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High pressure effects on the Raman spectrum of CsC60 polymer
Physica B 265 (1999) 234—238
High pressure effects on the Raman spectrum of CsC polymer J. Arvanitidis *, K. Papagelis , I. Tsilika , G. Kanellis , S. Ves , G.A. Kourouklis, K. Tanigaki, K. Prassides Physics Department, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Fundamental Research Laboratory, NEC Corporation, 34 Miyukigaoka, Tsukuba 305, Japan School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, UK
Abstract The effect of high pressure (up to 11 GPa) on the phonon modes of o-CsC polymer has been studied by means of Raman spectroscopy. The pressure dependence of all the observed phonon frequencies exhibits reversible changes in the pressure region 4.3$0.5 GPa. The width of the H (8) mode increases almost linearly with increasing pressure up to &4.3 GPa, above which it seems to remain constant. All the above changes may be related to the metal— insulator transition observed below 50 K for the material under investigation and/or with the structural changes, induced by pressure and observed also by X-ray diffraction measurements. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fullerides; Polymeric fullerides and fullerite derivatives; CsC ; Raman spectroscopy
1. Introduction The alkali doped A C (A: K, Rb, Cs) fullerides V have been studied extensively, especially because superconductivity was observed at rather high temperatures in some of the A C compounds [1]. The AC fullerides have also attracted attention because of their interesting phase diagrams. For ¹'400 K they consist of monomers with FCC rocksalt structure (space group: Fm3 m) and they are electrical conductors [2]. When slowly cooled,
* Correspondence address. Fax: #30-31-995928; e-mail: [email protected].
below 400 K, they adopt a stable orthorhombic polymer phase (space group: Pmnn), forming linear chains of C molecules along the a direction (the face-diagonal direction of the FCC phase) [3]. For the polymerization process a 2#2 cycloaddition between two neighboring C molecules appears to be the most likely reaction [4]. The reaction is expected to occur between the double bonds on two molecules which connect two adjacent hexagons. Quenching of AC fullerides in the high temperature FCC phase induces mostly a dimer phase [5]. At room temperature, the CsC polymer be haves as a quasi-1-D conductor which becomes an insulator after a well-defined phase transition
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 3 8 2 - 9
J. Arvanitidis et al. / Physica B 265 (1999) 234—238
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around 50 K [6]. This transition is connected with the formation of a spin density wave (SDW)-like state which is expected to have strong influence on the structure of CsC polymer [7]. Also, synchrotron X-ray diffraction (XRD) measurements under high pressure showed that for pressures higher than 3.5 GPa this material goes to a more isotropic phase [8]. In this work, we present an investigation of the effect of high hydrostatic pressure (up to &11 GPa) on the Raman spectrum of (CsC ) . L 2. Experiments The (CsC ) powder samples were prepared by L the reaction of stoichiometric amount of C with Cs metal in sealed, evacuated quartz tubes. Final annealing was carried out at 800 K for four weeks. XRD data reveal excellent quality single phase material with an orthorhombic structure (a"9.095 As , b"10.225 As and c"14.173 As ) at normal conditions [5]. High pressure was generated using the diamond anvil cell (DAC) of Mao—Bell type [9]. Glycerol was used as the pressure transmitting medium because the traditional 4 : 1 methanol—ethanol mixture seems to dissolve our samples. The well-known ruby fluorescence technique was used for pressure calibration [10]. Raman spectra were recorded using a triple monochromator (DILOR XY-500) equipped with a CCD liquid-nitrogen-cooled detector system. The spectral width of the system was &5 cm\. The 514.5 nm line of an Ar> laser was used for excitation, with the laser power kept less than 5 mW, measured directly before the cell, in order to avoid laser heating effects analogous to the ones leading to the softening of the A (2) pentagon-pinch (PP) mode observed in C [11]. 3. Results and Discussion The Raman spectra of CsC polymer in the fre quency regions 200—630 cm\ and 1370—1800 cm\ at various pressures and room temperature are illustrated in Fig. 1. At normal pressure, the Raman
Fig. 1. Raman spectra of (CsC ) in the frequency regions L 200—630 cm\ and 1370—1800 cm\ at room temperature and for various pressures. The vertical lines indicate the observed phonon peaks while the numbers on the right indicate the relative scale of the spectra.
