- Email: [email protected]
Growth and properties of pure C60 single crystals from vapor
PHYSICA [x
Physica C 195 ( 1992) 205-208 North-Holland
Growth
and properties
of pure CbO single crystals from vapor
J. Li ‘,l, S. Komiya a, T. Tamura ‘, C. Nagasaki b, J. Kihara b, K. Kishio a and K. Kitazawa a a Department of Industrial Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan b Department of Metallurgy, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received
27 March
1992
CeO single crystals free from solvent contamination were grown from its vapor up to a size of 0.5 X 1.2X 1.6-2 mm3 by a method of periodic oscillation of the crystal temperature. The heat capacity of the crystals exhibited a sharp peak at 260 K with hysteresis of less than I .2 K. The XRD patterns from room-temperature to 50 K show a transition of structure from face-centered cubic to simple cubic at 260 K. Values of 14.5 & 0.6 and 17.5 * 0.5 for the Vickers hardness of Cer, crystals on the ( 100) and ( 111) faces, respectively, indicated that C6a is a very soft material.
1. Introduction The success in efficiently synthesizing C6,, [ 1,2] has spurred intense interest in the chemical, electronic and physical properties of this new class of molecular crystals. To date, most work has been done on polycrystalline samples. Crystals of CeO grown from solvents have been known to include a significant amount of the solvent molecules and to exhibit different structures depending on the solvent used [ 3-5 1. Fleming et al. [ 6 ] and Meng et al. [ 7 ] have recently grown CeOcrystals, up to a size of about 0.42 mm, by slow vaporization of solid CeO over a temperature gradient; the room-temperature structure was shown to be face-centered cubic (fee). Measurements of XRD, NMR, sound velocity and specific heat C, on C&, have revealed that an orientational-ordering-induced fee to simple cubic (SC) transition occurs in the temperature range 245-260 K [g-lo]. However, there are differences of up to 10 K in the transition temperature between XRD and C, measurements by Heiney et al. [ 81 and a broadened transition, 245-255 K, from C, studies by Atake et al. [ 111 and by Fortune et al. [ 5 1. This is thought to be related to the presence of residual solvent molecules in the samples. Hence, it is desirable to obtain ’
Permanent University
address: Department of Chemistry, Central of Technology, Changsha, Hunan, China.
0921-4534/92/$05.00
0 1992 Elsevier Science Publishers
South
highly pure crystals of sufficient size for various property measurements. In this paper, we report the growth of C&, single crystals of millimeter size from vapor in a sealed quartz tube through a novel method of periodic oscillation of the crystal temperature. Employing these pure crystals, some of the property measurements are described in order to examine how the purity of the crystals affects the essential properties. Measurements of the XRD and heat capacity below room temperature indicated that the behavior of the phase transition near 260 K was different from what has previously been reported. Also, the values of the Vickers hardness are measured on different crystalline planes.
2. Experimental A commercial fullerene mixture of about 80% C&, 18-l 9% CT0 and l-2% of other higher fullerenes was dissolved in benzene and separated chromatographically in a 7 cm diameter neutral alumina column [ 121 with hexane and toluene (hexane/toluene= 9 : 1 in volume for elutriation of C6a and 7 : 3 for elutriation of &,). The C6,, powder so obtained was degassed at 350°C under a dynamic vacuum for a few hours to remove the solvent, yielding the raw material for crystal growth. 30-50 mg of C6,, powder was sealed in a 1 cm ID x lo- 12 cm long quartz tube
B.V. All rights reserved,
206
J. Li et al. /Growth and properties of C6,,single crystals
under vacuum. The tube was placed in a horizontal furnace with two temperature zones so that the temperatures at the two ends of the tube could be controlled independently. The source temperature was kept at 600 “C, while the temperature at the other end was held at 500°C with a periodic intermittent oscillation to 520-540°C in order to eliminate undesired tiny nuclei and selectively promote the growth of larger crystals. The crystal growth was continued for about 3-5 days with an oscillation period of l2 h and a temperature pulse duration of 30 min at the crystal growth site. A few milligrams of the resulting small crystals were ground into powder and held on a glass holder for XRD. This was mounted in a closed-cycle helium refrigerator for the measurement of XRD below roomtemperature, using a Mac Science MXP 18 X-ray diffractometer with a rotating Cu anode. A rather flat crystal with a size of 0.5 x 0.4 X 0.1 mm3 was selected for measurement in an AC heat capacity calorimeter at a temperature scanning rate of 0.4 K/min. The calorimeter was equipped with an optical chopper, operating at 1.5 Hz, to periodically heat the sample. The Vickers hardness, H,, on the ( 100) and ( 111) crystal faces was measured by the standard diamond indentation method under an optical microscope. Crystals with well-developed ( 100) and ( 111) faces were selected for the hardness measurements and fixed on a copper substrate by silver paste to keep the desired crystal faces perpendicular to the diamond indenter. The samples were held for 30 s under 25 g of load.
