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High pressure optical properties of fullerene and fullerene derivatives
Physica B 265 (1999) 214—222
High pressure optical properties of fullerene and fullerene derivatives G.A. Kourouklis *, S. Ves, K.P. Meletov Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Physics Department, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Institute of the Solid State Physics RAS, Chernogolovka, Moscow region 142432, Russia
Abstract High-pressure Raman spectroscopy is a useful tool to investigate various pressure-induced effects on fullerene and its chemical derivatives. In the case of pristine C the pressure-induced freezing of molecular rotations causes the softening, while the lowering of symmetry causes the splitting of several phonon modes. In the neutral state donor—acceptor molecular complex C *TMTSF*2(CS ) high pressure leads to an irreversible charge-transfer phase transition, asso ciated with the transfer of one electron from TMTSF donor to C acceptor molecule. In the case of azafullerene (C N) , the pressure dependencies of the phonon frequencies exhibit reversible changes in their slopes, associated with the attainment of the ideal HCP structure (originally c/a"1.623), as well as with the starting of the intradimer bridge shortening. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fullerene and fullerene derivatives; Raman spectroscopy; High pressure
1. Introduction The interest in molecular dynamics and phonon spectra of fullerenes and their chemical derivatives is connected with the unique molecular structure of C and the versatility of its phonon spectrum. Various perturbations in the structure of the C cage, caused by external disturbances like pres sure and temperature application, chemical bond formation, etc., are manifested in the phonon spec-
* Corresponding author. Tel.: #30-31995947; fax: #3031995928; e-mail: [email protected].
trum. Thus, Raman scattering has become a very powerful and useful tool for the investigation of the physical properties, phase transitions and material transformations [1—5] for the huge family of fullerene-based materials. In particular, Raman scattering coupled with the application of external pressure has yielded a wealth of information on phase transitions, pressure-induced chemical transformations and charge transfer transitions [6—9]. In this paper we present a summary of the experimental results obtained on pressure-induced effects on pristine fullerene and some fullerene derivatives studied by means of Raman scattering, giving important information on their structural, physical and chemical properties.
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 7 6 - 3
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In particular, for fullerite we have determined the pressure dependence of the intramolecular phonon modes [7,8] and we have shown that changes, observed in the Raman spectra, are related to phase transitions at 0.4 and 2.5 GPa. These transitions are associated with the pressure-induced freezing of molecular rotations, manifested by the softening and splitting of several phonon modes as well as changes in their pressure coefficients. In the Raman spectra of the molecular donor—acceptor complex of fullerene, C *C H Se *2(CS ) [C H Se , tetramethyl-tetra-selenafulvalene (TMTSF)], the pressure dependence of the frequencies, of almost all intramolecular phonon modes, exhibits irreversible changes at 5.0$0.5 GPa [9]. These changes include splitting and softening of various modes, the more characteristic of them being the irreversible softening of the A (2) pentagon pinch (PP) mode by 9 cm\, observed upon total release of pressure. The residual softening of this mode is almost the same as in the case of the potassiumdoped fullerene KC . These observations may be attributed to a pressure-induced irreversible phase transition, associated with the transfer of one electron from TMTSF donor to C acceptor molecule. In the case of azafullerene (C N) , a fullerene de rivative containing a hetero-atom in the cage of C , the pressure dependencies of the H (7), A (2), and H (8) phonon frequencies exhibit reversible changes in their pressure coefficients in the region 6.0$0.5 GPa. In addition, the A (1) breathing mode shows two clear changes in the dependence of its pressure coefficient, one at &3 and another at &6 GPa. The changes at &6 GPa can be attributed to the fact that (C N) attains the ideal HCP structure (c/a"1.633) in this pressure region (originally c/a"1.623), while the changes of A (1) mode at 3 GPa might imply the starting of the intradimer bridge shortening.
2. Pressure-induced phase transitions in pristine C60 probed by Raman scattering The Raman spectra of C and its derivatives were measured in two frequency regions from 200 to 800 cm\ and from 1350 to 1700 cm\, in the intermediate region the Raman mode of diamond
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appears (1333.4 cm\) making it difficult to record spectra. The vibrational spectrum of a free C mol ecule contains 46 modes, with symmetries: C"2A #3F #4F #6G #8H #A #4F #5F #6G #7H , and with allowance for the high symmetry (I ) and degeneracy the total number of modes becomes 174 [1,3], from these only the two A and eight H are Raman active modes. At ambient pressure, the Raman spectrum of pristine C contains eight intramolecular modes: H (1)"272 cm\, H (2)" 435 cm\, A (1)"496 cm\, H (3)"710 cm\, H (4)"772 cm\, H (7)"1422 cm\, A (2)" 1467 cm\ and H (8)"1570 cm\. The most in tense A (2) and A (1) modes of the spectrum corres pond to the out-of-phase and in-phase dilatory vibration of the pentagonal and hexagonal rings of the molecule, respectively. The frequencies of the observed phonon modes at ambient pressure are close to the previously reported ones [1,3]. It is important to note that the Raman measurements for all materials were performed with special care in setting the laser power at such a level in order to get the maximum value (&1467 cm\ at normal pressure) for the frequency of the A (2) PP-mode. For higher laser power, a decrease in the frequency of the A (2) mode, up to 4 cm\, was observed. The laser power induced reversible softening of the A (2) mode is related to the local overheating of the sample in the laser spot as experimental and theoretical investigations have shown [10]. Whenever the laser power density was higher than some critical value this decrease of the A (2) mode frequency for pristine C became irreversible and was ac companied by the damage on the crystal surface. This phenomenon is associated with the photodimerization of the C molecules in the FCC phase of pristine C [4]. The laser power density was kept at a minimum level for all measurements to exclude the softening of PP-mode, both at normal and high pressure. The behavior of the phonon modes observed in the low and middle frequency region, namely, H (1), H (2), A (1), H (3) and H (4) as well as the nine new modes, u(1)—u(9), appeared in the partially ordered simple cubic phase is shown in Fig. 1. In Fig. 2a, the
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pressure dependence of H (7), A (2), H (8) and a new mode, marked u(10), is shown by open and solid symbols for increasing and decreasing pressure, respectively. The experimental data for A (2) and H (8) modes were fitted linearly in the pressure region above 0.4 GPa, giving slopes 5.5 cm\/GPa and 4.8 cm\/GPa, respectively. The solid lines correspond to this fit. The H (7) mode shows also a linear pressure dependence, but its slope changes drastically at a pressure of 2.5 GPa. Below this pressure, the slope is 9.3 cm\/GPa, while above the slope decreases by more than a factor of 2 and becomes 4.1 cm\/GPa. The pressure coefficients of the phonon frequencies change at pressures 0.4 and 2.5 GPa. In the region 0—0.4 GPa, several of the observed phonon modes exhibit negative frequency shifts amounting to 10 cm\ for H (1) and A (2) down to 5 cm\ for H (2), A (1) and H (3) modes. For some of them this softening is shown in Fig. 2(b), in which it is clearly shown that the same pressure-induced softening behavior of the A (2), is also exhibited by several other modes. Above 0.4 GPa the pressure-induced shift of the phonon modes becomes positive except for the H (3), H (4), and two new modes, which are clustered in the
Fig. 1. Pressure dependence of the intramolecular mode frequency of H (2), A (1), H (3), H (4) and for u(1)—u(9) modes for the pristine C . The modes u(1)—u(9) appear in the SC phase above 0.4 GPa. Data are taken in different experimental runs for increasing pressure (open symbols) and decreasing pressure (solid symbols) cycles. The lines are linear least-square fits to the experimental data. Vertical lines indicate the transition pressures 0.4 and 2.5 GPa.