spectrum of (CsC ) reveals the six intramolecular L modes of C located in the above frequency re gions, i.e. H (1), H (2), A (1), H (7), A (2) and H (8). In addition, two more modes marked by x(1) and x(2), are observed with frequencies 338, and 554 cm\, respectively. These two modes are also observed in fullerite C but for pressures higher than 0.4 GPa, where this material adopts the single cubic structure with the partial ordering of the molecular rotations [12]. Their presence in (CsC ) is consistent with the freezing of the free L rotations caused by polymerization. We note that the appearance of the x(1) mode at 338 cm\ should be due to the lowering of the symmetry of the C molecules and has been proposed
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J. Arvanitidis et al. / Physica B 265 (1999) 234—238
as the Raman signature of the C—C covalent bridge between the molecules [13,14]. The assignment of this Raman mode is still an open question [14]. At normal conditions the Raman spectrum of the crystalline C is dominated by the A (2) pentag onal pinch (PP) mode at &1469 cm\. In the case of the orthorhombic AC polymer phase (o-AC ) this mode appears to be split into two components with frequencies &1452 and &1462 cm\. The first component has been proposed to be the Raman signature of a polymerized state while the second has been assigned to the rest of the nonpolymerized C\ ions in the o-AC phase [14]. In our case, at normal conditions we observe one rather single but relatively broad peak at
&1458 cm\. The H (7) and, in particular, the H (8) modes appear to be very broad in compari son with those in pristine C crystals. This is the reason why it was very difficult to follow the pressure evolution of the first one which is very close to the intense and broad PP-mode. The broadening of the high frequency H modes is supposed to originate from the interaction with free carriers [15]. Also, the H (8) mode seems to be comprised of more than one component, as expected in the o-AC phase where the formation of linear chains, below 400 K, results in the lowering of the C molecular symmetry from I to D [16]. The pressure dependence of the observed phonon frequencies is shown in Figs. 2 and 3.
Fig. 2. The pressure dependence of the H (1), x(1), H (2), A (1) and x(2) intramolecular Raman modes of (CsC ) . The open L (solid) symbols denote data taken for increasing (decreasing) pressure runs. The shaded area denotes the changes in the slope of the pressure dependence for these modes.
Fig. 3. The pressure dependence of the H (7), A (2) and H (8) intramolecular Raman modes of (CsC ) . The open (solid) sym L bols denote data taken for increasing (decreasing) pressure runs. The shaded area denotes the changes in the slope of the pressure dependence for A (2) and H (8) modes.
J. Arvanitidis et al. / Physica B 265 (1999) 234—238
The open (solid) symbols correspond to spectra recorded with increasing (decreasing) pressure. The peak positions were determined by fitting of Lorentzian (Gaussian for (H (8)) line shapes to the experi mental data. All the observed modes exhibit a positive response to pressure, in analogy to the pristine C [12]. The pressure coefficients of all the observed intramolecular modes exhibit reversible changes in the pressure region 4.3$0.5 GPa which is more pronounced for the H (1), x(2) and H (8) modes (Figs. 2 and 3). The critical pressure regime at 4.3$0.5 GPa is in agreement with the results obtained by synchrotron X-ray diffraction measurements showing that for P'3.5 GPa the (CsC ) L goes to a more isotropic phase [8]. One would expect that in the initial anisotropic orthorhombic structure of (CsC ) , the increase of pressure re L sults in significant changes in the van der Waals interactions between the linear chains (along the band c-axis) while the C—C covalent bonds in the chains (a-axis) must stay essentially unaffected. This anisotropy in compressibility may be responsible for the formation of the more isotropic phase at higher pressures. The fact that the material remains essentially incompressible along the C chains (a-axis) is well-substantiated by the pressure behavior of the total symmetric A (1) breathing mode. We note that in (C N) this mode shows a fast hardening at P'3 GPa where a considerable change in the intramolecular C—C bridge starts [13]. In the case of (CsC ) L we did not observe, up to 11 GPa, any fast hardening of the A (1) mode. This can be understood by the fact that in this material there are two C—C bridges between two C molecules with smaller length (1.44 As [17]) compared to the (C N) (1.61 As [18]). Therefore, in CsC polymer higher pressures are required to observe possible significant changes in the C—C bonds along the linear chains. If we assume that a metal—insulator transition takes place in (CsC ) at P+4.3 GPa, then it L should be expected that the higher frequency Raman peaks of H symmetry would become sharper due to the decreasing of the free carriers’ concentration [15]. In order to verify this assumption we studied the pressure response of the
