Josep Ll. Tamarit

A new hexagonal phase of fullerene C60

Abstract A new phase of fullerene C 60 with a simple hexagonal unit cell with 6/mmm symmetry was grown by slowly evaporating solutions of C 60 in dichloromethane. X-ray measurements reveal that the c / a ratio is 1.616 at 298 K and increases as temperature decreases. At low temperature, it seems to extrapolate to 1.633, the ideal ratio for a close-packed hexagonal lattice. The unit cell volume which is higher than that of the usual cubic C 60 phases at high temperature decreases near 90 K. A structural model is proposed according to which a 3-fold molecular axis is parallel to the 6-fold crystallographic z -axis. This suggests that the molecules could reorient around this axis at high temperature.

Binary mixtures of nCB and nOCB liquid crystals. Two experimental evidences for a smectic A–nematic tricritical point

Modulated differential scanning calorimetry (MDSC) has been used to obtain specific heat measurements on octyloxycyanobiphenyl (8OCB), octylcyanobiphenyl (8CB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and on several binary mixtures of 8OCB + 9OCB and 8CB + 9OCB. The order of the mesophase transitions, smectic A(SmA) to nematic(N) and N to isotropic (I) on pure components and on binary mixtures has been studied and, in addition, both binary mesophase diagrams have been built. For each set of mixtures, there exists a tricritical point (TCP) composition at the SmA–N transition. The TCP compositions have been found to be X9OCB = 0.63 for the system 8OCB + 9OCB and X9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (α) through second order SmA to N transition have been obtained between 8OCB(or 8CB) and the TCP composition for the two sets of binary mixtures. When these α-values were plotted against the normalised nematic range, a common and uniform crossover trend was found. Indeed, this fact suggests a uniform behaviour in terms of the McMillan ratio for the nCB and nOCB series of liquid crystals. Moreover, for these compounds there exists a common value of (TAN/TNI)TCP of 0.99 and also a value of (TAN/TNI)3D-XY of about 0.96.

Two-component systems of isomorphous orientationally disordered crystals. Part 1: Packing of the mixed crystals

Five experimental two-component phase diagrams between orientationally disordered crystals (ODICs) have been established from the low-temperature phase to the liquid state using thermal analysis and X-ray powder diffraction techniques. The high-temperature orientationally disordered phases for the pure components, which belong to the series (CH 3 ) 4–n1 C(CH 2 OH) n1 (n 1 =1,2,3) and (NO 2 )(CH 3 ) 3–n2 C(CH 2 OH) n2 (n 2 =0,1) are all face centered cubic. Continuous disordered mixed crystals in the whole range of concentration have been found, which indicates the existence of an isomorphism relationship. The intermolecular interactions in the ODIC state of these systems and other related two-component systems are discussed using the evolution of the packing coefficient as a function of the composition.

Degree of miscibility between non-isomorphous plastic phases: binary system NPG (neopentyl glycol)-TRIS[tris(hydroxymethyl)aminomethane]

The experimental phase diagram between the plastic crystals neopentyl glycol (NPG) and tris(hydroxymethyl)aminomethane (TRIS) has been established. The importance of the use of complementary techniques with different timescales in order to obtain the most realistic phase diagram is underlined. The miscibility is discussed on the basis of the geometric conditions (size and shape of molecules and unit-cell dimensions) and of the role of the intermolecular interactions in the structure of the mixed crystals. Binary phase diagrams formed by related, previously reported neopentane derivatives are also analysed in the same manner.

C60CCl4 phase diagram: polythermal behaviour of solvates C60, 12 CCl4 and C60, 2 CCl4

Abstract Polythermal investigation of the C 60 CCl 4 system resulted in two phase diagrams. The stable one involves two solvates with 1:12 and 1:2 molar ratios, respectively. A metastable diagram, only showing the 1:2 solvate, has been observed. C 60 , 12 CCl 4 and C 60 , 2 CCl 4 decompose peritectically at 299 and 402 K, with ΔH = 15.2 and 61 J g −1 , respectively. C 60 , 12 CCl 4 is formed with a positive excess volume while C 60 , 2 CCl 4 does so with a negative one. The desolvation enthalpy for C 60 , 2CCl 4 and sublimation enthalpy for pure CCl 4 are close.

