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Temperature dependence of the infrared spectra of C60: orientational transition and freezing
Volume 196, number 6
CHEMICAL PHYSICS LETTERS
28 August 1992
Temperature dependence of the infrared spectra of Cho: orientational transition and freezing V. Suresh Babu and Mohindar S. Seehra Physics Department, West Virginia University, Morgantown. WV26506. USA Received 19 May 1992; in final form 12 June 1992
The temperature dependence (8-350 K) of the position and intensity of the IR modes of Cso near 526, 576, 1182 and 1430 cm-’ is reported. Data indicate a phase transition T,, near 260 K and support the model of a glassy state below z 100 K predicted recently by molecular dynamic calculations. The first-order nature of To is smeared by impurities in the sample.
1. Introduction During the past year, several theoretical and experimental investigations of the orientational ordering transition near T,-,x 260 K in CbOhave been reported [ l-9 1. These include studies by X-ray and neutron diffraction [ 1,2], nuclear magnetic resonance (NMR) [ 3,4], differential scanning calorimetry [ 11, thermal conductivity [ 5 ] and specific heat measurements [ 6 1. At room temperature ChO has fee structure with lattice constant a0 x 14.2 A and it has a high degree of orientational disorder. On cooling through To, Ceo undergoes a phase transition to simple cubic structure as a consequence of orientational ordering of the Ceo molecules. Besides To, a glassy transition (orientational freezing) has been predicted from theoretical considerations [ 7-91. In support of this prediction, it has been shown that below To, orientational dynamics changes from continuous rotational diffusion to a jumping motion between discrete orientations [4] so that some degree of orientation disorder is present at all temperatures and below 85 K, the probability of misoriented molecules is frozen at x 17W [ 51 leading to a glassy behavior. Infrared (IR) spectroscopy has been widely used to identify fullerenes in the soot obtained by arcing Correspondence to: MS. Seehra, Physics Department, West Virginia University, Morgantown, WV 26506, USA.
of graphite [ 10 1. Pressure dependence of the IR-active modes of CGoat 1430, 1182, 576 and 526 cm-’ has been reported by Huang et al. [ 111 and a number of papers have reported calculations of the vibrational modes of Ceo [ 12-141. To the best of our knowledge, a temperature dependence of the IR-active modes has not been reported so far. In this Letter, we report a detailed temperature-dependent study (8-350 K) of the four IR modes near 1430, 1182, 576 and 526 cm-’ and identify the temperature zones where major changes in the orientational dynamics of Cso occur. These results indicate a phase transition near 260 K and support the model of orientational freezing below z 100 K.
2. Experimental procedures The sample of Cso used in these investigations was obtained from Strem Chemicals Inc. and was used as obtained. This sample contained a few percent of CT0 and may also contain some residual organic solvents. For the IR studies, a 10% pellet ( 1.5 cm diameter, 100 mg) of Cho was made by mixing with pure KBr powder in the ratio of 1: 9 by weight. The IR spectra were investigated with a Mattson Cygnus 100 FAIR spectrometer (also equipped with a MTEClOO photoacoustic detector for room temperature studies). For temperature-dependent studies a closed-cycle helium refrigerator system (Air
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
569
28 August 1992
CHEMICAL PHYSICS LETTERS
Volume 196, number 6
Products Model lR02A) was used, using KBr windows, an MCT detector and the standard transmission mode. The spectra were recorded in the scan range of 400-4000 cm-‘, under a dynamic vacuum of 10m3Torr and the temperature range of 8-350 K using an associated heater assembly. All recorded spectra were taken using 16 scans at resolution of 1 cm-‘.
