Comment on "Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2 yH2O"

0.13 MB·English

by  P. Lemmens

Comment on ”Raman spectroscopy study of Na CoO and superconducting
x 2
Na CoO ·yH O”
x 2 2
P.Lemmens,1,∗P.Scheib,1Y.Krockenberger,2,3L.Alff,3F.C.Chou,4C.T.Lin,2H.-U.Habermeier,2andB.Keimer2
1Institute for Physics of Condensed Matter, TU Braunschweig, D-38106 Braunschweig, Germany
2Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany
3Institute of Materials Science, TU Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany
7 4Center for Materials Science and Engineering, MIT, Cambridge, MA 02139, USA
0 (Dated: February 6, 2008)
0
2 The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed
and compared to experimental results from Co3O4 single crystals. We conclude that on NaCl flux
n
grown NaxCoO2 crystals a surface layer of Co3O4 easily forms that leads to the observation of an
a intense phonon around 700 cm−1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly
J
cleavedcrystalsfromopticalfloatingzoneovensdonotshowsucheffectsandhaveahighfrequency
4 phonon cut-off at approximately 600 cm−1 [Phys. Rev. Lett 96, 167204 (2006)]. We discuss the
2
relation of structural dimensionality, electronic correlations and the high frequency phonon cut-off

of thethermolectric cobaltite.
]
l
e PACSnumbers: 72.80.Ga, 75.30.-m,71.30.+h,78.30.-j
-
r
t
s
Ramanscatteringisawellestablishedprobeforstruc- outfromthe fluxinwater. Thelatterstepmayleadto a
.
t
a turalandelectronic propertiesof solidsas, e.g. composi- Nanonstoichiometry. Evidencefordegradationandcrys-
m tionalandsymmetryinformationcanbegainedfromthe tallographic changes of NaxCoO2 and NaxCoO2·yH2O
- numberandfrequencyoftheobservedphononmodes[1]. on time scales from minutes to weeks exist in literature.
d On the other side its high surface sensitivity may also [5, 6].
n
lead to challenges in sample preparation. The cobaltite
o InarecentRamanscatteringinvestigationofNaCl-flux
c NaxCoO2·yH2O is a correlatedelectron system with an grownNaxCoO2crystals,Shietal.havereportedRaman
[ enormous thermopower for large x > 0.7 and supercon-
spectra that show 5 phonons with in-plane polarization
ductivity for smaller x = 1/3 and hydration, y = 1.3.
1 [7], see Fig. 1, curve a). These modes are attributed
Due to the large mobility of Na on different sites and
v to five Raman active modes corresponding to displace-
3 the mixed nominal Co valency NaxCoO2 has a complex mentsofsodiumandoxygen[5]. Incontrasttothesedata
8 defect chemistry. In the presence of CO and humidity
2 Raman scattering investigations on freshly cleaved TFZ
5 surface layers are formed that consists of, e.g. CoCO ,
3 growncrystals give only two modes with largerintensity
1
Na CO and Co O . The latter compound is also used
0 2 3 3 4 [8]. These modes are attributed to oxygen in-plane and
as an ingot material in sample preparation [2].
7 out-of-plane displacements. While the non-observance
0 The preparation of large single crystals has been re- of the low frequency Na modes is attributed to disorder
t/ ported from optical traveling floating zone (TFZ) ovens on the partially occupied Na sites [9], the vibrations of
ma [3] and from NaCl flux (NaCl, Na2CO3, and B2O3 in oxygen within the CoO2 layers should have character-
varyingcompositions)[4]. TFZgrowncrystalscaneasily istic frequencies. Indeed a linear frequency shift of the
d- be cleaved, while samples from NaCl flux are washed- highestfrequency, out-of-plane mode at590cm−1 by 5%
n has been found with increasing Na content in the TFZ
o crystals[8]. Theshiftimpliesthattheoxygenmodesonly
c
weaklydependonthestackingoftheCoO layersandthe
: 2
v occupationofNasitesthatcharacterizethe(α,β,γ type)
i
X crystalstructure[9]. Withthisrespectthecompoundcan
be considered as two-dimensional and the evolution of
r
a electroniccorrelationswithdopingdominatesthephonon