Fig. 2. (a) Pressure dependence of the intramolecular mode frequency of H (7), A (2), H (8) and u(10) for the pristine C . The mode u(10) appears in the SC phase above 0.4 GPa. The lines are linear least-square fits to the experimental data. Vertical lines indicate the transition pressures 0.4 GPa and 2.5 GPa. (b) Pressure dependence of the mode frequency of H (8), A (2), A (1) and H (1) of pristine C showing in more detail the pressure-induced softening in the 0—0.4 GPa pressure region. Data are taken in different experimental runs for increasing pressure (open symbols) and decreasing pressure (solid symbols) cycles.
700—800 cm\ region, exhibiting a softening up to the highest pressure reached in our experiments. It must be emphasized that all the pressure induced changes are reversible with pressure. The changes in the pressure dependence of the Raman modes of pristine C at 0.4 GPa and the appearance of new modes above this pressure is connected with the orientational ordering phase transition from the FCC to the SC crystal structure. The first Raman indication of this transition was observed by Chandrabhas et al. [6] as a softening of the A (2) PP-mode at pressures up to 0.6 GPa. At normal pressure this transition takes place at 249 K [11] and the transition temperature increases with pressure at a rate of 10.4 K/kbar [12]. The appearance of the additional Raman modes in the low temperature SC phase of C was reported for the first time by van Loosdrecht et al. [2]. We have observed all typical features of the lowtemperature SC phase in the pressure-induced orientationally ordered SC phase at room temperature. It is known that the photodimerization of
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C under intense UV illumination, results in sim ilar changes to the Raman spectra, namely, the softening of A (2) PP-mode and the appearance of new modes in the spectrum [4]. The fundamental difference between the pressure-induced and photodimerization effects, is the reversibility of all pressure-induced effects. Therefore, the observed pressure behavior of the Raman spectra near 0.4 GPa is due to the orientational ordering phase transition. An interesting result, in our opinion, is the softening of almost all intramolecular modes of C in the pressure region below the orientational ordering phase transition. The softening of the intramolecular modes of C has also been observed in the intercalated C compounds A C , A"K, Rb, Cs [13]. In this case the Raman active modes are insensitive to the A atom, and therefore to the lattice expansion caused by doping with alkali metal atoms with larger ionic radii. This softening has been attributed [13] to the elongation of the intramolecular bond lengths induced by charge transfer and the resulting softening of the force constants. Charge transfer may also cause the pressure-induced softening of the intramolecular modes in pristine C . The orientational ordering phase transition leads to the freezing of random rotations of the C molecules. In the SC phase the molecules are oriented in one of the two equivalent standard orientations, in which the two-fold molecular axes are aligned along the crystallographic axes. The simple cubic structure requires that each of the four molecules in the unit cell rotates about different [1 1 1] direction preserving the threefold axes along the [1 1 1] direction required by the cubic symmetry [14]. The neutron powder diffraction patterns showed that the principal rotation angle was about 98° [15]. This angle is such that the six electron-poor centers of the pentagonal faces of a given molecule face the short, electron-rich bonds of the neighboring molecules. This, precisely, may lead to the elongation of the intramolecular bond lengths and the resulting softening of the force constants. The pressure dependence of the observed phonon modes exhibits drastic changes in the pressure coefficients of a number of modes at 2.5 GPa. X-ray powder diffraction study [16] of C under
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Fig. 3. Phase diagram of fullerite.
pressure showed that, at room temperature, the rotation free orientationally ordered SC phase occurs at pressures higher than 2.5 GPa. The X-ray results obtained for 0.55 GPa and 1.3 GPa indicate the presence of a reduced degree of orientational order reflecting the rotation of molecules along [1 1 1] directions. At ambient pressure this transition occurs near 85 K [17]. The change near 2.5 GPa is the manifestation of this phase transition at room temperature. All these observations could lead to the construction of a phase diagram for fullerite as is shown in Fig. 3.
3. Pressure-induced charge transfer and chemical modifications in fullerene complexes The interest in the pressure behavior of the neutral state donor—acceptor molecular complexes of C is associated with the pressure-induced enhance ment of the donor—acceptor interaction resulting in charge transfer between donor and acceptor molecules. The charge reorganization changes the bond lengths and, therefore, is reflected in the
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phonon spectrum. As an example of a neutral state molecular complex of C , we have studied the C *C H Se *2(CS ) [C H Se , tetramethyl tetra-selenafulvalene (TMTSF)] compound. The crystal structure of this compound, determined by X-ray diffraction, is monoclinic with lattice parameters: a"1.5407, b"1.2934, c"1.2026 nm, b"108.39°, »"2.2741 nm, space group C ,
Z"2, o "1.929 g/cm, R"0.047 [18]. The conformation of the C molecule and the distances between them are not changed significantly in comparison to the pristine C crystals. The conforma tion of TMTSF molecule is significantly altered and becomes non-planar [18]. The Raman spectrum of C *TMTSF*2(CS ) complex, in the frequency region from 100 to 1700 cm\, contains the eight principal molecular modes of C with practically the same frequencies as in the case of pristine C [1,2,8]. In addition to these bands, there are some very weak Raman peaks, whose origin may be of a second-order scattering from the C molecular vibrations, since the observed frequencies do not resemble those observed in TMTSF powder or in TMTSF\ in solution [19]. The pressure dependence of the H (3), H (4) H (7), A (2) and H (8) phonon frequencies is shown in Fig. 4. The additional weak Raman peak observed at x"755 cm\, at normal pressure, is related with two-phonon scattering from the combination of [F (1)#G (1)] modes [20]. All modes exhibit a positive response (&5.2 cm\/GPa) to pressure except H (3) and the combination mode, which exhibit a negative response in the whole pressure range investigated. The pressure dependence of both modes is practically the same with values of the pressure coefficients !1.4 cm\/GPa for increasing and !0.9 cm\/GPa for releasing pressure. When pressure increases, in the region of 5.0$0.5 GPa, the A (2) PP-mode softens, whereas the H (7) mode hardens considerably. Upon pres sure release no such peculiarities in this pressure region, or in the rest pressure range is observed. All these changes are irreversible. The pressure coefficients of the dependencies for A (2) and H (7) modes are practically the same below and above 5.0 GPa, both for increasing and releasing pressure, and are equal to 5.2 cm\/GPa. The value of
Fig. 4. Pressure dependence of H (7), A (2), H (8), H (3), H (4) and the two-phonon [F (1)#G (1)] intramolecular phonon frequencies of the C *TMTSF*2(CS ) molecular complex. Open (solid) symbols correspond to data taken at three pressure runs for increasing (decreasing) pressure. Solid lines (dashed lines) are guides to the eye for increasing (decreasing) pressure. The shaded area shows the pressure region of the phase transition.