First experimental demonstration of crossed isodimorphism: (CH3)3CCl + CCl4 melting phase diagram

The melting phase diagram of the two-component system 2-methyl-2-chloro-propane ((CH3)3CCl) and carbon tetrachloride (CCl4) has been determined by combining X-ray powder diffraction and thermal analysis. The isomorphism relation between the orientationally disordered (OD) stable face-centered cubic (FCC) phase of (CH3)3CCl and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component. This equilibrium interferes with a rhombohedral plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant. A thermodynamic analysis, in terms of the crossed isodimorphism, has also been performed in order to obtain the excess properties of the FCC and R OD phases.

Polymorphism in 2-X-Adamantane Derivatives (X = Cl, Br)

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group P212121, Z = 4) and a high-temperature plastic phase (Fm3̅m, Z = 4) from 277.9 ± 1.0 K to the melting point at 413.4 ± 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (P1̅, Z = 2), which transforms to a monoclinic phase (C2/c, Z = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (P63/mcm, Z = 6) at ca. 241 K and the highest one, cubic (Fm3̅m, Z = 4), being stable from 244 ± 1.0 K up to the melting point at 467.5 ± 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br···Br as well as C-Br···H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks.

High-pressure differential thermal analysis study of the phase behaviour in sometert-butyl compounds: pivalic acid, 2-methylpropane-2-thiol and tert-butylamine

Pressure–temperature phase diagrams of thetert-butyl compounds (Bu t X) pivalic acid (X=CO 2 H), 2-methylpropane-2-thiol (X=SH) and tert-butylamine (X=NH 2 ) have been determined at temperatures between 100 K and the melting curve and up to 300 MPa with the aid of differential thermal analysis (DTA) under pressure. New normal as well as high-pressure induced phases have been found for 2-methylpropane-2-thiol and tert-butylamine. The results for the studied compounds are compared to those previously obtained for compounds belonging to the same tert-butyl family (i.e. X=Cl, Br, CH 2 OH, NO 2 , Me and CN) in order to establish similar trends in the phase behaviour as well as in the thermodynamic properties of the phase transitions. For the related tert-butyl compounds, with the exception of the X=CN compound, the average of the slopes (dT/dp) of the melting and the II to I transition curves have been determined to be (0.64±0.14) and (0.27±0.13) K MPa -1 respectively.

Thermodynamic scaling of the dynamics of a strongly hydrogen-bonded glass-former

We probe the temperature- and pressure-dependent specific volume (v) and dipolar dynamics of the amorphous phase (in both the supercooled liquid and glass states) of the ternidazole drug (TDZ). Three molecular dynamic processes are identified by means of dielectric spectroscopy, namely the α relaxation, which vitrifies at the glass transition, a Johari-Goldstein βJG relaxation, and an intramolecular process associated with the relaxation motion of the propanol chain of the TDZ molecule. The lineshapes of dielectric spectra characterized by the same relaxation time (isochronal spectra) are virtually identical, within the studied temperature and pressure ranges, so that the time-temperature-pressure superposition principle holds for TDZ. The α and βJG relaxation times fulfil the density-dependent thermodynamic scaling: master curves result when they are plotted against the thermodynamic quantity Tvγ, with thermodynamic exponent γ approximately equal to 2. These results show that the dynamics of TDZ, a system characterized by strong hydrogen bonding, is characterized by an isomorphism similar to that of van-der-Waals systems. The low value of γ can be rationalized in terms of the relatively weak density-dependence of the dynamics of hydrogen-bonded systems.

Disentangling the secondary relaxations in the orientationally disordered mixed crystals: cycloheptanol + cyclooctanol two-component system

The dynamics of the pure compounds and mixed crystals formed between cycloheptanol (cC7-ol) and cyclooctanol (cC8-ol) has been studied by means of broadband dielectric spectroscopy at temperatures near and above the orientational glass transition temperature. Both compounds are known to display at least one orientationally disordered (OD) phase of simple cubic symmetry, and within this phase, a continuous formation of mixed crystals was demonstrated in the past (Rute, M. A. et al. J. Phys. Chem. B 2003, 107, 5914). The dielectric loss spectra of cC7-ol and cC8-ol show, in addition to the well-pronounced alpha-relaxation peaks with a continuous temperature shift (characteristic of the freezing of the molecular dynamics), secondary relaxations (beta and gamma for cC8-ol and gamma for cC7-ol) which are intramolecular in nature. The dynamics of several OD mixed crystals was recently studied (Singh, L. P.; Murthy, S. S. N. J. Phys. Chem. B 2008, 112, 2606), and surprisingly enough one of the secondary relaxations was not evidenced. We show here by means of a careful set of measurements for several mixed crystals and of a detailed analysis procedure the existence of the secondary relaxations for the mixed crystals. The results, moreover, doubtless reinforce the physical origin of each of the secondary relaxations.