237
FWHM of the H (8) mode which is well-defined in our spectra. The width of this mode increases almost linearly (from 75 up to 100 cm\) with pressure up to 4.3 GPa. At that pressure we observe an abrupt decrease, by &10 cm\, in the FWHM of this mode and for higher pressures remains nearly constant. The existence of free electrons in the structure of fullerides (e.g. Alkali doped) causes the softening of certain modes [16]. One, therefore, might expect the opposite effect in a metal— insulator transition. This is consistent with the observed faster hardening of the H (1), H (2), x(2), A (2) and H (8) modes for P'4.3 GPa in (CsC ) . This observation along with the FWHM L of H (8) behavior may be attributed to the metal— insulator transition induced by pressure in this material. In conclusion, our results clearly show reversible changes in the pressure coefficients and the width of the observed intramolecular modes at P+ 4.3 GPa. These findings may be attributed to the metal—insulator transition and/or to structural modification from an initial highly anisotropic phase to a more isotropic one, induced by pressure in the (CsC ) . L
Acknowledgements Support by the General Secretariat for Research and Technology, Greece and NATO Grant (HTECH.CRG 97-2317) is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
R.M. Fleming et al., Nature 352 (1991) 787. M.C. Martin et al., Phys. Rev. B 49 (1994) 10818. P.W. Stephens et al., Nature 370 (1994) 636. S. Pekker et al., Solid State Commun. 90 (1994) 349. M. Kosaka et al., Phys. Rev. B 51 (1995) 12018. H. Kuzmany et al., Synth. Metals 85 (1997) 1173. O. Chauvet et al., Phys. Rev. Lett. 72 (1994) 2721. K. Prassides et al., to be published. A. Jayaraman, Rev. Sci. Instrum. 57 (1986) 1013. D. Barnett et al., Rev. Sci. Instrum. 44 (1973) 1.
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[11] K.P. Meletov et al., Chem. Phys. Lett. 290 (1998) 125. [12] K.P. Meletov et al., Phys. Rev. B 52 (1995) 10090. [13] J. Arvanitidis et al., Phys. Stat. Sol. (B) 211 (1999) 435. [14] D. Bormann et al., Phys. Rev. B 54 (1996) 14139.
[15] H. Kuzmany et al., Synth. Metals 85 (1997) 1173. [16] M.C. Martin et al., Phys. Rev. B 51 (1995) 3210. [17] A. Lappas et al., J. Am. Chem. Soc. 117 (1995) 7560. [18] C.M. Brown et al., Chem. Mater. 8 (1996) 2548.
High pressure effects on the Raman spectrum of CsC polymer J. Arvanitidis *, K. Papagelis , I. Tsilika , G. Kanellis , S. Ves , G.A. Kourouklis, K. Tanigaki, K. Prassides Physics Department, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Fundamental Research Laboratory, NEC Corporation, 34 Miyukigaoka, Tsukuba 305, Japan School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, UK
Abstract The effect of high pressure (up to 11 GPa) on the phonon modes of o-CsC polymer has been studied by means of Raman spectroscopy. The pressure dependence of all the observed phonon frequencies exhibits reversible changes in the pressure region 4.3$0.5 GPa. The width of the H (8) mode increases almost linearly with increasing pressure up to &4.3 GPa, above which it seems to remain constant. All the above changes may be related to the metal— insulator transition observed below 50 K for the material under investigation and/or with the structural changes, induced by pressure and observed also by X-ray diffraction measurements. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fullerides; Polymeric fullerides and fullerite derivatives; CsC ; Raman spectroscopy
1. Introduction The alkali doped A C (A: K, Rb, Cs) fullerides V have been studied extensively, especially because superconductivity was observed at rather high temperatures in some of the A C compounds [1]. The AC fullerides have also attracted attention because of their interesting phase diagrams. For ¹'400 K they consist of monomers with FCC rocksalt structure (space group: Fm3 m) and they are electrical conductors [2]. When slowly cooled,
* Correspondence address. Fax: #30-31-995928; e-mail: [email protected].