the residual softening of the PP-mode, after total release of pressure, is &9 cm\. The residual hardening of the H (7) mode is of the same value, whereas the softening of the H (8) mode is approx imately two times smaller. The pressure dependence of the H (3), H (4) and the combination mode frequencies shows the same irreversible changes in the pressure region 5.0$0.5 GPa. Their pressure dependencies for increasing and releasing pressure differ considerably at pressures below 5.0 GPa (above 5.0 GPa these dependencies are the same). In addition, the H (4) splits in two modes upon releasing pressure below 5.0 GPa. All three modes, upon total release of pressure, exhibit a residual softening equal to 9 cm\ for the H (3) and the two-phonon combination [F u(1)#G (1)] modes, while for the two components resulting from the
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splitting of the H (4) mode, this residual softening is equal to 18 and 9 cm\, respectively. From these results it is obvious that in the transition pressure range, 5.0$0.5 GPa (shaded area in Fig. 5), take place changes in the pressure dependence of phonon modes and these effects are irreversible. The pressure dependence of the intramolecular phonon frequencies of the C *TMTSF*2(CS ) complex in the pressure region 0—3.0 GPa do not show any reversible changes, associated with orientational ordering of C molecular rotations as in pristine C . This is compatible with the X-ray scattering data, indicating that the C molecules in the C *TMTSF*2(CS ) complex are orienta tionally ordered at room temperature and normal pressure [18]. The most interesting features of the pressure dependence of the phonon modes of C *TMTSF*2(CS ) are the prominent changes at 5.0$0.5 GPa, associated with the softening of all phonon modes and the splitting of some of them. These changes are irreversible and the residual softening, observed in the recovered material upon total release of pressure, vary from 5 up to 18 cm\ for the various modes. The softening of the A (2) PP-mode, observed under various degrees of illumination, is reversible under conditions of low illumination and has been associated with the sample overheating [10]. We have always checked the laser power density in our experiments, making sure that no overheating effects were observed. It must be emphasized that the photodimerization of C molecules within the C *TMTSF*2(CS ) complex and consequently the irreversible softening of the PP-mode is not possible because of the total orientational order of the C molecules in the structure of the complex at normal conditions [18]. The softening of the intramolecular modes of C has been observed in the intercalated C compounds. This softening has been attributed to the elongation of the intramolecular bond lengths induced by charge transfer from alkali metal atoms to the C molecule and the resulting softening of the force constants [13]. Winter and Kuzmany [21] have studied the softening of the PP-mode for the K C series of comV pounds in the region 0)x)6. The main result of their study is that the PP-mode softens almost linearly with increasing x, following the charge
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transfer from K to C molecule. They have found that in the case of the KC compound the soften ing of PP-mode is equal to &9 cm\. The pressure-induced softening of phonon modes in the C *TMTSF*2(CS ) molecular com plex could be associated with the charge transfer between the TMTSF donor and C acceptor mol ecules. At normal pressure the van der Waals-type interaction between donor and acceptor molecules is too weak so they form the neutral state molecular complex with weak charge transfer. The gradual reduction of the intermolecular donor—acceptor distances and the enhancement of the interaction between them at higher pressures, results in the charge-transfer phase transition, taking place in the region 5.0$0.5 GPa. During the phase transition of one electron from TMTSF is transfered to the C molecule forming a new anion state C\ charge-transfer complex. As a result of this, the transformation of the material becomes irreversible and the PP-mode exhibits a residual softening by 9 cm\, observed upon total release of pressure, in complete analogy to the case of KC [21]. Similar studies [22] have shown that pressure can induced neutral to ionic state phase transition in the case of tetrathiofulvalene—haloquinone mixed stack charge transfer crystal.
4. Dimerization effects on the phonon spectrum-Azafullerene (C59N)2 As a result of the trivalency of nitrogen, compared to the tetravalency of carbon, nitrogen substitution of a carbon atom in the C cage leads to the azafullerene radical, C N* — isoelectronic with the C\ anion — which has been found to rapidly dimerize, yielding the (C N) dimer [23]. Thus, solid (C N) consists of dimerized molecular units linked by C—C bonds between carbon atoms adjacent to the N atom (the C—N bond is between two six-membered rings) at each monomer [23—25]. Conductivity measurements have confirmed that the material is an insulator, while the EPR spectra consist of only one sharp narrow line, attributed to a low concentration of defect spins [26]. Electron energyloss and photoemission spectroscopy measurements revealed little mixing between the
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N- and C-electronic states with strong localization of the excess electron at the N atom [27]. At ambient pressure, the Raman spectrum of (C N) contains the same eight modes H (1), H (2), A (1), H (3), H (4), H (7), A (2) and H (8), as that of pristine C . Three additional modes, u(1), u(2) and u(3), observed at 344, 395 and 730 cm\, respectively, were observed in pristine C at pres sures higher than 0.4 GPa, where the free rotations of the C molecules are partially frozen [8]. The behavior and appearance, in various fullerene derivatives, of the mode at 344 cm\ poses an important problem as to the origin of this mode. Its presence has been associated with the polymeric phase in AC [28]. The presence of u(1)—u(3) in (C N) is consistent with the freezing of free rota tion caused by dimerization. The frequencies of the observed phonon modes in the Raman spectra of
Fig. 5. The pressure dependence of the A (1), H (3), x(3), H (4), H (7), A (2) and H (8), intramolecular Raman modes of (C N) . The open (solid) symbols denote data taken for increasing (decreasing) pressure runs. Shaded areas near &6.0, &3 and &5 GPa denote the change in the slopes of the pressure dependencies of different modes.
