcobalt oxides have attracted the highest attention: NaxCoO2,Ca3Co4O9,2 Bi2Sr3Co2O9.3 In NaxCoO2 TE oxide the sodiumlayers are sandwiched between CoO2 blocks and these arestacked along the c axis. CoO2 layers are responsible forelectrical conductivity and Seebeck coefficient while the Nalayers are insulating and determine thermal conductivity ofthe oxide. Fouassier et al.4 reported that a large amount ofvacancies can easily be introduced into the sodium layer,giving a possibility for a high Na nonstoichiometry. It wasalso shown that highly sodium deficient and water interca-lated NaxCoO2yH2O is superconducting at lowtemperatures.5 Moreover, Balsys et al.6 reported that due tothe peculiar layered structure, Na ions can freely migrate inbetween the oxygen layers. Such migration of Na ions mightbe responsible for structural phase transitions close to roomtemperature which were described by Tojo et al.7 They havefound existence of three first order transitions at 288.7,296.3, and 302.1 K. This result is supported by Motohashi etal.,8 who showed anomalous behavior in electrical resistivitycurves for NaxCoO2 polycrystalline samples.As being a transition metal oxide, NaxCoO2 is very at-tractive for chemical substitution studies. Both cation sites,Na and Co, can be substituted by various elements. Kawataet al.9 showed enhanced TE properties for Ca-substitutedsamples. On the other hand Sr does not go into Na site, butforms an individual compound SrxCoO2.10 Other studiesshowed that Mn, Ru, and Rh act as strong scatters to increaseelectrical resistivity rapidly, while Cu, Zn, and Pd substitu-tion does not alter the electric conduction.11,12 In this articlewe present synthesis and characterization of alkali metaldoped NaxRyCoO2 R=K,Rb polycrystalline samples. Theintroduction of bigger alkali metal ions might decrease Namigration in the layer and thus improve transport propertiesof the material.All samples were prepared applying “rapid heat-up”technique.8 Phase purity was checked using x-ray powderdiffraction MAC Science, MXP18VAHF, Cu K1 radia-tion. Phase formation was studied by means of thermogravi-metric TG analysis Perkin Elmer, Pyris 1. For full syn-thesis simulation, a 5–30 mg amount of sample was put intothe thermobalance. In order to avoid fast alkali metal evapo-ration, the heating rate of 2 °C /min was adopted. The mor-phology and microstructure of the samples was investigatedby scanning electron microscopy SEM Hitachi, S-4500.Chemical composition of the samples was determined usinginductively coupled plasma atomic emission spectrometerICP-AES Seiko, SPS1500VR. Transport properties wereanalyzed by physical property measurement system Quan-tum Design in a temperature range of 5–350 K. For magne-toresistance studies the applied field was varied from 0to 7 T.aAuthor to whom correspondence should be addressed. Electronic mail:peleckis@uow.edu.au.JOURNAL OF APPLIED PHYSICS 107, 09D716 20100021-8979/2010/1079/09D716/3/$30.00 © 2010 American Institute of Physics107, 09D716-1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.233.216.27 On: Tue, 13 Oct 2015 07:10:24Figure 1 shows x-ray diffraction patterns of undoped andK/Rb doped NaxCoO2 samples after sintering. The samplesare of single phase corresponding to that of hexagonal-Na0.75CoO2 oxide. The 002 and 004 peaks at 2=16.2°and 33.4° are much higher in intensity for the Rb dopedsample as opposed to other ones indicating a more pro-nounced c axis orientation. Lattice parameters calculatedfrom the x-ray diffraction patterns revealed that introductionof bigger alkali ion in the system caused elongation of thelattice parameter c, i.e., cun-doped=10.912 Å and cRb-doped=10.946 Å, respectively.Figures 2a and 2b show SEM micrographs ofNa0.75Rb0.25CoO2 calcined powders and sintered pellets, re-spectively. As we can see, the sample morphology has verydistinctive differences. For calcined powders the particles areembedded in some sort of melt and the shape of the particlesis somewhat circular. In contrary, the particles in the sinteredsample Fig. 2b are well formed and separated, have clearsigns of layered structure, and are elongated in shape. Someresidue of melting or sublimation can be seen on the surfaceof the crystallites. The average particle size is about 20 m.Furthermore, it was found that for K-doped samples, singlecrystallinelike formations have been observed Fig. 2c.Two kinds of distinctive products were formed: whisker-typeand petal-like crystals as indicated in Fig. 2c. The forma-tion of the product was found to be highly dependant on Kamount in the samples and sample sintering temperature, i.e.,higher K content and higher temperatures caused largeramount of petal-like crystals to be formed. Detailed phaseformation analysis and TE properties of whisker type crystalsis reported elsewhere.13 We were unable to measure electricproperties of petal like crystals as they were