3. Results and discussion Single crystals of Ce,, up to a size of 0.5 x 1.2~ 1.62 mm3 were obtained. The crystals grown under a smaller temperature gradient have a polyhedral shape with well-developed, shiny crystal habit faces such as ( loo), ( 111) of the fee structure, assigned by the transmission Laue method. However, the crystals grown under larger temperature gradients, up to 25”C/cm, developed a flat triangular or trapezoidal shape which showed a three-fold symmetry in the Laue photograph, suggesting that the ( 111) face has the slowest growth speed. Figure 1 shows the typical crystals obtained.
Fig. 1. Typical crystals grown from vapor.
30
I 250
260
270
280
T'K Fig. 2. Temperature dependence of heat capacity of C,, crystal; inset shows the data as reported by Atake et al. [ 111.
The temperature dependence of the specific heat in fig. 2 shows a sharp phase transition at 260 K. It is in good agreement with the previous differential scanning calorimetry measurement by Heiney et al. [ 8 1, although only the warming curve was reported in that work, and is also in agreement with the transition temperature determined from the elastic constant measurement by Shi et al. [ 10 1. The difference in the transition temperature for warming and cooling curves was less than 1.2 K. This contrasts with previous results in which the transition was much broader [ 5,111. The sharp transition at 260 K with significantly smaller hysteresis observed in the present measurements can be regarded as an indication of the high purity and homogeneity of the crystals.
J. Li et al. /Growth
and properties of C,, single crystals
The results from a heat capacity measurement of a powder sample by Atake et al. [ 111 gave a 255 K transition temperature with a second peak at 250 K, while C, measurement of CbOcrystals grown from CS, solvent gave an even lower temperature of about 240 K [ 5,101. It is evident that these lower transition temperatures can be attributed to impurities or solvents in the samples. Figure 3 shows the change in the XRD patterns measured from room-temperature to 50 K. All the peaks at room-temperature can be indexed as fee with a0 = 14.16 A. Many new peaks appeared below 260 K and their intensity increased with decrease of temperature. Below 260 K, all of the XRD peaks can be indexed as simple cubic, as previously reported [ 8 1. The T-dependence of the intensity, relative to the 3 11 peak, of the 451 peak, which is fee forbidden, and 111, 222 and 333 peaks, shown in fig. 4, also indicates a transition at 260 K rather than 249 K as re-
80
i
L_ d h^
20
30
40
10
50
2i3/Cu-Ka
Fig. 3. Powder X-ray diffraction patterns: ( 1) room temperature, (2) 270 K, (3) 265 K, (4) 260 K, (5) 255 K, (6) 250 K, (7)240K, (8) 200K, (9) lOOK, (10)50K.
20-l
ported by Heiney et al. [ 81. Figure 5 shows the change of the unit cell parameter around T,. It has not become clear whether the phase transition at 260 K is first order, as suggested by Shi et al. from their elastic constant measurement [ 3 1, or second order. The small hysteresis of the transition, the L-type C, curve and the continuous change of lattice constants around T, seem to suggest a second-order nature of the transition. Even if it is of the first order, it should be a weak first order transition as suggested by Heiney et al. [ 8 1. One of the most widely adopted microscopic pictures for the transition at 260 K of the Cbo molecular crystal has been that the transition removes all of the rotational degrees of freedom of the molecules, leading to an orientationally ordered SC structure. Further experimental evidence is necessary to conclude whether the transition of Cho at 260 K is first order or second order. As has been discussed, the present crystals seem to exhibit significantly different properties from those obtained from solutions. And, since the crystals are large enough for Vickers hardness measurements, it is of interest to know this essential mechanical property because it is known to be quite sensitive to the presence of impurities. The values of the Vickers hardness, H,, of Cbo crystals for the ( 100) and ( 111) faces were 14.5 & 0.6 and 17.5 f 0.5, respectively. Compared with H,=25 of pure gold [ 131, Cho appears to be a very soft crystal. Some systematic crack lines were observed on the ( 100) and ( 111) faces after indentation, as shown in fig. 6. The crack lines were oriented with an angle of 60” between them on the ( 111) face and along the diagonal direction on the (100) face. These crack lines evidently propagated under the indenter along ( 110) directions as
14.16 14.14 Q -n
14.12
I
14.06 ’ 0
50
100
150
200
250
300
T /K
Fig. 4. Temperature dependence of XRD intensity of Ihlr1/13,, peaks.