(C N) are close to the corresponding ones in pristine C [1,2,8]. The observed differences do not exceed the spectral resolution of the apparatus (&3 cm\), except for the H (1), A (2) and u(2) modes, where the corresponding differences are slightly larger, in the range of 6—8 cm\. It is worth pointing out that the frequency of the A (2) mode, at ambient pressure, is&1460 cm\, exactly the same as for the photodimerized C [4]. Therefore, dimerization results in the softening of the A (2) mode in C and its derivatives. Another interest ing observation in the Raman spectrum is the broad (width &40 cm\) asymmetric feature at 265 cm\. In photodimerized C , the H (1) intra molecular mode appears to be split into two components with frequencies 258 and 271 cm\ [4]. Thus we assume that this feature in the spectrum of azafullerene has the same origin. The above observations are clear manifestations of the dimeric character of azafullerene in its Raman spectra. The pressure dependence of the observed phonon mode frequencies is shown in Fig. 5 for the high and middle frequency regions. The open (solid) symbols denote data taken for increasing (decreasing) pressure measurements. All modes, except H (3) and H (4), exhibit a positive response to pres sure, in analogy to pristine C . Note that the u(3) mode exhibits positive pressure coefficient for P(5 GPa, while in fullerite its slope has opposite sign. The shaded areas denote the pressure regions where changes of the pressure coefficients occur. The data obtained show that (C N) does not exhibit pressure induced phase transitions, driven by changes in the fullerene rotational state, similar to those observed in C at 0.4 and 2.5 GPa [8]. This behavior is consistent with the existence of the intradimer C—C bridge in azafullerene, which prevents such molecular reorientations. These results are consistent with those also found in XRD measurements [27]. The strongest lines of the Raman spectrum of (C N) , corresponding to the H (7), A (2) and H (8) intramolecular phonon modes, exhibit a monotonic increase with pressure (Fig. 5). The pressure coefficients (*u/*p) of these phonons for P(6 GPa are systematically smaller than those of the corresponding modes of pristine C . This can be rationalized by taking into account the lower
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compressibility of (C N) [29,30] compared to that of pristine C . However, when the pressure reaches the region 6.0$0.5 GPa, a reversible change in their pressure response occurs, without any frequency discontinuity or any residual softening after pressure release. Their pressure coefficients above that pressure region are drastically reduced. The effect is most pronounced for the H (7) mode, whose pressure coefficient is reduced by more than 50%. The critical pressure regime, 6.0$0.5 GPa, is in excellent agreement with structural results obtained by synchrotron X-ray diffraction measurements. These have shown that the (c/a) ratio in azafullerene solid increases monotonically with increasing pressure until, at &6.5 GPa, it reaches the ideal hcp (c/a) value, and then remains unchanged to much higher pressures (20 GPa) [26,29]. The observed behavior was rationalised by noting that, at this pressure (&6.5 GPa), the interdimer centreto-centre distance is compressed to &9.3—9.4 As , essentially identical to the corresponding intradimer distance. Similar is the behaviour of the u(3) mode in this pressure region. Therefore, the Raman data are also sensitive to these structural “modifications” of (C N) in the same pressure region. The A (1) breathing mode, which is expected to be the most sensitive to the formation of the intramolecular C—C bridge, exhibits two reversible changes with pressure, one at &3.0 and one at &6 GPa (Fig. 6). A change at &3 GPa has been also observed in the XRD-measurements for the (1 0 1 1) reflection [29]. This behavior was interpreted as reflecting the significant difference between the compressibility of the molecular units and that of the bulk solid. The pressure dependence of the A (1) mode supports this interpretation. For pressure P(3 GPa, the breathing mode hardens at a slower rate than that for C as the decrease in volume is mainly absorbed by the weak van der Waals forces between the dimeric units and has little effect on the intradimer C—C bridge. For 3( P(6 GPa, the A (1) mode hardens faster, while for P'6 GPa, its frequency remains practically unchanged. For pressures above 3 GPa, the change of the intradimer bridge becomes more pronounced leading to the faster hardening of A (1) mode in this pressure region. In the end, for pressures higher than 6 GPa, where the structure of the material
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Fig. 6. The pressure dependence of the A (1), H (3) and H (4), intramolecular Raman modes of (C N) , showing in detail the characteristic behavior of A (1) with pressure. Shaded areas near &6.0 and &3 GPa denote the change in the slope of its pressure dependence. The open (solid) symbols denote data taken for increasing (decreasing) pressure runs.
approaches the ideal HCP one, its lower compressibility results in the frequency invariance of this mode. The H (3) and H (4) intramolecular modes of (C N) (Fig. 6) soften with increasing pressure almost linearly in the whole pressure region investigated, as in the case of pristine C but with smaller pressure coefficients. By comparing the pressure dependence of the modes observed in the C *TMTSF complex and in the (C N) to that of the corresponding modes in C , we find that both materials show the same signs in the frequency shifts (*u/*p). This indicates that the structure of the material does not play a significant role in the pressure response of the intramolecular modes with its role limited to modifying the magnitude of (*u/*p). Charge transfer comes into play only after a sufficient volume reduction and its strongest effect is to produce
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irreversible softening in most of the Raman modes. Dimerization, on the other hand, mainly leads to two effects observed in the Raman spectra. First, it induces a red shift to the frequencies of certain modes and second, it separates the modes into two categories: those, which exhibit pressure coefficient changes, and those, which do not. We suggest that the modes showing pressure coefficient changes are those either having their eigenvectors strongly directed along the intradimer bridge, e.g. A (1), or having large components in that direction, e.g. A (2), H (1), H (7), H (8). Acknowledgements The authors thank Dr. O. Zharikov, Dr. I. Kremenskaya, Dr. N. Spitsina, Dr. E. Yagubskii, Dr. F. Wudl, and Prof. K. Prassides for providing the samples of C and its derivatives used in this work. Financial support from the Secretariat for Research and Development, Greece, and NATO HTECH. CRG C 972317 is kindly acknowledged. References [1] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, W.G. Golden, H. Seki, C.A. Brown, M.S. de Vries, Chem. Phys. Lett. 179 (1991) 181. [2] P.H.M. van Loosdrecht, P.J.M. van Bentum, G. Meijer, Phys. Rev. Lett. 68 (1992) 1176. [3] P.H.M. van Loosdrecht, P.J.M. van Bentum, M.A. Verheijen, G. Meijer, Chem. Phys. Lett. 198 (1992) 87. [4] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X-Xiu Bi, P.C. Eklund, D.S. Cornett, W.A. Duncan, I.J. Amster, Science 259 (1993) 955. [5] D.W. Snoke, Y.S. Raptis, K. Syassen, Phys. Rev. B 45 (1992) 14419. [6] N. Chandrabhas, M.N. Shashikala, D.V.S. Muthy, A.K. Sood, C.N.R. Rao, Chem. Phys. Lett. 197 (1992) 319. [7] K.P. Meletov, D. Christofilos, G.A. Kourouklis, S. Ves, Chem. Phys. Lett. 236 (1995) 265. [8] K.P. Meletov, D. Christofilos, S. Ves, G.A. Kourouklis, Phys. Rev. B 52 (1995) 10090. [9] K.P. Meletov, V.K. Dolganov, N.G. Spitsina, E.B. Yagubskii, J. Arvanitidis, Papagelis, S. Ves, G.A. Kourouklis, Chem. Phys. Lett. 281 (1998) 360.
