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. 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