below 400 K, they adopt a stable orthorhombic polymer phase (space group: Pmnn), forming linear chains of C molecules along the a direction (the face-diagonal direction of the FCC phase) [3]. For the polymerization process a 2#2 cycloaddition between two neighboring C molecules appears to be the most likely reaction [4]. The reaction is expected to occur between the double bonds on two molecules which connect two adjacent hexagons. Quenching of AC fullerides in the high temperature FCC phase induces mostly a dimer phase [5]. At room temperature, the CsC polymer be haves as a quasi-1-D conductor which becomes an insulator after a well-defined phase transition
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 3 8 2 - 9
J. Arvanitidis et al. / Physica B 265 (1999) 234—238
235
around 50 K [6]. This transition is connected with the formation of a spin density wave (SDW)-like state which is expected to have strong influence on the structure of CsC polymer [7]. Also, synchrotron X-ray diffraction (XRD) measurements under high pressure showed that for pressures higher than 3.5 GPa this material goes to a more isotropic phase [8]. In this work, we present an investigation of the effect of high hydrostatic pressure (up to &11 GPa) on the Raman spectrum of (CsC ) . L 2. Experiments The (CsC ) powder samples were prepared by L the reaction of stoichiometric amount of C with Cs metal in sealed, evacuated quartz tubes. Final annealing was carried out at 800 K for four weeks. XRD data reveal excellent quality single phase material with an orthorhombic structure (a"9.095 As , b"10.225 As and c"14.173 As ) at normal conditions [5]. High pressure was generated using the diamond anvil cell (DAC) of Mao—Bell type [9]. Glycerol was used as the pressure transmitting medium because the traditional 4 : 1 methanol—ethanol mixture seems to dissolve our samples. The well-known ruby fluorescence technique was used for pressure calibration [10]. Raman spectra were recorded using a triple monochromator (DILOR XY-500) equipped with a CCD liquid-nitrogen-cooled detector system. The spectral width of the system was &5 cm\. The 514.5 nm line of an Ar> laser was used for excitation, with the laser power kept less than 5 mW, measured directly before the cell, in order to avoid laser heating effects analogous to the ones leading to the softening of the A (2) pentagon-pinch (PP) mode observed in C [11]. 3. Results and Discussion The Raman spectra of CsC polymer in the fre quency regions 200—630 cm\ and 1370—1800 cm\ at various pressures and room temperature are illustrated in Fig. 1. At normal pressure, the Raman
Fig. 1. Raman spectra of (CsC ) in the frequency regions L 200—630 cm\ and 1370—1800 cm\ at room temperature and for various pressures. The vertical lines indicate the observed phonon peaks while the numbers on the right indicate the relative scale of the spectra.
spectrum of (CsC ) reveals the six intramolecular L modes of C located in the above frequency re gions, i.e. H (1), H (2), A (1), H (7), A (2) and H (8). In addition, two more modes marked by x(1) and x(2), are observed with frequencies 338, and 554 cm\, respectively. These two modes are also observed in fullerite C but for pressures higher than 0.4 GPa, where this material adopts the single cubic structure with the partial ordering of the molecular rotations [12]. Their presence in (CsC ) is consistent with the freezing of the free L rotations caused by polymerization. We note that the appearance of the x(1) mode at 338 cm\ should be due to the lowering of the symmetry of the C molecules and has been proposed
236
J. Arvanitidis et al. / Physica B 265 (1999) 234—238
as the Raman signature of the C—C covalent bridge between the molecules [13,14]. The assignment of this Raman mode is still an open question [14]. At normal conditions the Raman spectrum of the crystalline C is dominated by the A (2) pentag onal pinch (PP) mode at &1469 cm\. In the case of the orthorhombic AC polymer phase (o-AC ) this mode appears to be split into two components with frequencies &1452 and &1462 cm\. The first component has been proposed to be the Raman signature of a polymerized state while the second has been assigned to the rest of the nonpolymerized C\ ions in the o-AC phase [14]. In our case, at normal conditions we observe one rather single but relatively broad peak at
&1458 cm\. The H (7) and, in particular, the H (8) modes appear to be very broad in compari son with those in pristine C crystals. This is the reason why it was very difficult to follow the pressure evolution of the first one which is very close to the intense and broad PP-mode. The broadening of the high frequency H modes is supposed to originate from the interaction with free carriers [15]. Also, the H (8) mode seems to be comprised of more than one component, as expected in the o-AC phase where the formation of linear chains, below 400 K, results in the lowering of the C molecular symmetry from I to D [16]. The pressure dependence of the observed phonon frequencies is shown in Figs. 2 and 3.
Fig. 2. The pressure dependence of the H (1), x(1), H (2), A (1) and x(2) intramolecular Raman modes of (CsC ) . The open L (solid) symbols denote data taken for increasing (decreasing) pressure runs. The shaded area denotes the changes in the slope of the pressure dependence for these modes.
Fig. 3. The pressure dependence of the H (7), A (2) and H (8) intramolecular Raman modes of (CsC ) . The open (solid) sym L bols denote data taken for increasing (decreasing) pressure runs. The shaded area denotes the changes in the slope of the pressure dependence for A (2) and H (8) modes.