frequency[10]. Ourexperimentsarefurthersupportedby
recentinelasticX-rayscatteringthatshowabendoverof
the highest phononbranchatabout 70meV ≡ 583cm−1
[11].
In contrast, the three-dimensional Co O has a very
3 4
intense Raman mode at a higher frequency (690cm−1),
Figure 1: Raman scattering spectra of (a) NaCl flux grown i.e. in the same frequency regime as Raman data [7]
Na0.75CoO2 at RT (Ref. 7), (b) Co3O4 at T =200K and (c) of NaCl-flux grown NaxCoO2 crystals. In Fig. 1 we
TFZ grown Na0.83CoO2 at T =90K (Ref. 8). show respective spectra. The small frequency shift and
2
broadening of curve a) compared to b) is attributed to Lin, Phys. Rev.B 70, 024506 (2004)
an oxygen deficiency or a small thickness of the surface [4] K. Fujita, T. Mochida, and K. Nakamura, Jpn. J. Appl.
layer. Similar data on Co O have been reported earlier Phys. 40, 4644 (2001); M. Mikami, M. Yoshimura, Y.
3 4
Mori, T.Sasaki,R.FunahashiandI.Matsubara,Jpn.J.
by Hadjiev et al. [12] and more recently by Qu et al.
Appl. Phys.Vol. 41, L777 (2002).
discussing phase separation [13]. We conclude that
[5] M.N. Iliev, A.P. Litvinchuk, R.L. Meng, Y.Y. Sun, J.
the Raman data [7] of NaCl-flux grown NaxCoO2 are Cmaidalka, C.W. Chu, Physica C 402 (2004) 239-242.
most probably interferedby a degradationof the sample [6] N. Oeschler, R. A. Fisher, N. E. Phillips, J. E. Gordon,
leading to a surface layer of Co3O4. We highlight that M.-L. Foo, and R. J. Cava, cond-mat/0503690 (2005);
although from symmetry analysis the same number of M. Karppinen, I. Asako, T. Motohashi, and H. Ya-
Raman active modes are expected, the frequency of mauchi,Phys.Rev.B71,092105(2005);A.Zorkovsk´,M.
Orenda´cˇ,J.Sˇebek,E.Sˇantava´,P.Svoboda,I.Bradaric´,
tThheemiondteensseinCCoo3OO4 manoddeNaaxtCo6O902cmdi−ffe1rccaonnsibdeeraubselyd. I. Savic´, and A. Feher, Phys. Rev. B 72, 132412 (2005);
3 4 C. de Vaulx, M.-H. Julien, C. Berthier, M. Horvatic´, P.
as a quality measure of cobaltates in thermoelectric
Bordet, V. Simonet, D. P. Chen, and C. T. Lin, Phys.
applications. Rev. Lett. 95, 186405 (2005); P.W. Barnes, M. Avdeev,
J.D. Jorgensen, D.G. Hinks, H.Claus, S. Short, Phys.
Acknowledgement: Weacknowledgesupportbythe Rev.B 72, 134515 (2005); X.N.Zhang,P. Lemmens,B.
DFGwithintheprojectLe967/4-1andtheESFprogram Keimer, D.P. Chen, C.T. Lin, K.Y. Choi, V. Gnezdilov,
F.C. Chou, Physica B 359-361, 424 (2005).
Highly Frustrated Magnetism.
[7] Y. G. Shi, Y. L. Liu, H. X. Yang, C. J. Nie, R. Jin, and
J. Q.Li, Phys.Rev. B 70, 052502 (2004).
[8] P. Lemmens, K.Y. Choi, V. Gnezdilov, E.Ya. Sherman,
D.P.Chen,C.T.Lin,F.C.Chou,andB.Keimer,Phys.
Rev. Lett 96, 167204 (2006).
∗ Electronic address: [email protected]
[9] Q. Huang, M. L. Foo, R. A. Pascal, Jr., J. W. Lynn, B.
[1] See, e.g. contributions to Light Scattering in Solids,
H. Toby, Tao He, H. W. Zandbergen, and R. J. Cava,
edited by G. Gu¨ntherodt and M. Cardona (Springer, Phys. Rev.B 70, 184110 (2004).
Berlin, 1984-2004).
[2] At high temperatures (T >850◦C) Na2O has a large [10] Edr.eSchhte,rmGa.nG,nMth.eFroisdcth,eEr,uPro.pLheyms.mLeentst,.P4.8H,.M64.8v(a1n99L9o)o.s-
volatility. As a result the more stable Co3O4 forms in- [11] J.-P.Rueff,M.Calandra,M.d’Astuto,Ph.Leininger,A.
steadofNaxCoO2 withsmallerorvaryingx.Atambient Shukla,A.Bosak,M.Krisch,H.Ishii,Y.Cai,P.Badica,
conditions the remaining Na2O together with CO2 and T.Sasaki,K.Yamada,andK.Togano,Phys.Rev.B74,
H2O forms Na2CO3. 020504(R) (2006).
[3] F. C. Chou, E. T. Abel, J. H. Cho, Y. S. Lee, J. Phys.
[12] V.G.Hadjiev,M.N.IlievandI.V.Vergilov,J.Phys.C:
Chem.Sol.66, 155 (2005); F.C. Chou,J.H.Cho, P.A. Solid StatePhys.21L190(1988) andreferencestherein.
Lee, E. T. Abel, K. Matan, and Y. S. Lee, Phys. Rev.
[13] J.F.Qu,W.Wang,Y.Chen,G.Li,andX.G.Li,Phys.
Lett. 92, 157004 (2004); D. P. Chen, H. C. Chen, A. Rev. B 73, 092518 (2006).
Maljuk, A. Kulakov, H. Zhang, P. Lemmens, and C. T.