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• 1. [Article] Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2}

  We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 ...

  Citation × Citation Citation Abstract Copy Title: Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2} Author: Stetzer, M. R., Heiney, Paul A., Stephens, Peter W., Dinnebier, R. E., Zhu, Q., McGhie, Andrew R., Strongin, Robert M., Brandt, B. M., Smith, Amos B., III Year: 2000 We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈b Title: Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2} Author: Stetzer, M. R., Heiney, Paul A., Stephens, Peter W., Dinnebier, R. E., Zhu, Q., McGhie, Andrew R., Strongin, Robert M., Brandt, B. M., Smith, Amos B., III Year: 2000 We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈b Close

• 2. [Article] Structure, Dynamics, and Phase Transitions in the Fullerene Derivatives C_{60}O and C_{61}H_{2}

  The effect of perturbing the icosohedral symmetry of C60 by the addition of the side groups -O and -CH2 upon orientational order-disorder and glass transitions in solid C60 has been studied by a combination ...

  Citation × Citation Citation Abstract Copy Title: Structure, Dynamics, and Phase Transitions in the Fullerene Derivatives C_{60}O and C_{61}H_{2} Author: Meingast, C., Roth, G., Pintschovius, L., Michel, R. H., Stoermer, C., Kappes, M. M., Heiney, Paul A., Brard, Laurent, Strongin, Robert M., Smith, Amos B., III Year: 1996 The effect of perturbing the icosohedral symmetry of C60 by the addition of the side groups -O and -CH2 upon orientational order-disorder and glass transitions in solid C60 has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives C60O (epoxide) and C61H2 (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure C60, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the exact origin of the glass transition in C61H2 is unclear, the glass transition in C60O has the same origin as that in C60, with a significantly higher degree of order due to a larger energy difference between pentagon and hexagon orientations. The dilatometric data at the glass transition indicate that, in contrast to C60, the ground-state orientation of both C60O and C61H2 molecules is that with the smallest volume, also demonstrating a significant influence of the side groups upon the details of the structure. A possible explanation of these differences in terms of steric effects is proposed. Title: Structure, Dynamics, and Phase Transitions in the Fullerene Derivatives C_{60}O and C_{61}H_{2} Author: Meingast, C., Roth, G., Pintschovius, L., Michel, R. H., Stoermer, C., Kappes, M. M., Heiney, Paul A., Brard, Laurent, Strongin, Robert M., Smith, Amos B., III Year: 1996 The effect of perturbing the icosohedral symmetry of C60 by the addition of the side groups -O and -CH2 upon orientational order-disorder and glass transitions in solid C60 has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives C60O (epoxide) and C61H2 (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure C60, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the exact origin of the glass transition in C61H2 is unclear, the glass transition in C60O has the same origin as that in C60, with a significantly higher degree of order due to a larger energy difference between pentagon and hexagon orientations. The dilatometric data at the glass transition indicate that, in contrast to C60, the ground-state orientation of both C60O and C61H2 molecules is that with the smallest volume, also demonstrating a significant influence of the side groups upon the details of the structure. A possible explanation of these differences in terms of steric effects is proposed. Close

• 3. [Article] Existence of High-Order Superlattices in Orientationally Ordered C_{60}

  We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. ...

  Citation × Citation Citation Abstract Copy Title: Existence of High-Order Superlattices in Orientationally Ordered C_{60} Author: Fischer, J. E., Luzzi, D. E., Kniaź, K., McGhie, A. R., Ricketts-Foot, D. A., Romanow, W. R., Vaughan, Gavin B. M., Li, Dayin, Smith, Allan L., Strongin, Robert M., Cichy, M. A., Bard, Laurent, Smith, Amos B., III Year: 1993 We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. We have searched for, but not observed, the 2a face-centered cubic structure recently observed in low-temperature electron and single-crystal x-ray diffraction. We conclude that the intrinsic ground state is the previously reported simple cubic structure [space groupPa3¯, a(14 K)=14.06 Å], and that other structures may be stabilized by defects or impurities. Title: Existence of High-Order Superlattices in Orientationally Ordered C_{60} Author: Fischer, J. E., Luzzi, D. E., Kniaź, K., McGhie, A. R., Ricketts-Foot, D. A., Romanow, W. R., Vaughan, Gavin B. M., Li, Dayin, Smith, Allan L., Strongin, Robert M., Cichy, M. A., Bard, Laurent, Smith, Amos B., III Year: 1993 We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. We have searched for, but not observed, the 2a face-centered cubic structure recently observed in low-temperature electron and single-crystal x-ray diffraction. We conclude that the intrinsic ground state is the previously reported simple cubic structure [space groupPa3¯, a(14 K)=14.06 Å], and that other structures may be stabilized by defects or impurities. Close

• 4. [Article] Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs).

  X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit ...

  Citation × Citation Citation Abstract Copy Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Close

• 5. [Article] Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2}

  We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered ...

  Citation × Citation Citation Abstract Copy Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Close

• 6. [Article] Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60}

  We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from ...

  Citation × Citation Citation Abstract Copy Title: Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60} Author: Reznik, D., Kamitakahara, W. A., Neumann, D. A., Copley, J. R. D., Fischer, J. E., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1994 We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in K3C60, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied M3C60 compounds; modes at 53 and 66 meV in pure C60 are not observed in Rb2.6K0.4C60. Title: Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60} Author: Reznik, D., Kamitakahara, W. A., Neumann, D. A., Copley, J. R. D., Fischer, J. E., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1994 We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in K3C60, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied M3C60 compounds; modes at 53 and 66 meV in pure C60 are not observed in Rb2.6K0.4C60. Close

• 7. [Article] Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation

  Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence ...

  Citation × Citation Citation Abstract Copy Title: Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation Author: Kiefl, R. F., MacFarlane, W. A., Chow, K. H., Dunsiger, S., Duty, T. L., Johnston, T. M. S., Schneider, J. W., Sonier, J., Brard, Laurent, Strongin, Robert M., Fischer, J. E., Smith, Amos B., III Year: 1993 Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence of T1-1 shows a coherence peak just below Tc and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/kB=53(4) K or 2Δ/ kBTc=3.6(3), is consistent with the BCS weak coupling limit. Title: Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation Author: Kiefl, R. F., MacFarlane, W. A., Chow, K. H., Dunsiger, S., Duty, T. L., Johnston, T. M. S., Schneider, J. W., Sonier, J., Brard, Laurent, Strongin, Robert M., Fischer, J. E., Smith, Amos B., III Year: 1993 Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence of T1-1 shows a coherence peak just below Tc and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/kB=53(4) K or 2Δ/ kBTc=3.6(3), is consistent with the BCS weak coupling limit. Close