Fig. 5. Temperature dependence of the lattice constant of the CGO crystal.
208
J. Li et al. /Growth and properties of C,, single crystals
hysteresis than in previous reports. The values of the Vickers hardness of Ceo crystals on the ( 100) and ( 111) faces indicate that CGOis a very soft material.
Acknowledgements We thank D.M. Pooke for valuable discussions and T. Nagano and K. Suenaga for their technical assistance in XRD measurements.
References
Fig. 6. Crack lines under the indenter on the (100) face.
(a) on the ( I 11) face; (b)
well as along the directions of the maximum stress distribution. This observation is typical of other crystals with fee structure. Details .nre under further investigation.
4. Conclusions Pure single crystals have been grown from vapor upto a size of 0.5 x 1.2~ 1.6-2 mm3 by the method of periodic oscillation of the crystal temperature. The measurement of the heat capacity from 300 K to 4.3 K and XRD patterns from room-temperature to 50 K on the resulting C6,, crystals show a sharp phase transition from fee to SCat 260 K with a much smaller
[ 1] H.W. Kroto, J.R. Heath, SC. O’Brien, R.F. Curl and R.E. Smalley, Nature 318 (1985) 162. [2] W. Kratschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347 (1990) 354. [ 3 ] R.M. Fleming, A.R. Kortan, B. Hessen, T. Siegrist, F.A. Thiel, R.C. Haddon, R. Tycko, G. Dabbagh, M.L. Kaplan and A.M. Mujsce, Phys. Rev. B 44 ( 199 1) 888. 41 H. Ogata, T. Inabe, H. Hoshi, Y. Maruyama, Y. Achiba, S. Suzuki, K. Kikuchi and I. Ikemoto, Jpn. J. Appl. Phys. 31 (1992) L166. 51 N.A. Fortune, K. Murata, F. Iga, Y. Nishihara, K. Kikuchi, S. Suzuki, I. Ikemoto and Y. Achiba, Physica C 185-189 (1991) 425. [6] R.M. Fleming, T. Siegrist, P.M. Marsh, B. Hessen, A.R. Kortan, D.W. Murphy, R.C. Haddon, R. Tycko, G. Dabbagh, A.M. Mujsce, M.L. Kaplan and S.M. Zahurak, Mater. Res. Sot. Symp. Proc. 206 ( 199 1) 69 1. [ 71 R.L. Meng, D. Ramirez, X. Jiang, P.C. Chow, C. Diaz, K. Matsuishi, S.C. Moss, P.H. Hor and C.W. Chu, Appl. Phys. Lett. 59 ( 1991) 3402. 81 P.A. Heiney, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley, A.B. Smith and D.E. Cox, Phys. Rev. Lett. 66 ( 199 1) 29 11. 91 R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhija and S.M. Zahurak, Phys. Rev. Lett. 67 ( 1991) 1886. 1OlX.D. Shi, A.R. Kortan, J.M. Williams, A.M. Kini, B.M. Savall and P.M. Chaikin, Phys. Rev. Lett. 68 (1992) 827. 111 T. Atake, T. Tanaka, H. Kawaji, K. Kikuchi, K. Saito, S. Suzuki, I. Ikemoto and Y. Achiba, Physica C 185-189 (1991) 427. 12 H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, F. Diederich, K. Fostiropoulos, D.R. Huffman, W. Kratschmer, Y. Rubin, K.E. Schriver, D. Sensharma and R.L. Whetten, J. Phys. Chem. 94 (1990) 8630. ]13 1G.P. Upit and S.A. Varchenya, The Science of Hardness Testing and Its Research Applications, eds. J.H. Westbrook and H. Conrad, USA ( 197 1) p. 137.
