The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of

A(4)C(60) (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the

molecular Jahn-Teller distortion and the distorting potential field of the counterions, we measured

the mid-IR spectra of A(4)C(60) compounds in the temperature range 90 - 300 K and found significant

spectral changes with temperature in all three compounds. We also compare these spectra to that

of Na(4)C(60) in its room-temperature polymeric phase, where the distortion is more pronounced and

evident from the structure

Borondics, F.

Kamarás, Katalin

Klupp, Gyöngyi

Oszlányi, Gábor

English

Repository of the Academy's Library

Distortions of C4−60 studied by infraredspectroscopyG. Klupp, F. Borondics, G. Oszlányi and K. KamarásResearch Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P. O. Box49, H-1525 Budapest, HungaryAbstract. The Jahn-Teller effect plays a crucial role in the explanation of the insulating character ofA4C60 (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between themolecular Jahn-Teller distortion and the distorting potential field of the counterions, we measuredthe mid-IR spectra of A4C60 compounds in the temperature range 90 - 300 K and found significantspectral changes with temperature in all three compounds. We also compare these spectra to thatof Na4C60 in its room-temperature polymeric phase, where the distortion is more pronounced andevident from the structure.INTRODUCTIONThe A4C60 (A = K, Rb, Cs) fullerides are insulating, despite the predictions of bandstructure calculations. As an explanation, the concept of the Mott-Jahn-Teller insulatorhas been proposed [1]. In the C4−60 molecule the coupling of the threefold degenerate telectronic state with the Hg vibrational modes will lead to a Jahn-Teller distortion thatcan change the molecular symmetry from Ih to either D5d or D3d (uniaxially distorted),or to D2h (biaxially distorted) [2]. In Cs4C60 there is direct evidence for the distortion:Dahlke et al. observed D2h geometry at both 5 and 293 K by neutron diffraction [3].Na4C60 has a different structure at room temperature from the above three: it formstwo dimensional polymer sheets [4]. In this polymer the fullerene balls are distorted toCi geometry via the sigma bonds connecting the adjacent monomer units.Here we present midinfrared (mid-IR) spectra of Na4C60, K4C60, Rb4C60 and Cs4C60to compare the distortions and to draw conclusions concerning the Jahn-Teller effectin A4C60. We find the spectral pattern consistent with Ci in Na4C60. In the other threecompounds, our results indicate much smaller distortion, varying with temperature. Thechanges suggest a staggered static distortion or a dynamic Jahn-Teller effect in the high-temperature state, and the dominance of the distortion caused by the potential field ofthe surrounding ions in the low temperature state.EXPERIMENTALFulleride salts with more than 95 % purity were prepared by reacting stoichiometricamounts of the alkali metal and C60 at 350 ◦C for K, Rb, Cs, and at 200 ◦C for Na4C60.Mid-IR transmittance of the fullerides were measured in KBr pellets pressed in dry box.Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp500 600 700 800 900 1000 1100 1200 1300 1400  Ih           T1u  D5d        A2u + E1u  D3d        A2u + Eu  D2h        B1u + B2u + B3u  Ci          3 AuCs4C60Rb4C60K4C60C60Na4C60IR Absorbance (a.u., baseline corrected)wavenumber (cm -1 )FIGURE 1. Left panel: Room temperature spectra of C60, K4C60, Rb4C60, Cs4C60 and Na4C60 withthe splitting of the highest frequency T1u mode shown with arrows. Right panel: splitting of T1u modes indifferent pointgroups.The IR measurements were performed in dynamic vacuum in a liquid nitrogen cooledflow-through cryostat with a Bruker IFS28 FT-IR spectrometer using 2 cm−1 resolution.RESULTSThe room temperature spectra of the four measured compounds compared to that ofC60 are shown in Fig.1. In K4C60 and Rb4C60 the two higher-frequency T1u modes ofC60 show a twofold splitting according to fits with Lorentzians (Fig.2). Assuming a JTdistortion twofold splitting occurs when the distortion is D3d or D5d (see right panel ofFig. 1). In Cs4C60 the splitting is threefold (see fits in Fig.2), indicating D2h geometry,which is in accordance with the result of neutron diffraction [3]. In Na4C60 the T1u modesalso show a threefold splitting in accordance with the expected Ci geometry [4].There are numerous spectral features associated with the chemical intermolecularbonds in Na4C60 and the strongly distorted molecular geometry they cause. In contrastto the other A4C60 fullerides, where splitting of the two lower frequency T1u modes isnot observed clearly with the present resolution, in Na4C60 all of the T1u modes are split.In addition, many strong new modes appear in the spectrum, while even Cs4C60 – theA4C60 compound having the lowest symmetry fulleride ion – exhibits only few weaknew modes. We regard the strong feature around 800 cm−1 as direct evidence for theDownloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp1200 1300 1400 100 200 300Rb4C6087 K300 KIR Absorbance (a.u., baselinecorrected)wavenumber (cm -1 )0.0Rb4C60 1325 cm -1 1349 cm -1 1364 cm -1Peak Height (a.u.)Temperature (K)1200 1300 1400Cs4C60475 K300 K96 KIR Absorbance (a.u., baseline corrected)wavenumber (cm -1 )FIGURE 2. Temperature dependence of the spectra of Rb4C60 and Cs4C60. First panel: the two highestfrequency T1u modes of Rb4C60 at 87 K and at 300 K (bold line) with fitted Lorentzian curves (thin line).Second panel: change of peak height of the components of the highest frequency T1u peak in Rb4C60,indicating a transition between 200 - 220 K. Third panel: the two highest frequency T1u modes of Cs4C60at 96 K, at 300 K and at 475 K (bold line) with fitted Lorentzian curves (thin line).single covalent bond, similar to those found in sigma-bonded dimers [5, 6].The temperature dependence of the IR spectrum of Rb4C60 and Cs4C60 is shown inFig. 