3. Experimental results and discussion In fig. 1 we show the FTIR-photoacoustic spectra of Cbo at room temperature recorded on the as-obtained sample along with the transmission FIIR spectra of the C6&Br pellet at two selected temperatures. The locations of the four prominent modes are at 1429.72, 1183.02, 576.78 and 526.95 cm-’ from the photoacoustic spectra and at 1428.24,
1181.81, 575.83 and 526.36 cm-’ from the transmission FIIR spectroscopy. The pressure needed to produce the pellet may account for the slight shift in the frequencies observed above and the observed frequency-dependent shift in the background reported also by others is perhaps related to the KBr pellet structure. However this shift in the background does not seriously affect the precise locations of the peak positions which are accurately measured with the software supplied with the CygnuslOO. In fig. 2, we show the temperature dependence of the positions of the four modes near 1430,1182,576 and 526 cm-‘. Beginning near 100 K, significant shifts in the positions of these bands are observed in three cases. Clearly, the higher frequency modes experience more shift. Near 260 K, the frequency shifts Aware: Ay= 1.5 cm-’ forthe y= 1430.3 cm-’ mode;
576'5j
g
\
576
cm-'
(
1.2
is a s a
0.8
0.4
0.6
14,,.0~ 0
1400
12bo
lOcl0
660
600
WAVENUMBER Fig. 1. The transmission FNR spectra of CW at 8.5 and 300 K and the FUR-photoacoustic spectrum (PAS) at 300 K. 570
50
100
150
200
250
300
350
TEMPERATURE (K) Fig. 2. The temperature dependence of the vibrational frequencies of four prominent IR modes of Cso. The lines are drawn connecting the points as guides. The arrow at 260 K locates the transition To.
CHEMICAL PHYSICS LETTERS
Volume 196, number 6
A~z0.8 for the v= 1182.8 cm-’ mode; Av=OS for the v=576.8cm-’ modeandAv=O.l forthe v=526 cm-’ mode. These values give Av/v= 105X 10e3, 0.68x 10-3, 0.87x 10m3and 0.2x 1O-3 for the higher to the lower frequency modes. An interesting feature of these data is the variation of the rate of shift Avl AT in different temperature regions. In rig. 2, there is a clear inflection point near 260 K which coincides with the well-known T,, orientational transition in C6,,.The slopes of the data in fig. 2 except for the 526 cm-’ band will yield a peak at To whereas the temperature dependence of the vibrational frequency of the 526 cm-’ band shows a dip near To. Thus these measurements show the existence of To in CbO.Furthermore, the data in fig. 2 show that the shifts in the modes continue down to % 100 K, and below this temperature there is no further significant shift with lowering temperatures. The temperature dependence of the peak height for the four modes is shown in fig. 3. Here the temperature dependence of the 576 cm-’ mode is significantly different from those of the other three in
0.88 0
I 50
I 100
1 150
I 200
TEMPERATURE
I 250
I
1 300
I 350
I
(K)
Fig. 3. The temperature dependence of the peak value of the absorbance of the four IR modes of Cm. The lines are drawn as guides and the arrow indicates the transition To at 260 K.
28 August 1992
that for the former, the intensity of the modes decreases with lowering temperatures although in three cases, and inflection in the curves is evident near 260 K. Though the peak absorbance change of the 526 cm-l mode over the temperature range of the measurements is too small, relatively abrupt changes are observed near 250 and 100 K. For the 1183 and 1430 cm-’ modes an additional change is evident below x 100 K. This may be supportive of the glassy state discussed in recent papers [ 5,7-91. The IR data reported, above provide clear evidence that between % 100 K and T,,= 260 K, there are dynamic changes occurring in CsO.In this region, the temperature is still high enough to thermally activate the misoriented molecules into other orientations. Of course above To, molecules rotate freely and the average structure is fee. This view is supported well by molecular dynamics simulations [ 7,8 1. The glassy transition in the 100 K region was estimated from the observation that each CbOmolecule has a unique orientation, and that there are many local minima close to the ground state but separated by a large potential barrier E,, x 300 meV [ 7 1. Using the transition rate equation 7-l = v exp( - Eb/kBT) and V.Y10” Hz and Eb= 300 meV, one gets r~ 1 s at 130 K and TN 1 day at 90 K. Thus as the temperature is lowered below 130 K, the time scale becomes longer and longer as compared to the experimental time, leading to a glassy transition near 100 K. Our observations reported in figs. 2 and 3 are supportive of this description. From X-ray diffraction measurements in a well-annealed sample, Heiney et al. [ 1,2] reported a discontinuous change in the lattice constant a0 at To, suggesting that the transition at To is first order in nature. Our IR measurements reported above in the as-obtained sample show the transition to be more gradual. It is very likely that the first-order nature of the transition near To in the IR data has been smeared by the impurities in the sample. In summary, the temperature dependence of the position and intensity of the IR modes of Cho indicate a phase transition near 260 K and support the picture of a glassy state below 100 K. The first-order nature of the transition near 260 K is somewhat smeared in our measurements presumably because of impurities in the sample. 571
Volume 196, number 6
CHEMICAL PHYSICS LETTERS
References [ 1 ] P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr., A.B. Smith III and D.E. Cox, Phys. Rev. Letters 66 (1991) 249. [2] P.A. Heiney, G.B.M. Vaughan, J.E. Fisher,N. Coustel, D.E. Cox, J.R.D. Copley, D.A. Neumann, W.A. Kamitakahara, KM. Creegan, D.M. Cox, J.P. McCauley Jr. and A.B. Smith III, Phys. Rev. B 45 ( 1992) 4544. [ 3 ] C.S. Yannoni, R.D. Johnson, G. Meijer, D.S. Bethune and J.R. Salem, J. Phys. Chem. 95 ( 1991) 9. [4] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglas and A.M. Mujsce, J. Phys. Chem. 95 ( 199 1) 5 18. [ 5 ] R.C. Yu. N. Tea, M.B. Salamon, D. Lorents and R. Malhotra, Phys. Rev. Letters 68 (1992) 2050. [6] W.P. Beyemtann, M.F. Hundley, J.D. Thompson, F.N. Diederich and G. Grtiner, Phys. Rev. Letters 68 ( 1992) 2046.