Physica C 195 ( 1992) 205-208 North-Holland
Growth
and properties
of pure CbO single crystals from vapor
J. Li ‘,l, S. Komiya a, T. Tamura ‘, C. Nagasaki b, J. Kihara b, K. Kishio a and K. Kitazawa a a Department of Industrial Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan b Department of Metallurgy, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received
27 March
1992
CeO single crystals free from solvent contamination were grown from its vapor up to a size of 0.5 X 1.2X 1.6-2 mm3 by a method of periodic oscillation of the crystal temperature. The heat capacity of the crystals exhibited a sharp peak at 260 K with hysteresis of less than I .2 K. The XRD patterns from room-temperature to 50 K show a transition of structure from face-centered cubic to simple cubic at 260 K. Values of 14.5 & 0.6 and 17.5 * 0.5 for the Vickers hardness of Cer, crystals on the ( 100) and ( 111) faces, respectively, indicated that C6a is a very soft material.
1. Introduction The success in efficiently synthesizing C6,, [ 1,2] has spurred intense interest in the chemical, electronic and physical properties of this new class of molecular crystals. To date, most work has been done on polycrystalline samples. Crystals of CeO grown from solvents have been known to include a significant amount of the solvent molecules and to exhibit different structures depending on the solvent used [ 3-5 1. Fleming et al. [ 6 ] and Meng et al. [ 7 ] have recently grown CeOcrystals, up to a size of about 0.42 mm, by slow vaporization of solid CeO over a temperature gradient; the room-temperature structure was shown to be face-centered cubic (fee). Measurements of XRD, NMR, sound velocity and specific heat C, on C&, have revealed that an orientational-ordering-induced fee to simple cubic (SC) transition occurs in the temperature range 245-260 K [g-lo]. However, there are differences of up to 10 K in the transition temperature between XRD and C, measurements by Heiney et al. [ 81 and a broadened transition, 245-255 K, from C, studies by Atake et al. [ 111 and by Fortune et al. [ 5 1. This is thought to be related to the presence of residual solvent molecules in the samples. Hence, it is desirable to obtain ’
Permanent University
address: Department of Chemistry, Central of Technology, Changsha, Hunan, China.
0921-4534/92/$05.00
0 1992 Elsevier Science Publishers
South
highly pure crystals of sufficient size for various property measurements. In this paper, we report the growth of C&, single crystals of millimeter size from vapor in a sealed quartz tube through a novel method of periodic oscillation of the crystal temperature. Employing these pure crystals, some of the property measurements are described in order to examine how the purity of the crystals affects the essential properties. Measurements of the XRD and heat capacity below room temperature indicated that the behavior of the phase transition near 260 K was different from what has previously been reported. Also, the values of the Vickers hardness are measured on different crystalline planes.
2. Experimental A commercial fullerene mixture of about 80% C&, 18-l 9% CT0 and l-2% of other higher fullerenes was dissolved in benzene and separated chromatographically in a 7 cm diameter neutral alumina column [ 121 with hexane and toluene (hexane/toluene= 9 : 1 in volume for elutriation of C6a and 7 : 3 for elutriation of &,). The C6,, powder so obtained was degassed at 350°C under a dynamic vacuum for a few hours to remove the solvent, yielding the raw material for crystal growth. 30-50 mg of C6,, powder was sealed in a 1 cm ID x lo- 12 cm long quartz tube
B.V. All rights reserved,
206
J. Li et al. /Growth and properties of C6,,single crystals
under vacuum. The tube was placed in a horizontal furnace with two temperature zones so that the temperatures at the two ends of the tube could be controlled independently. The source temperature was kept at 600 “C, while the temperature at the other end was held at 500°C with a periodic intermittent oscillation to 520-540°C in order to eliminate undesired tiny nuclei and selectively promote the growth of larger crystals. The crystal growth was continued for about 3-5 days with an oscillation period of l2 h and a temperature pulse duration of 30 min at the crystal growth site. A few milligrams of the resulting small crystals were ground into powder and held on a glass holder for XRD. This was mounted in a closed-cycle helium refrigerator for the measurement of XRD below roomtemperature, using a Mac Science MXP 18 X-ray diffractometer with a rotating Cu anode. A rather flat crystal with a size of 0.5 x 0.4 X 0.1 mm3 was selected for measurement in an AC heat capacity calorimeter at a temperature scanning rate of 0.4 K/min. The calorimeter was equipped with an optical chopper, operating at 1.5 Hz, to periodically heat the sample. The Vickers hardness, H,, on the ( 100) and ( 111) crystal faces was measured by the standard diamond indentation method under an optical microscope. Crystals with well-developed ( 100) and ( 111) faces were selected for the hardness measurements and fixed on a copper substrate by silver paste to keep the desired crystal faces perpendicular to the diamond indenter. The samples were held for 30 s under 25 g of load.