2. The spectra of K4C60 are similar to Rb4C60, and were shown in Ref. [7]. At lowtemperature the two highest frequency T1u modes of all three A4C60 salts are split intothree, corresponding to a D2h distortion (Fig. 1). On heating these three lines collapseinto two, corresponding to a uniaxial distortion. (The latter finding in Cs4C60 is contraryto our earlier statement [8] and reflects the results of the fits shown here.) The change ofthe spectra with temperature can be best followed by the disappearance of peaks, shownin Fig. 2 and in Ref. [8]. The transitions found occur in the following temperature ranges:K4C60: 260 - 280 K, Rb4C60: 200 - 220 K, Cs4C60: 400 K.DISCUSSIONIn Cs4C60 the low temperature crystal structure is orthorhombic (Immm) [3], thus theD2h fulleride ions are orientationally ordered. On heating the crystal structure changesto tetragonal (I4/mmm) [3]. By analogy, a similar structural phase transition in K4C60and Rb4C60 is also likely: NMR relaxation time measurements [9] point in this directionand this was the explanation put forward in our earlier publications [7, 8]. However,a gradual freezing of molecular motion below the IR time scale would also result insimilar spectral changes. In this case, the anions could still show orientational disorderoccupying two standard orientations in a tetragonal structure. The importance of timescales is also reflected in the fact that the line splitting in Cs4C60 occurs at a highertemperature than in NMR measurements, where a change has been detected betweenDownloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp300 and 350 K [10].As the potential energy surface of these systems has minima at either D5d or D3dsymmetry, and saddle points at D2h symmetry [2], a biaxial distortion can only berealized when the potential field of the surrounding ions lock the molecules in a D2hgeometry. As both the tetragonal and orthorhombic arrangements lead to such a potentialfield, the structure cannot be deducted from the spectra only. Further diffraction studiesare needed to clarify this point.In the high temperature A4C60 phases the crystal structure is bct [11], but the spectraare only consistent with D3d or D5d molecular geometry. As the main axis in thesepoint groups is neither two- nor fourfold, the distortion of the anion obviously is notdetermined by the crystal field of the solid alone. There are two possible scenarios forthe structure of this phase: a staggered static distortion [1, 8] where the molecule’smain axis does not coincide with the c axis of the crystal, or a dynamic Jahn-Tellereffect where pseudorotation occurs between uniaxial geometries with a characteristicfrequency lower than that of the infrared. The fact that the transition temperatures donot scale with cation size is another indication that more than one process is involved.Sorting out the contributions from molecular dynamics and crystal field is a complicatedprocedure, but exactly this perspective makes these materials interesting.ACKNOWLEDGMENTSThis work was supported by OTKA grants T 034198 and T 029931.REFERENCES1. Fabrizio, M., and Tosatti, E., Phys. Rev. B 55, 13465 (1997).2. Chancey, C. C., and O’Brien, M. C. M., The Jahn-Teller effect in C60 and Other IcosahedralComplexes (Princeton University Press, Princeton, 1997).3. Dahlke, P., and Rosseinsky, M. J., Chem. Mater. 14, 1285 (2002).4. Oszlányi, G., Baumgartner, G., Faigel, G., and Forró, L., Phys. Rev. Lett. 78, 4438 (1997).5. Kamarás, K., Tanner, D. B., and Forró, L., Fullerene Sci. Tech. 5, 465 (1997).6. Kürti, J., Borondics, F., and Klupp, Gy., AIP Conference Proceedings 591, 25 (2001).7. Kamarás, K., Klupp, G., Tanner, D. B., Hebard, A. F., Nemes, N. M., and Fischer, J. E., Phys. Rev. B65, 052103 (2002).8. Kamarás, K., Klupp, G., Borondics, F., Gránásy, L., and Oszlányi, G., AIP Conference Proceedings633, 55 (2002).9. Brouet, V., Alloul, H., Garaj, S. and Forró, L., Phys. Rev. B 66, 155122 (2002).10. Goze, C., Rachdi, F. and Mehring, M., Phys. Rev. B 54, 5164 (1996).11. Fleming, R. M., Rosseinsky, M. J., Ramirez, A. P., Murphy, D. W., Tully, J. C., Haddon, R. C.,Siegrist, T., Tycko, R., Glarum, S. H., Marsh, P., Dabbagh, G., Zahurak, S. M., Makhija, A. V., andHampton, C., Nature 352, 701 (1991).Downloaded 17 May 2007 to 128.227.127.61. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

Distortions of C-60(4-) studied by infrared spectroscopy

Abstract

Similar works

Full text

Available Versions

Repository of the Academy's Library