[10] K.P. Meletov, E. Liarokapis, J. Arvanitidis, K. Papagelis, D. Palles, G.A. Kourouklis, S. Ves, Chem. Phys. Lett. 290 (1998) 125. [11] P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley, A.B. Smith, D.E. Cox, Phys. Rev. Lett. 66 (1991) 2911. [12] G.A. Samara, J.E. Schirber, B. Morosin, L.V. Hansen, D. Loy, D. Sylwester, Phys. Rev. Lett. 67 (1991) 3136. [13] K.-A. Wang, Y. Wang, P. Zhou, J.M. Holden, S.-L. Ren, G.T. Hager, H.F. Ni, P.C. Eklund, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 45 (1992) 1955. [14] J.D. Axe, S.C. Moss, D.A. Neumann, in: H. Ehrenreich, F. Spaepen (Eds.), Solid State Physics, Vol. 48, Academic Press, New york, 1994, p. 171. [15] W.I.F. David, R.M. Ibberson, J.C. Matthewman, K. Prassides, T.J.S. Dennis, J.P. Hare, H.W. Kroto, R. Taylor, D.R.M. Walton, Nature 353 (1991) 147. [16] A.P. Jephcoat, J.A. Hriljac, L.W. Finger, D.E. Cox, Europhys. Lett. 25 (1994) 429. [17] W.I.F. David, R.M. Iberson, T.J.S. Dennis, J.R. Hare, K. Prassides, Europhys. Lett. 18 (1992) 219. [18] S.V. Konovalikhin, O.A. D’yachenko, G.V. Shilov, N.G. Spitsina, K.V. Van, E.B. Yagubskii, Russ. Chem. Bull. 46 (1997) 1415. [19] M. Meneghetti, R. Bozio, I. Zanon, C. Pecile, C. Ricotta, M. Zanetti, J. Chem. Phys. 80 (1984) 6210. [20] Z.-H. Dong, P. Zhou, J.M. Holden, P.C. Eklund, M.S. Dresselhaus, G. Dresselhaus, Phys. Rev. B 48 (1993) 2862. [21] J. Winter, H. Kuzmany, Solid State Commun. 84 (1992) 935. [22] M. Hanfland, K. Reimann, K. Syassen, A. Brillante, A. Girlando, Syn. Met. 27 (1988) B549. [23] J.C. Hummelen, B. Knight, J. Pavlovich, R. Gonzalez, F. Wudl, Science 269 (1995) 1554. [24] W. Andreoni, A. Curioni, K. Holczer, K. Prassides, M. Keshavarz-K, J.C. Hummelen, F. Wudl, J. Am. Chem. Soc. 118 (1996) 11335. [25] C.M. Brown, L. Cristofolini, K. Kordatos, K. Prassides, C. Bellavia, R. Gonzalez, M. Keshavarz-K, F. Wudl, A.K. Cheetham, J.P. Zhang, W. Andreoni, A. Curioni, A.N. Fitch, P. Pattison, Chem. Mater. 8 (1996) 2548. [26] K. Prassides, F. Wudl, W. Andreoni, Full. Sci. Technol. 5 (1997) 801. [27] T. Pichler, M. Knupfer, M.S. Golden, S. Haffner, R. Friedlein, J. Fink, W. Andreoni, A. Curioni, M. Keshavarz-K, C. Bellavia-Lund, A. Sastre, J.C. Hummelen, F. Wudl, Phys. Rev. Lett. 78 (1997) 4249. [28] D. Bormann, J.L. Sauvajol, G. Goze, F. Rachdi, A. Moreac, A. Girard, L. Forro, O. Chauvert, Phys. Rev. B 52 (1996) 14139. [29] C.M. Brown, E. Beer, C. Bellavia, L. Cristofolini, R. Gonzalez, M. Hanfland, D. Hausermann, M. Keshavarz-K, K. Kordatos, K. Prassides, F. Wudl, J. Am. Chem. Soc. 118 (1996) 8715. [30] S.Jd. Duclos, K. Brister, R.C. Haddon, A.R. Kortan, F.A. Thiel, Nature 351 (1991) 380.
High pressure optical properties of fullerene and fullerene derivatives G.A. Kourouklis *, S. Ves, K.P. Meletov Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Physics Department, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Institute of the Solid State Physics RAS, Chernogolovka, Moscow region 142432, Russia
Abstract High-pressure Raman spectroscopy is a useful tool to investigate various pressure-induced effects on fullerene and its chemical derivatives. In the case of pristine C the pressure-induced freezing of molecular rotations causes the softening, while the lowering of symmetry causes the splitting of several phonon modes. In the neutral state donor—acceptor molecular complex C *TMTSF*2(CS ) high pressure leads to an irreversible charge-transfer phase transition, asso ciated with the transfer of one electron from TMTSF donor to C acceptor molecule. In the case of azafullerene (C N) , the pressure dependencies of the phonon frequencies exhibit reversible changes in their slopes, associated with the attainment of the ideal HCP structure (originally c/a"1.623), as well as with the starting of the intradimer bridge shortening. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fullerene and fullerene derivatives; Raman spectroscopy; High pressure
1. Introduction The interest in molecular dynamics and phonon spectra of fullerenes and their chemical derivatives is connected with the unique molecular structure of C and the versatility of its phonon spectrum. Various perturbations in the structure of the C cage, caused by external disturbances like pres sure and temperature application, chemical bond formation, etc., are manifested in the phonon spec-
* Corresponding author. Tel.: #30-31995947; fax: #3031995928; e-mail: [email protected].
trum. Thus, Raman scattering has become a very powerful and useful tool for the investigation of the physical properties, phase transitions and material transformations [1—5] for the huge family of fullerene-based materials. In particular, Raman scattering coupled with the application of external pressure has yielded a wealth of information on phase transitions, pressure-induced chemical transformations and charge transfer transitions [6—9]. In this paper we present a summary of the experimental results obtained on pressure-induced effects on pristine fullerene and some fullerene derivatives studied by means of Raman scattering, giving important information on their structural, physical and chemical properties.
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 7 6 - 3
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In particular, for fullerite we have determined the pressure dependence of the intramolecular phonon modes [7,8] and we have shown that changes, observed in the Raman spectra, are related to phase transitions at 0.4 and 2.5 GPa. These transitions are associated with the pressure-induced freezing of molecular rotations, manifested by the softening and splitting of several phonon modes as well as changes in their pressure coefficients. In the Raman spectra of the molecular donor—acceptor complex of fullerene, C *C H Se *2(CS ) [C H Se , tetramethyl-tetra-selenafulvalene (TMTSF)], the pressure dependence of the frequencies, of almost all intramolecular phonon modes, exhibits irreversible changes at 5.0$0.5 GPa [9]. These changes include splitting and softening of various modes, the more characteristic of them being the irreversible softening of the A (2) pentagon pinch (PP) mode by 9 cm\, observed upon total release of pressure. The residual softening of this mode is almost the same as in the case of the potassiumdoped fullerene KC . These observations may be attributed to a pressure-induced irreversible phase transition, associated with the transfer of one electron from TMTSF donor to C acceptor molecule. In the case of azafullerene (C N) , a fullerene de rivative containing a hetero-atom in the cage of C , the pressure dependencies of the H (7), A (2), and H (8) phonon frequencies exhibit reversible changes in their pressure coefficients in the region 6.0$0.5 GPa. In addition, the A (1) breathing mode shows two clear changes in the dependence of its pressure coefficient, one at &3 and another at &6 GPa. The changes at &6 GPa can be attributed to the fact that (C N) attains the ideal HCP structure (c/a"1.633) in this pressure region (originally c/a"1.623), while the changes of A (1) mode at 3 GPa might imply the starting of the intradimer bridge shortening.