very brittle andthin.To establish and understand the proper synthesis proce-dure we have performed TG simulation of synthesis proce-dure. Figure 3 shows TG curve for the simulation of solid-state reaction, when Rb is doped into NaxCoO2 sample. TheTG curve has distinctive difference as compared to publisheddata for Na-pure samples.8 At around 600–700 °C we havestable triple cation system, i.e., Na–Rb–Co. However, we cansee a drop in weight at temperatures above 760 °C. This canbe attributed to the evaporation of Rb from the samples. Attemperature of 830 °C almost half of Rb content is alreadylost and is completely vanished at 860 °C. This is in contrastto K-doped samples, where stable Na–K–Co temperature re-gion is broader, i.e., 650–830 °C.13 The observed Rb evapo-ration is supported by ICP-AES results, which showed thatNa0.75Rb0.25CoO2 precursor powders region between steps2 and 3 after sintering after step 3 were in factNa0.73Rb0.02CoO2.Electrical properties measured revealed that all samplesare metallic, with lowest electrical resistivity  at 300 K forRb substituted Na0.75CoO2, i.e., =0.9 m cm. The Rbdoped sample does not show characteristic anomaly in theelectrical resistivity curve at room temperature which is incontrast to undoped8 Na0.75CoO2. Furthermore, we have in-FIG. 1. X-ray diffraction patterns for sintered NaxRyCoO2 R=K,Rbsamples.FIG. 2. Color online SEM pictures of a calcined, b sinteredNa0.75Rb0.25CoO2 samples, and c single crystalline products on the surfaceof the pellets for K-doped samples.09D716-2 Jood et al. J. Appl. Phys. 107, 09D716 2010 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.233.216.27 On: Tue, 13 Oct 2015 07:10:24vestigated magnetotransport properties of our samples at 5 KFig. 4b. The undoped sample shows positive magnetore-sistance MR effect reaching 11% at 5 K. In the case of Kdoping this MR response is suppressed to 7%. Lastly, for Rbdoped sample the MR effect is positive and negative, yetquite small, i.e., 4%. The origin of such suppression of theMR effect might be related to the structural reordering andelongation of lattice parameter c. It is possible that due to thelonger distances between CoO2 layers, the magnetic orderingin the material is lower than that for undoped compound.In summary, K and Rb doped samples of Na0.75CoO2 TEoxide were prepared via “rapid heat-up” synthesis technique.Single crystalline structures were observed on the surface ofthe pellets for K-doped samples. TG analysis showed deple-tion of doped alkali ion during sintering procedure. Allsamples were metallic with positive MR effect for undopedand K-doped samples reaching 11% and 7% at 5 K, respec-tively. In contrast, Rb doped sample showed weakly pro-nounced MR features which might be caused by significantstructural/microstructural changes.ACKNOWLEDGMENTSG.P. thanks the Australian Research Council for supportunder Discovery Grant No. DP0 879 714. P.J. thanks theUniversity of Wollongong for providing matching scholar-ship for her Ph.D. studies.1I. Terasaki, Y. Sasago, and K. Uchinokura, Phys. Rev. B 56, R126851997.2A. C. Masset, C. Michel, A. Magnan, M. Hevein, O. Ooulemonde, F.Studier, and B. Raveau, Phys. Rev. B 62, 166 2000.3J. M. Tarascon, R. Rajesh, P. Darboux, M. S. Hedge, G. W. Hull, L. H.Greene, M. Giroux, Y. Lesage, W. R. mcKinnon, J. V. Waszczak, and L. F.Schneemeyer, Solid State Commun. 71, 663 1989.4C. Fouassier, G. Matejka, J.-M. Reau, and P. Hagenmuller, J. Solid StateChem. 6, 532 1973.5K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian,and T. Sasaki, Nature London 422, 53 2003.6R. J. Balsys and R. L. Davis, Solid State Ionics 93, 279 1997.7T. Tojo, H. Kawaji, T. Atake, Y. Yamamura, M. Hashida, and T. Tsuji,Phys. Rev. B 65, 052105 2002.8T. Motohashi, E. Naujalis, R. Ueda, K. Isawa, M. Karppinen, and H.Yamauchi, Appl. Phys. Lett. 79, 1480 2001.9T. Kawata, Y. Iguchi, T. Itoh, K. Takahara, and I. Terasaki, Phys. Rev. B60, 10584 1999.10R. Ishikawa, Y. Ono, Y. Miyazaki, and T. Kajitani, Jpn. J. Appl. Phys., Part2 41, L337 2002.11R. Kitawaki and I. Terasaki, J. Phys.: Condens. Matter 14, 12495 2002.12S. Li, R. Funahashi, I. Matsubara, and S. Sodeoka, Mater. Res. Bull. 35,2371 2000.13G. Peleckis, M. Karppinen, and H. Yamauchi, Appl. Phys. Lett. 83, 54162003.FIG. 3. TG curve of a raw material powder mixture Na:Rb:Co=0.75:0.25:1 upon heating in O2 gas. ¬ Evaporation of H2O; − decar-bonation process; and ® depletion of Rb. Inset shows the same data withrespect to time.FIG. 4. MR for pure and alkali metal doped Na0.75CoO2 measured at 5 K.09D716-3 Jood et al. J. Appl. Phys. 107, 09D716 2010 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.233.216.27 On: Tue, 13 Oct 2015 07:10:24

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Phase formation and magnetotransport of alkali metal doped Na[sub 0.75]CoO[sub 2] thermoelectric oxide

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