3. Results and discussion Single crystals of Ce,, up to a size of 0.5 x 1.2~ 1.62 mm3 were obtained. The crystals grown under a smaller temperature gradient have a polyhedral shape with well-developed, shiny crystal habit faces such as ( loo), ( 111) of the fee structure, assigned by the transmission Laue method. However, the crystals grown under larger temperature gradients, up to 25”C/cm, developed a flat triangular or trapezoidal shape which showed a three-fold symmetry in the Laue photograph, suggesting that the ( 111) face has the slowest growth speed. Figure 1 shows the typical crystals obtained.
Fig. 1. Typical crystals grown from vapor.
30
I 250
260
270
280
T'K Fig. 2. Temperature dependence of heat capacity of C,, crystal; inset shows the data as reported by Atake et al. [ 111.
The temperature dependence of the specific heat in fig. 2 shows a sharp phase transition at 260 K. It is in good agreement with the previous differential scanning calorimetry measurement by Heiney et al. [ 8 1, although only the warming curve was reported in that work, and is also in agreement with the transition temperature determined from the elastic constant measurement by Shi et al. [ 10 1. The difference in the transition temperature for warming and cooling curves was less than 1.2 K. This contrasts with previous results in which the transition was much broader [ 5,111. The sharp transition at 260 K with significantly smaller hysteresis observed in the present measurements can be regarded as an indication of the high purity and homogeneity of the crystals.
J. Li et al. /Growth
and properties of C,, single crystals
The results from a heat capacity measurement of a powder sample by Atake et al. [ 111 gave a 255 K transition temperature with a second peak at 250 K, while C, measurement of CbOcrystals grown from CS, solvent gave an even lower temperature of about 240 K [ 5,101. It is evident that these lower transition temperatures can be attributed to impurities or solvents in the samples. Figure 3 shows the change in the XRD patterns measured from room-temperature to 50 K. All the peaks at room-temperature can be indexed as fee with a0 = 14.16 A. Many new peaks appeared below 260 K and their intensity increased with decrease of temperature. Below 260 K, all of the XRD peaks can be indexed as simple cubic, as previously reported [ 8 1. The T-dependence of the intensity, relative to the 3 11 peak, of the 451 peak, which is fee forbidden, and 111, 222 and 333 peaks, shown in fig. 4, also indicates a transition at 260 K rather than 249 K as re-
80
i
L_ d h^
20
30
40
10
50
2i3/Cu-Ka
Fig. 3. Powder X-ray diffraction patterns: ( 1) room temperature, (2) 270 K, (3) 265 K, (4) 260 K, (5) 255 K, (6) 250 K, (7)240K, (8) 200K, (9) lOOK, (10)50K.
20-l
ported by Heiney et al. [ 81. Figure 5 shows the change of the unit cell parameter around T,. It has not become clear whether the phase transition at 260 K is first order, as suggested by Shi et al. from their elastic constant measurement [ 3 1, or second order. The small hysteresis of the transition, the L-type C, curve and the continuous change of lattice constants around T, seem to suggest a second-order nature of the transition. Even if it is of the first order, it should be a weak first order transition as suggested by Heiney et al. [ 8 1. One of the most widely adopted microscopic pictures for the transition at 260 K of the Cbo molecular crystal has been that the transition removes all of the rotational degrees of freedom of the molecules, leading to an orientationally ordered SC structure. Further experimental evidence is necessary to conclude whether the transition of Cho at 260 K is first order or second order. As has been discussed, the present crystals seem to exhibit significantly different properties from those obtained from solutions. And, since the crystals are large enough for Vickers hardness measurements, it is of interest to know this essential mechanical property because it is known to be quite sensitive to the presence of impurities. The values of the Vickers hardness, H,, of Cbo crystals for the ( 100) and ( 111) faces were 14.5 & 0.6 and 17.5 f 0.5, respectively. Compared with H,=25 of pure gold [ 131, Cho appears to be a very soft crystal. Some systematic crack lines were observed on the ( 100) and ( 111) faces after indentation, as shown in fig. 6. The crack lines were oriented with an angle of 60” between them on the ( 111) face and along the diagonal direction on the (100) face. These crack lines evidently propagated under the indenter along ( 110) directions as
14.16 14.14 Q -n
14.12
I
14.06 ’ 0
50
100
150
200
250
300
T /K
Fig. 4. Temperature dependence of XRD intensity of Ihlr1/13,, peaks.