572
28 August 1992
[7] J.P. Lu, X.-P. Li and R.M. Martin, Phys. Rev. Letters 68 (1992) 1050. [8] M.P. Gelfand and J.P. Lu, Phys. Rev. Letters 68 (1992) 1050. [9] A. Cheng and M.L. Klein, Phys. Rev. B 45 (1992) 1889. [ lo] W. KnZitschmer,K. Fostiropoulos and D.R. HufTman, Chem. Phys. Letters 170 ( 1990) 167. [ 111 Y. Huang, D.F.R. Gilson and I.S. Butler, J. Phys. Chem. 95 (1991) 5723. [ 121 K. Raghavachari and C.M. Rohlfing, J. Phys. Chem. 95 (1991) 5768. [ 13 ] S.J. Cyvin, E. Brendsdal, B.N. Cyvin and J. Brunvoll, Chem. Phys. Letters 143 (1988) 377. [ 1412. Slanina, J.M. Rudzinski, M. Togasi and E. Gsawa, J. Mol. Struct. THEOCHEM 202 (1989) 169.
CHEMICAL PHYSICS LETTERS
28 August 1992
Temperature dependence of the infrared spectra of Cho: orientational transition and freezing V. Suresh Babu and Mohindar S. Seehra Physics Department, West Virginia University, Morgantown. WV26506. USA Received 19 May 1992; in final form 12 June 1992
The temperature dependence (8-350 K) of the position and intensity of the IR modes of Cso near 526, 576, 1182 and 1430 cm-’ is reported. Data indicate a phase transition T,, near 260 K and support the model of a glassy state below z 100 K predicted recently by molecular dynamic calculations. The first-order nature of To is smeared by impurities in the sample.
1. Introduction During the past year, several theoretical and experimental investigations of the orientational ordering transition near T,-,x 260 K in CbOhave been reported [ l-9 1. These include studies by X-ray and neutron diffraction [ 1,2], nuclear magnetic resonance (NMR) [ 3,4], differential scanning calorimetry [ 11, thermal conductivity [ 5 ] and specific heat measurements [ 6 1. At room temperature ChO has fee structure with lattice constant a0 x 14.2 A and it has a high degree of orientational disorder. On cooling through To, Ceo undergoes a phase transition to simple cubic structure as a consequence of orientational ordering of the Ceo molecules. Besides To, a glassy transition (orientational freezing) has been predicted from theoretical considerations [ 7-91. In support of this prediction, it has been shown that below To, orientational dynamics changes from continuous rotational diffusion to a jumping motion between discrete orientations [4] so that some degree of orientation disorder is present at all temperatures and below 85 K, the probability of misoriented molecules is frozen at x 17W [ 51 leading to a glassy behavior. Infrared (IR) spectroscopy has been widely used to identify fullerenes in the soot obtained by arcing Correspondence to: MS. Seehra, Physics Department, West Virginia University, Morgantown, WV 26506, USA.
of graphite [ 10 1. Pressure dependence of the IR-active modes of CGoat 1430, 1182, 576 and 526 cm-’ has been reported by Huang et al. [ 111 and a number of papers have reported calculations of the vibrational modes of Ceo [ 12-141. To the best of our knowledge, a temperature dependence of the IR-active modes has not been reported so far. In this Letter, we report a detailed temperature-dependent study (8-350 K) of the four IR modes near 1430, 1182, 576 and 526 cm-’ and identify the temperature zones where major changes in the orientational dynamics of Cso occur. These results indicate a phase transition near 260 K and support the model of orientational freezing below z 100 K.