2. Pressure-induced phase transitions in pristine C60 probed by Raman scattering The Raman spectra of C and its derivatives were measured in two frequency regions from 200 to 800 cm\ and from 1350 to 1700 cm\, in the intermediate region the Raman mode of diamond
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appears (1333.4 cm\) making it difficult to record spectra. The vibrational spectrum of a free C mol ecule contains 46 modes, with symmetries: C"2A #3F #4F #6G #8H #A #4F #5F #6G #7H , and with allowance for the high symmetry (I ) and degeneracy the total number of modes becomes 174 [1,3], from these only the two A and eight H are Raman active modes. At ambient pressure, the Raman spectrum of pristine C contains eight intramolecular modes: H (1)"272 cm\, H (2)" 435 cm\, A (1)"496 cm\, H (3)"710 cm\, H (4)"772 cm\, H (7)"1422 cm\, A (2)" 1467 cm\ and H (8)"1570 cm\. The most in tense A (2) and A (1) modes of the spectrum corres pond to the out-of-phase and in-phase dilatory vibration of the pentagonal and hexagonal rings of the molecule, respectively. The frequencies of the observed phonon modes at ambient pressure are close to the previously reported ones [1,3]. It is important to note that the Raman measurements for all materials were performed with special care in setting the laser power at such a level in order to get the maximum value (&1467 cm\ at normal pressure) for the frequency of the A (2) PP-mode. For higher laser power, a decrease in the frequency of the A (2) mode, up to 4 cm\, was observed. The laser power induced reversible softening of the A (2) mode is related to the local overheating of the sample in the laser spot as experimental and theoretical investigations have shown [10]. Whenever the laser power density was higher than some critical value this decrease of the A (2) mode frequency for pristine C became irreversible and was ac companied by the damage on the crystal surface. This phenomenon is associated with the photodimerization of the C molecules in the FCC phase of pristine C [4]. The laser power density was kept at a minimum level for all measurements to exclude the softening of PP-mode, both at normal and high pressure. The behavior of the phonon modes observed in the low and middle frequency region, namely, H (1), H (2), A (1), H (3) and H (4) as well as the nine new modes, u(1)—u(9), appeared in the partially ordered simple cubic phase is shown in Fig. 1. In Fig. 2a, the
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pressure dependence of H (7), A (2), H (8) and a new mode, marked u(10), is shown by open and solid symbols for increasing and decreasing pressure, respectively. The experimental data for A (2) and H (8) modes were fitted linearly in the pressure region above 0.4 GPa, giving slopes 5.5 cm\/GPa and 4.8 cm\/GPa, respectively. The solid lines correspond to this fit. The H (7) mode shows also a linear pressure dependence, but its slope changes drastically at a pressure of 2.5 GPa. Below this pressure, the slope is 9.3 cm\/GPa, while above the slope decreases by more than a factor of 2 and becomes 4.1 cm\/GPa. The pressure coefficients of the phonon frequencies change at pressures 0.4 and 2.5 GPa. In the region 0—0.4 GPa, several of the observed phonon modes exhibit negative frequency shifts amounting to 10 cm\ for H (1) and A (2) down to 5 cm\ for H (2), A (1) and H (3) modes. For some of them this softening is shown in Fig. 2(b), in which it is clearly shown that the same pressure-induced softening behavior of the A (2), is also exhibited by several other modes. Above 0.4 GPa the pressure-induced shift of the phonon modes becomes positive except for the H (3), H (4), and two new modes, which are clustered in the
Fig. 1. Pressure dependence of the intramolecular mode frequency of H (2), A (1), H (3), H (4) and for u(1)—u(9) modes for the pristine C . The modes u(1)—u(9) appear in the SC phase above 0.4 GPa. Data are taken in different experimental runs for increasing pressure (open symbols) and decreasing pressure (solid symbols) cycles. The lines are linear least-square fits to the experimental data. Vertical lines indicate the transition pressures 0.4 and 2.5 GPa.
Fig. 2. (a) Pressure dependence of the intramolecular mode frequency of H (7), A (2), H (8) and u(10) for the pristine C . The mode u(10) appears in the SC phase above 0.4 GPa. The lines are linear least-square fits to the experimental data. Vertical lines indicate the transition pressures 0.4 GPa and 2.5 GPa. (b) Pressure dependence of the mode frequency of H (8), A (2), A (1) and H (1) of pristine C showing in more detail the pressure-induced softening in the 0—0.4 GPa pressure region. Data are taken in different experimental runs for increasing pressure (open symbols) and decreasing pressure (solid symbols) cycles.
700—800 cm\ region, exhibiting a softening up to the highest pressure reached in our experiments. It must be emphasized that all the pressure induced changes are reversible with pressure. The changes in the pressure dependence of the Raman modes of pristine C at 0.4 GPa and the appearance of new modes above this pressure is connected with the orientational ordering phase transition from the FCC to the SC crystal structure. The first Raman indication of this transition was observed by Chandrabhas et al. [6] as a softening of the A (2) PP-mode at pressures up to 0.6 GPa. At normal pressure this transition takes place at 249 K [11] and the transition temperature increases with pressure at a rate of 10.4 K/kbar [12]. The appearance of the additional Raman modes in the low temperature SC phase of C was reported for the first time by van Loosdrecht et al. [2]. We have observed all typical features of the lowtemperature SC phase in the pressure-induced orientationally ordered SC phase at room temperature. It is known that the photodimerization of
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C under intense UV illumination, results in sim ilar changes to the Raman spectra, namely, the softening of A (2) PP-mode and the appearance of new modes in the spectrum [4]. The fundamental difference between the pressure-induced and photodimerization effects, is the reversibility of all pressure-induced effects. Therefore, the observed pressure behavior of the Raman spectra near 0.4 GPa is due to the orientational ordering phase transition. An interesting result, in our opinion, is the softening of almost all intramolecular modes of C in the pressure region below the orientational ordering phase transition. The softening of the intramolecular modes of C has also been observed in the intercalated C compounds A C , A"K, Rb, Cs [13]. In this case the Raman active modes are insensitive to the A atom, and therefore to the lattice expansion caused by doping with alkali metal atoms with larger ionic radii. This softening has been attributed [13] to the elongation of the intramolecular bond lengths induced by charge transfer and the resulting softening of the force constants. Charge transfer may also cause the pressure-induced softening of the intramolecular modes in pristine C . The orientational ordering phase transition leads to the freezing of random rotations of the C molecules. In the SC phase the molecules are oriented in one of the two equivalent standard orientations, in which the two-fold molecular axes are aligned along the crystallographic axes. The simple cubic structure requires that each of the four molecules in the unit cell rotates about different [1 1 1] direction preserving the threefold axes along the [1 1 1] direction required by the cubic symmetry [14]. The neutron powder diffraction patterns showed that the principal rotation angle was about 98° [15]. This angle is such that the six electron-poor centers of the pentagonal faces of a given molecule face the short, electron-rich bonds of the neighboring molecules. This, precisely, may lead to the elongation of the intramolecular bond lengths and the resulting softening of the force constants. The pressure dependence of the observed phonon modes exhibits drastic changes in the pressure coefficients of a number of modes at 2.5 GPa. X-ray powder diffraction study [16] of C under
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Fig. 3. Phase diagram of fullerite.
pressure showed that, at room temperature, the rotation free orientationally ordered SC phase occurs at pressures higher than 2.5 GPa. The X-ray results obtained for 0.55 GPa and 1.3 GPa indicate the presence of a reduced degree of orientational order reflecting the rotation of molecules along [1 1 1] directions. At ambient pressure this transition occurs near 85 K [17]. The change near 2.5 GPa is the manifestation of this phase transition at room temperature. All these observations could lead to the construction of a phase diagram for fullerite as is shown in Fig. 3.