Fig. 5. Temperature dependence of the lattice constant of the CGO crystal.
208
J. Li et al. /Growth and properties of C,, single crystals
hysteresis than in previous reports. The values of the Vickers hardness of Ceo crystals on the ( 100) and ( 111) faces indicate that CGOis a very soft material.
Acknowledgements We thank D.M. Pooke for valuable discussions and T. Nagano and K. Suenaga for their technical assistance in XRD measurements.
References
Fig. 6. Crack lines under the indenter on the (100) face.
(a) on the ( I 11) face; (b)
well as along the directions of the maximum stress distribution. This observation is typical of other crystals with fee structure. Details .nre under further investigation.
4. Conclusions Pure single crystals have been grown from vapor upto a size of 0.5 x 1.2~ 1.6-2 mm3 by the method of periodic oscillation of the crystal temperature. The measurement of the heat capacity from 300 K to 4.3 K and XRD patterns from room-temperature to 50 K on the resulting C6,, crystals show a sharp phase transition from fee to SCat 260 K with a much smaller
[ 1] H.W. Kroto, J.R. Heath, SC. O’Brien, R.F. Curl and R.E. Smalley, Nature 318 (1985) 162. [2] W. Kratschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347 (1990) 354. [ 3 ] R.M. Fleming, A.R. Kortan, B. Hessen, T. Siegrist, F.A. Thiel, R.C. Haddon, R. Tycko, G. Dabbagh, M.L. Kaplan and A.M. Mujsce, Phys. Rev. B 44 ( 199 1) 888. 41 H. Ogata, T. Inabe, H. Hoshi, Y. Maruyama, Y. Achiba, S. Suzuki, K. Kikuchi and I. Ikemoto, Jpn. J. Appl. Phys. 31 (1992) L166. 51 N.A. Fortune, K. Murata, F. Iga, Y. Nishihara, K. Kikuchi, S. Suzuki, I. Ikemoto and Y. Achiba, Physica C 185-189 (1991) 425. [6] R.M. Fleming, T. Siegrist, P.M. Marsh, B. Hessen, A.R. Kortan, D.W. Murphy, R.C. Haddon, R. Tycko, G. Dabbagh, A.M. Mujsce, M.L. Kaplan and S.M. Zahurak, Mater. Res. Sot. Symp. Proc. 206 ( 199 1) 69 1. [ 71 R.L. Meng, D. Ramirez, X. Jiang, P.C. Chow, C. Diaz, K. Matsuishi, S.C. Moss, P.H. Hor and C.W. Chu, Appl. Phys. Lett. 59 ( 1991) 3402. 81 P.A. Heiney, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley, A.B. Smith and D.E. Cox, Phys. Rev. Lett. 66 ( 199 1) 29 11. 91 R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhija and S.M. Zahurak, Phys. Rev. Lett. 67 ( 1991) 1886. 1OlX.D. Shi, A.R. Kortan, J.M. Williams, A.M. Kini, B.M. Savall and P.M. Chaikin, Phys. Rev. Lett. 68 (1992) 827. 111 T. Atake, T. Tanaka, H. Kawaji, K. Kikuchi, K. Saito, S. Suzuki, I. Ikemoto and Y. Achiba, Physica C 185-189 (1991) 427. 12 H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, F. Diederich, K. Fostiropoulos, D.R. Huffman, W. Kratschmer, Y. Rubin, K.E. Schriver, D. Sensharma and R.L. Whetten, J. Phys. Chem. 94 (1990) 8630. ]13 1G.P. Upit and S.A. Varchenya, The Science of Hardness Testing and Its Research Applications, eds. J.H. Westbrook and H. Conrad, USA ( 197 1) p. 137.