2. Experimental procedures The sample of Cso used in these investigations was obtained from Strem Chemicals Inc. and was used as obtained. This sample contained a few percent of CT0 and may also contain some residual organic solvents. For the IR studies, a 10% pellet ( 1.5 cm diameter, 100 mg) of Cho was made by mixing with pure KBr powder in the ratio of 1: 9 by weight. The IR spectra were investigated with a Mattson Cygnus 100 FAIR spectrometer (also equipped with a MTEClOO photoacoustic detector for room temperature studies). For temperature-dependent studies a closed-cycle helium refrigerator system (Air
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
569
28 August 1992
CHEMICAL PHYSICS LETTERS
Volume 196, number 6
Products Model lR02A) was used, using KBr windows, an MCT detector and the standard transmission mode. The spectra were recorded in the scan range of 400-4000 cm-‘, under a dynamic vacuum of 10m3Torr and the temperature range of 8-350 K using an associated heater assembly. All recorded spectra were taken using 16 scans at resolution of 1 cm-‘.
3. Experimental results and discussion In fig. 1 we show the FTIR-photoacoustic spectra of Cbo at room temperature recorded on the as-obtained sample along with the transmission FIIR spectra of the C6&Br pellet at two selected temperatures. The locations of the four prominent modes are at 1429.72, 1183.02, 576.78 and 526.95 cm-’ from the photoacoustic spectra and at 1428.24,
1181.81, 575.83 and 526.36 cm-’ from the transmission FIIR spectroscopy. The pressure needed to produce the pellet may account for the slight shift in the frequencies observed above and the observed frequency-dependent shift in the background reported also by others is perhaps related to the KBr pellet structure. However this shift in the background does not seriously affect the precise locations of the peak positions which are accurately measured with the software supplied with the CygnuslOO. In fig. 2, we show the temperature dependence of the positions of the four modes near 1430,1182,576 and 526 cm-‘. Beginning near 100 K, significant shifts in the positions of these bands are observed in three cases. Clearly, the higher frequency modes experience more shift. Near 260 K, the frequency shifts Aware: Ay= 1.5 cm-’ forthe y= 1430.3 cm-’ mode;
576'5j
g
\
576
cm-'
(
1.2
is a s a
0.8
0.4
0.6
14,,.0~ 0
1400
12bo
lOcl0
660
600
WAVENUMBER Fig. 1. The transmission FNR spectra of CW at 8.5 and 300 K and the FUR-photoacoustic spectrum (PAS) at 300 K. 570
50
100
150
200
250
300
350
TEMPERATURE (K) Fig. 2. The temperature dependence of the vibrational frequencies of four prominent IR modes of Cso. The lines are drawn connecting the points as guides. The arrow at 260 K locates the transition To.
CHEMICAL PHYSICS LETTERS
Volume 196, number 6
A~z0.8 for the v= 1182.8 cm-’ mode; Av=OS for the v=576.8cm-’ modeandAv=O.l forthe v=526 cm-’ mode. These values give Av/v= 105X 10e3, 0.68x 10-3, 0.87x 10m3and 0.2x 1O-3 for the higher to the lower frequency modes. An interesting feature of these data is the variation of the rate of shift Avl AT in different temperature regions. In rig. 2, there is a clear inflection point near 260 K which coincides with the well-known T,, orientational transition in C6,,.The slopes of the data in fig. 2 except for the 526 cm-’ band will yield a peak at To whereas the temperature dependence of the vibrational frequency of the 526 cm-’ band shows a dip near To. Thus these measurements show the existence of To in CbO.Furthermore, the data in fig. 2 show that the shifts in the modes continue down to % 100 K, and below this temperature there is no further significant shift with lowering temperatures. The temperature dependence of the peak height for the four modes is shown in fig. 3. Here the temperature dependence of the 576 cm-’ mode is significantly different from those of the other three in
0.88 0
I 50
I 100
1 150
I 200
TEMPERATURE
I 250
I
1 300
I 350
I
(K)
Fig. 3. The temperature dependence of the peak value of the absorbance of the four IR modes of Cm. The lines are drawn as guides and the arrow indicates the transition To at 260 K.