3. Pressure-induced charge transfer and chemical modifications in fullerene complexes The interest in the pressure behavior of the neutral state donor—acceptor molecular complexes of C is associated with the pressure-induced enhance ment of the donor—acceptor interaction resulting in charge transfer between donor and acceptor molecules. The charge reorganization changes the bond lengths and, therefore, is reflected in the
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phonon spectrum. As an example of a neutral state molecular complex of C , we have studied the C *C H Se *2(CS ) [C H Se , tetramethyl tetra-selenafulvalene (TMTSF)] compound. The crystal structure of this compound, determined by X-ray diffraction, is monoclinic with lattice parameters: a"1.5407, b"1.2934, c"1.2026 nm, b"108.39°, »"2.2741 nm, space group C ,
Z"2, o "1.929 g/cm, R"0.047 [18]. The conformation of the C molecule and the distances between them are not changed significantly in comparison to the pristine C crystals. The conforma tion of TMTSF molecule is significantly altered and becomes non-planar [18]. The Raman spectrum of C *TMTSF*2(CS ) complex, in the frequency region from 100 to 1700 cm\, contains the eight principal molecular modes of C with practically the same frequencies as in the case of pristine C [1,2,8]. In addition to these bands, there are some very weak Raman peaks, whose origin may be of a second-order scattering from the C molecular vibrations, since the observed frequencies do not resemble those observed in TMTSF powder or in TMTSF\ in solution [19]. The pressure dependence of the H (3), H (4) H (7), A (2) and H (8) phonon frequencies is shown in Fig. 4. The additional weak Raman peak observed at x"755 cm\, at normal pressure, is related with two-phonon scattering from the combination of [F (1)#G (1)] modes [20]. All modes exhibit a positive response (&5.2 cm\/GPa) to pressure except H (3) and the combination mode, which exhibit a negative response in the whole pressure range investigated. The pressure dependence of both modes is practically the same with values of the pressure coefficients !1.4 cm\/GPa for increasing and !0.9 cm\/GPa for releasing pressure. When pressure increases, in the region of 5.0$0.5 GPa, the A (2) PP-mode softens, whereas the H (7) mode hardens considerably. Upon pres sure release no such peculiarities in this pressure region, or in the rest pressure range is observed. All these changes are irreversible. The pressure coefficients of the dependencies for A (2) and H (7) modes are practically the same below and above 5.0 GPa, both for increasing and releasing pressure, and are equal to 5.2 cm\/GPa. The value of
Fig. 4. Pressure dependence of H (7), A (2), H (8), H (3), H (4) and the two-phonon [F (1)#G (1)] intramolecular phonon frequencies of the C *TMTSF*2(CS ) molecular complex. Open (solid) symbols correspond to data taken at three pressure runs for increasing (decreasing) pressure. Solid lines (dashed lines) are guides to the eye for increasing (decreasing) pressure. The shaded area shows the pressure region of the phase transition.
the residual softening of the PP-mode, after total release of pressure, is &9 cm\. The residual hardening of the H (7) mode is of the same value, whereas the softening of the H (8) mode is approx imately two times smaller. The pressure dependence of the H (3), H (4) and the combination mode frequencies shows the same irreversible changes in the pressure region 5.0$0.5 GPa. Their pressure dependencies for increasing and releasing pressure differ considerably at pressures below 5.0 GPa (above 5.0 GPa these dependencies are the same). In addition, the H (4) splits in two modes upon releasing pressure below 5.0 GPa. All three modes, upon total release of pressure, exhibit a residual softening equal to 9 cm\ for the H (3) and the two-phonon combination [F u(1)#G (1)] modes, while for the two components resulting from the
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splitting of the H (4) mode, this residual softening is equal to 18 and 9 cm\, respectively. From these results it is obvious that in the transition pressure range, 5.0$0.5 GPa (shaded area in Fig. 5), take place changes in the pressure dependence of phonon modes and these effects are irreversible. The pressure dependence of the intramolecular phonon frequencies of the C *TMTSF*2(CS ) complex in the pressure region 0—3.0 GPa do not show any reversible changes, associated with orientational ordering of C molecular rotations as in pristine C . This is compatible with the X-ray scattering data, indicating that the C molecules in the C *TMTSF*2(CS ) complex are orienta tionally ordered at room temperature and normal pressure [18]. The most interesting features of the pressure dependence of the phonon modes of C *TMTSF*2(CS ) are the prominent changes at 5.0$0.5 GPa, associated with the softening of all phonon modes and the splitting of some of them. These changes are irreversible and the residual softening, observed in the recovered material upon total release of pressure, vary from 5 up to 18 cm\ for the various modes. The softening of the A (2) PP-mode, observed under various degrees of illumination, is reversible under conditions of low illumination and has been associated with the sample overheating [10]. We have always checked the laser power density in our experiments, making sure that no overheating effects were observed. It must be emphasized that the photodimerization of C molecules within the C *TMTSF*2(CS ) complex and consequently the irreversible softening of the PP-mode is not possible because of the total orientational order of the C molecules in the structure of the complex at normal conditions [18]. The softening of the intramolecular modes of C has been observed in the intercalated C compounds. This softening has been attributed to the elongation of the intramolecular bond lengths induced by charge transfer from alkali metal atoms to the C molecule and the resulting softening of the force constants [13]. Winter and Kuzmany [21] have studied the softening of the PP-mode for the K C series of comV pounds in the region 0)x)6. The main result of their study is that the PP-mode softens almost linearly with increasing x, following the charge
219
transfer from K to C molecule. They have found that in the case of the KC compound the soften ing of PP-mode is equal to &9 cm\. The pressure-induced softening of phonon modes in the C *TMTSF*2(CS ) molecular com plex could be associated with the charge transfer between the TMTSF donor and C acceptor mol ecules. At normal pressure the van der Waals-type interaction between donor and acceptor molecules is too weak so they form the neutral state molecular complex with weak charge transfer. The gradual reduction of the intermolecular donor—acceptor distances and the enhancement of the interaction between them at higher pressures, results in the charge-transfer phase transition, taking place in the region 5.0$0.5 GPa. During the phase transition of one electron from TMTSF is transfered to the C molecule forming a new anion state C\ charge-transfer complex. As a result of this, the transformation of the material becomes irreversible and the PP-mode exhibits a residual softening by 9 cm\, observed upon total release of pressure, in complete analogy to the case of KC [21]. Similar studies [22] have shown that pressure can induced neutral to ionic state phase transition in the case of tetrathiofulvalene—haloquinone mixed stack charge transfer crystal.
4. Dimerization effects on the phonon spectrum-Azafullerene (C59N)2 As a result of the trivalency of nitrogen, compared to the tetravalency of carbon, nitrogen substitution of a carbon atom in the C cage leads to the azafullerene radical, C N* — isoelectronic with the C\ anion — which has been found to rapidly dimerize, yielding the (C N) dimer [23]. Thus, solid (C N) consists of dimerized molecular units linked by C—C bonds between carbon atoms adjacent to the N atom (the C—N bond is between two six-membered rings) at each monomer [23—25]. Conductivity measurements have confirmed that the material is an insulator, while the EPR spectra consist of only one sharp narrow line, attributed to a low concentration of defect spins [26]. Electron energyloss and photoemission spectroscopy measurements revealed little mixing between the
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N- and C-electronic states with strong localization of the excess electron at the N atom [27]. At ambient pressure, the Raman spectrum of (C N) contains the same eight modes H (1), H (2), A (1), H (3), H (4), H (7), A (2) and H (8), as that of pristine C . Three additional modes, u(1), u(2) and u(3), observed at 344, 395 and 730 cm\, respectively, were observed in pristine C at pres sures higher than 0.4 GPa, where the free rotations of the C molecules are partially frozen [8]. The behavior and appearance, in various fullerene derivatives, of the mode at 344 cm\ poses an important problem as to the origin of this mode. Its presence has been associated with the polymeric phase in AC [28]. The presence of u(1)—u(3) in (C N) is consistent with the freezing of free rota tion caused by dimerization. The frequencies of the observed phonon modes in the Raman spectra of
Fig. 5. The pressure dependence of the A (1), H (3), x(3), H (4), H (7), A (2) and H (8), intramolecular Raman modes of (C N) . The open (solid) symbols denote data taken for increasing (decreasing) pressure runs. Shaded areas near &6.0, &3 and &5 GPa denote the change in the slopes of the pressure dependencies of different modes.
