title: Orientational ordering and low-temperature libration in the rotor-stator cocrystals of fullerenes and cubane.
abstract: Cocrystals of cubane and fullerenes, C60-cubane and C70-cubane, show distinct rotational ordering transitions. We studied the corresponding structural changes with temperature-dependent X-ray diffraction and the thermodynamics of the phase transitions with adiabatic microcalorimetry and differential scanning calorimetry. C60-cubane has one phase transition around 130 K from a high-temperature fcc phase with freely rotating C60 to a low-temperature orthorombic phase in which the fullerene rotation is frozen. The corresponding enthalpy change is approximately 1170 J/mol, and the entropy change is 9.6 J/(mol K). C70-cubane has two phase transitions. Around 380 K, the high-temperature fcc phase with freely rotating C70 transforms into a bct phase in which the C70 rotates uniaxially around an axis that precesses around the c direction with a full opening angle of 40 degree. Around 170 K, the uniaxial rotation also freezes out, with an accompanying structural transition to monoclinic and enthalpy and entropy changes of 620 J/mol and 8.7 J/(mol K), respectively. The low-temperature specific heat was analyzed in terms of the Debye-Einstein model to estimate the librational energies of the fullerenes and Debye temperatures. We found very similar values for the two cocrystals, approximately Elib = 2.2 meV and TDebye = 23 K. For reference, we also measured the specific heats of pure C60 and C70 and found Elib = 2.96 meV and TDebye = 32 K for C60 and Elib = 1.9 meV and TDebye = 20 K for C70.
authors.affiliation: Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Cantoblanco Madrid 28040, Spain. nmnemes@fis.ucm.es
authors.forename: Norbert M
authors.initials: NM
authors.lastname: Nemes
authors.forename: Mar
authors.initials: M
authors.lastname: García-Hernández
authors.forename: Gábor
authors.initials: G
authors.lastname: Bortel
authors.forename: Eva
authors.initials: E
authors.lastname: Kováts
authors.forename: Bence J
authors.initials: BJ
authors.lastname: Nagy
authors.forename: István
authors.initials: I
authors.lastname: Jalsovszky
authors.forename: Sándor
authors.initials: S
authors.lastname: Pekker
mesh_terms: D002151:Calorimetry; D003460:Crystallization; D037741:Fullerenes / Q000737:chemistry*; D015394:Molecular Structure; D044367:Phase Transition; D012399:Rotation; D013696:Temperature; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D037741:Fullerenes; C069836:fullerene C70; C069837:fullerene C60