28 August 1992
that for the former, the intensity of the modes decreases with lowering temperatures although in three cases, and inflection in the curves is evident near 260 K. Though the peak absorbance change of the 526 cm-l mode over the temperature range of the measurements is too small, relatively abrupt changes are observed near 250 and 100 K. For the 1183 and 1430 cm-’ modes an additional change is evident below x 100 K. This may be supportive of the glassy state discussed in recent papers [ 5,7-91. The IR data reported, above provide clear evidence that between % 100 K and T,,= 260 K, there are dynamic changes occurring in CsO.In this region, the temperature is still high enough to thermally activate the misoriented molecules into other orientations. Of course above To, molecules rotate freely and the average structure is fee. This view is supported well by molecular dynamics simulations [ 7,8 1. The glassy transition in the 100 K region was estimated from the observation that each CbOmolecule has a unique orientation, and that there are many local minima close to the ground state but separated by a large potential barrier E,, x 300 meV [ 7 1. Using the transition rate equation 7-l = v exp( - Eb/kBT) and V.Y10” Hz and Eb= 300 meV, one gets r~ 1 s at 130 K and TN 1 day at 90 K. Thus as the temperature is lowered below 130 K, the time scale becomes longer and longer as compared to the experimental time, leading to a glassy transition near 100 K. Our observations reported in figs. 2 and 3 are supportive of this description. From X-ray diffraction measurements in a well-annealed sample, Heiney et al. [ 1,2] reported a discontinuous change in the lattice constant a0 at To, suggesting that the transition at To is first order in nature. Our IR measurements reported above in the as-obtained sample show the transition to be more gradual. It is very likely that the first-order nature of the transition near To in the IR data has been smeared by the impurities in the sample. In summary, the temperature dependence of the position and intensity of the IR modes of Cho indicate a phase transition near 260 K and support the picture of a glassy state below 100 K. The first-order nature of the transition near 260 K is somewhat smeared in our measurements presumably because of impurities in the sample. 571
Volume 196, number 6
CHEMICAL PHYSICS LETTERS
References [ 1 ] P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr., A.B. Smith III and D.E. Cox, Phys. Rev. Letters 66 (1991) 249. [2] P.A. Heiney, G.B.M. Vaughan, J.E. Fisher,N. Coustel, D.E. Cox, J.R.D. Copley, D.A. Neumann, W.A. Kamitakahara, KM. Creegan, D.M. Cox, J.P. McCauley Jr. and A.B. Smith III, Phys. Rev. B 45 ( 1992) 4544. [ 3 ] C.S. Yannoni, R.D. Johnson, G. Meijer, D.S. Bethune and J.R. Salem, J. Phys. Chem. 95 ( 1991) 9. [4] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglas and A.M. Mujsce, J. Phys. Chem. 95 ( 199 1) 5 18. [ 5 ] R.C. Yu. N. Tea, M.B. Salamon, D. Lorents and R. Malhotra, Phys. Rev. Letters 68 (1992) 2050. [6] W.P. Beyemtann, M.F. Hundley, J.D. Thompson, F.N. Diederich and G. Grtiner, Phys. Rev. Letters 68 ( 1992) 2046.
572
28 August 1992
[7] J.P. Lu, X.-P. Li and R.M. Martin, Phys. Rev. Letters 68 (1992) 1050. [8] M.P. Gelfand and J.P. Lu, Phys. Rev. Letters 68 (1992) 1050. [9] A. Cheng and M.L. Klein, Phys. Rev. B 45 (1992) 1889. [ lo] W. KnZitschmer,K. Fostiropoulos and D.R. HufTman, Chem. Phys. Letters 170 ( 1990) 167. [ 111 Y. Huang, D.F.R. Gilson and I.S. Butler, J. Phys. Chem. 95 (1991) 5723. [ 121 K. Raghavachari and C.M. Rohlfing, J. Phys. Chem. 95 (1991) 5768. [ 13 ] S.J. Cyvin, E. Brendsdal, B.N. Cyvin and J. Brunvoll, Chem. Phys. Letters 143 (1988) 377. [ 1412. Slanina, J.M. Rudzinski, M. Togasi and E. Gsawa, J. Mol. Struct. THEOCHEM 202 (1989) 169.