(C N) are close to the corresponding ones in pristine C [1,2,8]. The observed differences do not exceed the spectral resolution of the apparatus (&3 cm\), except for the H (1), A (2) and u(2) modes, where the corresponding differences are slightly larger, in the range of 6—8 cm\. It is worth pointing out that the frequency of the A (2) mode, at ambient pressure, is&1460 cm\, exactly the same as for the photodimerized C [4]. Therefore, dimerization results in the softening of the A (2) mode in C and its derivatives. Another interest ing observation in the Raman spectrum is the broad (width &40 cm\) asymmetric feature at 265 cm\. In photodimerized C , the H (1) intra molecular mode appears to be split into two components with frequencies 258 and 271 cm\ [4]. Thus we assume that this feature in the spectrum of azafullerene has the same origin. The above observations are clear manifestations of the dimeric character of azafullerene in its Raman spectra. The pressure dependence of the observed phonon mode frequencies is shown in Fig. 5 for the high and middle frequency regions. The open (solid) symbols denote data taken for increasing (decreasing) pressure measurements. All modes, except H (3) and H (4), exhibit a positive response to pres sure, in analogy to pristine C . Note that the u(3) mode exhibits positive pressure coefficient for P(5 GPa, while in fullerite its slope has opposite sign. The shaded areas denote the pressure regions where changes of the pressure coefficients occur. The data obtained show that (C N) does not exhibit pressure induced phase transitions, driven by changes in the fullerene rotational state, similar to those observed in C at 0.4 and 2.5 GPa [8]. This behavior is consistent with the existence of the intradimer C—C bridge in azafullerene, which prevents such molecular reorientations. These results are consistent with those also found in XRD measurements [27]. The strongest lines of the Raman spectrum of (C N) , corresponding to the H (7), A (2) and H (8) intramolecular phonon modes, exhibit a monotonic increase with pressure (Fig. 5). The pressure coefficients (*u/*p) of these phonons for P(6 GPa are systematically smaller than those of the corresponding modes of pristine C . This can be rationalized by taking into account the lower
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compressibility of (C N) [29,30] compared to that of pristine C . However, when the pressure reaches the region 6.0$0.5 GPa, a reversible change in their pressure response occurs, without any frequency discontinuity or any residual softening after pressure release. Their pressure coefficients above that pressure region are drastically reduced. The effect is most pronounced for the H (7) mode, whose pressure coefficient is reduced by more than 50%. The critical pressure regime, 6.0$0.5 GPa, is in excellent agreement with structural results obtained by synchrotron X-ray diffraction measurements. These have shown that the (c/a) ratio in azafullerene solid increases monotonically with increasing pressure until, at &6.5 GPa, it reaches the ideal hcp (c/a) value, and then remains unchanged to much higher pressures (20 GPa) [26,29]. The observed behavior was rationalised by noting that, at this pressure (&6.5 GPa), the interdimer centreto-centre distance is compressed to &9.3—9.4 As , essentially identical to the corresponding intradimer distance. Similar is the behaviour of the u(3) mode in this pressure region. Therefore, the Raman data are also sensitive to these structural “modifications” of (C N) in the same pressure region. The A (1) breathing mode, which is expected to be the most sensitive to the formation of the intramolecular C—C bridge, exhibits two reversible changes with pressure, one at &3.0 and one at &6 GPa (Fig. 6). A change at &3 GPa has been also observed in the XRD-measurements for the (1 0 1 1) reflection [29]. This behavior was interpreted as reflecting the significant difference between the compressibility of the molecular units and that of the bulk solid. The pressure dependence of the A (1) mode supports this interpretation. For pressure P(3 GPa, the breathing mode hardens at a slower rate than that for C as the decrease in volume is mainly absorbed by the weak van der Waals forces between the dimeric units and has little effect on the intradimer C—C bridge. For 3( P(6 GPa, the A (1) mode hardens faster, while for P'6 GPa, its frequency remains practically unchanged. For pressures above 3 GPa, the change of the intradimer bridge becomes more pronounced leading to the faster hardening of A (1) mode in this pressure region. In the end, for pressures higher than 6 GPa, where the structure of the material
221
Fig. 6. The pressure dependence of the A (1), H (3) and H (4), intramolecular Raman modes of (C N) , showing in detail the characteristic behavior of A (1) with pressure. Shaded areas near &6.0 and &3 GPa denote the change in the slope of its pressure dependence. The open (solid) symbols denote data taken for increasing (decreasing) pressure runs.
approaches the ideal HCP one, its lower compressibility results in the frequency invariance of this mode. The H (3) and H (4) intramolecular modes of (C N) (Fig. 6) soften with increasing pressure almost linearly in the whole pressure region investigated, as in the case of pristine C but with smaller pressure coefficients. By comparing the pressure dependence of the modes observed in the C *TMTSF complex and in the (C N) to that of the corresponding modes in C , we find that both materials show the same signs in the frequency shifts (*u/*p). This indicates that the structure of the material does not play a significant role in the pressure response of the intramolecular modes with its role limited to modifying the magnitude of (*u/*p). Charge transfer comes into play only after a sufficient volume reduction and its strongest effect is to produce
222
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irreversible softening in most of the Raman modes. Dimerization, on the other hand, mainly leads to two effects observed in the Raman spectra. First, it induces a red shift to the frequencies of certain modes and second, it separates the modes into two categories: those, which exhibit pressure coefficient changes, and those, which do not. We suggest that the modes showing pressure coefficient changes are those either having their eigenvectors strongly directed along the intradimer bridge, e.g. A (1), or having large components in that direction, e.g. A (2), H (1), H (7), H (8). Acknowledgements The authors thank Dr. O. Zharikov, Dr. I. Kremenskaya, Dr. N. Spitsina, Dr. E. Yagubskii, Dr. F. Wudl, and Prof. K. Prassides for providing the samples of C and its derivatives used in this work. Financial support from the Secretariat for Research and Development, Greece, and NATO HTECH. CRG C 972317 is kindly acknowledged. References [1] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, W.G. Golden, H. Seki, C.A. Brown, M.S. de Vries, Chem. Phys. Lett. 179 (1991) 181. [2] P.H.M. van Loosdrecht, P.J.M. van Bentum, G. Meijer, Phys. Rev. Lett. 68 (1992) 1176. [3] P.H.M. van Loosdrecht, P.J.M. van Bentum, M.A. Verheijen, G. Meijer, Chem. Phys. Lett. 198 (1992) 87. [4] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X-Xiu Bi, P.C. Eklund, D.S. Cornett, W.A. Duncan, I.J. Amster, Science 259 (1993) 955. [5] D.W. Snoke, Y.S. Raptis, K. Syassen, Phys. Rev. B 45 (1992) 14419. [6] N. Chandrabhas, M.N. Shashikala, D.V.S. Muthy, A.K. Sood, C.N.R. Rao, Chem. Phys. Lett. 197 (1992) 319. [7] K.P. Meletov, D. Christofilos, G.A. Kourouklis, S. Ves, Chem. Phys. Lett. 236 (1995) 265. [8] K.P. Meletov, D. Christofilos, S. Ves, G.A. Kourouklis, Phys. Rev. B 52 (1995) 10090. [9] K.P. Meletov, V.K. Dolganov, N.G. Spitsina, E.B. Yagubskii, J. Arvanitidis, Papagelis, S. Ves, G.A. Kourouklis, Chem. Phys. Lett. 281 (1998) 360.
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