J. Arvanitidis et al. / Physica B 265 (1999) 234—238
The open (solid) symbols correspond to spectra recorded with increasing (decreasing) pressure. The peak positions were determined by fitting of Lorentzian (Gaussian for (H (8)) line shapes to the experi mental data. All the observed modes exhibit a positive response to pressure, in analogy to the pristine C [12]. The pressure coefficients of all the observed intramolecular modes exhibit reversible changes in the pressure region 4.3$0.5 GPa which is more pronounced for the H (1), x(2) and H (8) modes (Figs. 2 and 3). The critical pressure regime at 4.3$0.5 GPa is in agreement with the results obtained by synchrotron X-ray diffraction measurements showing that for P'3.5 GPa the (CsC ) L goes to a more isotropic phase [8]. One would expect that in the initial anisotropic orthorhombic structure of (CsC ) , the increase of pressure re L sults in significant changes in the van der Waals interactions between the linear chains (along the band c-axis) while the C—C covalent bonds in the chains (a-axis) must stay essentially unaffected. This anisotropy in compressibility may be responsible for the formation of the more isotropic phase at higher pressures. The fact that the material remains essentially incompressible along the C chains (a-axis) is well-substantiated by the pressure behavior of the total symmetric A (1) breathing mode. We note that in (C N) this mode shows a fast hardening at P'3 GPa where a considerable change in the intramolecular C—C bridge starts [13]. In the case of (CsC ) L we did not observe, up to 11 GPa, any fast hardening of the A (1) mode. This can be understood by the fact that in this material there are two C—C bridges between two C molecules with smaller length (1.44 As [17]) compared to the (C N) (1.61 As [18]). Therefore, in CsC polymer higher pressures are required to observe possible significant changes in the C—C bonds along the linear chains. If we assume that a metal—insulator transition takes place in (CsC ) at P+4.3 GPa, then it L should be expected that the higher frequency Raman peaks of H symmetry would become sharper due to the decreasing of the free carriers’ concentration [15]. In order to verify this assumption we studied the pressure response of the
237
FWHM of the H (8) mode which is well-defined in our spectra. The width of this mode increases almost linearly (from 75 up to 100 cm\) with pressure up to 4.3 GPa. At that pressure we observe an abrupt decrease, by &10 cm\, in the FWHM of this mode and for higher pressures remains nearly constant. The existence of free electrons in the structure of fullerides (e.g. Alkali doped) causes the softening of certain modes [16]. One, therefore, might expect the opposite effect in a metal— insulator transition. This is consistent with the observed faster hardening of the H (1), H (2), x(2), A (2) and H (8) modes for P'4.3 GPa in (CsC ) . This observation along with the FWHM L of H (8) behavior may be attributed to the metal— insulator transition induced by pressure in this material. In conclusion, our results clearly show reversible changes in the pressure coefficients and the width of the observed intramolecular modes at P+ 4.3 GPa. These findings may be attributed to the metal—insulator transition and/or to structural modification from an initial highly anisotropic phase to a more isotropic one, induced by pressure in the (CsC ) . L
Acknowledgements Support by the General Secretariat for Research and Technology, Greece and NATO Grant (HTECH.CRG 97-2317) is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
R.M. Fleming et al., Nature 352 (1991) 787. M.C. Martin et al., Phys. Rev. B 49 (1994) 10818. P.W. Stephens et al., Nature 370 (1994) 636. S. Pekker et al., Solid State Commun. 90 (1994) 349. M. Kosaka et al., Phys. Rev. B 51 (1995) 12018. H. Kuzmany et al., Synth. Metals 85 (1997) 1173. O. Chauvet et al., Phys. Rev. Lett. 72 (1994) 2721. K. Prassides et al., to be published. A. Jayaraman, Rev. Sci. Instrum. 57 (1986) 1013. D. Barnett et al., Rev. Sci. Instrum. 44 (1973) 1.
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J. Arvanitidis et al. / Physica B 265 (1999) 234—238
[11] K.P. Meletov et al., Chem. Phys. Lett. 290 (1998) 125. [12] K.P. Meletov et al., Phys. Rev. B 52 (1995) 10090. [13] J. Arvanitidis et al., Phys. Stat. Sol. (B) 211 (1999) 435. [14] D. Bormann et al., Phys. Rev. B 54 (1996) 14139.
[15] H. Kuzmany et al., Synth. Metals 85 (1997) 1173. [16] M.C. Martin et al., Phys. Rev. B 51 (1995) 3210. [17] A. Lappas et al., J. Am. Chem. Soc. 117 (1995) 7560. [18] C.M. Brown et al., Chem. Mater. 8 (1996) 2548.