At room temperature, the hexagonal C-60. 2(CH3)CCl3 solvate (a = 10.13(1) Angstrom, c = 10.84(1) Angstrom), made of alternating layers of C-60 and solvent molecules, forms with a negative excess volume, and its desolvation enthalpy is virtually the same as the sublimation enthalpy of the pure solvent. Crystallographic and calorimetric studies vs temperature indicate that hexagonal C-60. 2(CH3)CCl3 changes at 211.7 K (1.3 kJ mol(-1) of solvate) into an intermediate triclinic phase which transforms at 189.7 K (4.1 kJ mol(-1) of solvate) into another triclinic phase.A crystallographic analysis in the series of hexagonal C-60. 2 YCCl3 solvates (Y = H, Cl, Br, CH3) reveals that: (i) the change in the unit-cell volume values is due to a change in axis c whose value depends on the size of Y, (ii) the molar volume of the solvates depends linearly on the molar volume of the solvents.Ageing studies at room temperature show that C-60. 2(CH3)CCl3 loses its solvent molecules within a few days or a few months, depending on storage conditions. (C) 2004 Elsevier Ltd. All rights reserved.

The orientationally disordered stable and metastable mixed crystals of the two-component system (CH3)CCl3 (1,1,1-trichloroethane) + CBrCl3(bromotrichloro-methane) have been characterised by means of X-ray powder diffraction and thermal analysis techniques. The thermodynamic assessment coherently reproduces the melting equilibria (the stable [R + L] and the partially metastable [FCC + L]) and provides a coherent set of data for the thermodynamic properties of non-experimentally available phase transitions of pure compounds which agree perfectly with those properties obtained from the pressure–temperature phase diagrams.

Two-orientational glass transitions of two different orientationally disordered states of cycloheptanol are investigated and compared through static (thermodynamic and X-ray diffraction) as well as dynamic (dielectric spectroscopy) experimental techniques. What makes cycloheptanol unique is that one of these disordered states presents an isotropic lattice while for the other the lattice is anisotropic and the orientational glass transition temperature of the anisotropic-glassy state is found to be higher than that of the isotropic one. This result is discussed through the fragility concept, being consistent with the Adam–Gibbs theory. In addition, the kinetic fragilities through (F1/2-metric) as well as the thermodynamic fragilities through ((ΔTgl/TglA)-metric) of both states seem also to correlate in a consistent way.

The two-component system octyloxycyanobiphenyl (8OCB) + decyloxycyanobiphenil (10OCB) has been studied by means of modulated differential scanning calorimetry as well as optical microscopy. The general trends of the phase diagram are similar to the two-component system octylcyanobiphenyl (8CB) + decylcyanobiphenil (10CB), previously published. Evidence for the existence of a TCP have been reported, the molar composition being about 0.33 of 10OCB. Additionally, the smectic mesophase of the 8OCB + 10OCB mixtures has been unmistakably characterized through optical measurements as smectic A for the whole composition range.

Measurements of the specific heat and the static dielectric permittivity of heptyloxycyanobiphenyl (7OCB) confined to the 0.2 microm diameter parallel cylindrical pores of Anopore membranes in the isotropic phase and nematic mesophase, are presented. A comparison between the bulk and the confined 7OCB in treated and untreated pore wall surfaces using a chemical surfactant (HTBA) is performed. Both the treated and untreated membrane confinements seem to affect the nematic-to-isotropic phase transition by a downshift in transition temperature and some rounding at the specific-heat maximum, in a way similar to that which was earlier published for other liquid crystals confined in the same geometry. The static dielectric measurements clearly point out that untreated membrane confinement is axial, with the nematic director aligned parallel to the pore axis being homeotropic bulklike, i.e., with the nematic director aligned perpendicular to the electrode cell surfaces. After chemical surfactant treatment, the nematic director is constrained in a radial alignment being perpendicular to the pore walls. The dielectric measurements are revealed to be specially sensible to analyze the surface-induced nematic order due to the pore wall. The tricritical nature of the nematic-to-isotropic phase transition in bulk 7OCB as well as in treated and untreated Anopore confined geometries is discussed through both the specific heat and the static dielectric data.

The experimental systems studied are the five methylchloromethanes [(CH3)(4-n)CCln, n = 0, ..., 4] and seven of their binary combinations, the accent being on the thermodynamic mixing properties in the plastic crystalline (orientationally disordered) state with its three forms (rhombohedral, face-centered cubic and simple cubic). It is demonstrated that a uniform description can be given of the thermodynamic excess properties characterized by a uniform temperature (420 K) of enthalpy/entropy compensation. The magnitude of the excess properties is related to volumetric mismatch between the components of a system, and, to a relatively small extent, to the dipolar nature of the molecules.

The polymorphism of bromotrichloromethane (CBrCl3) has been investigated by X-ray powder diffraction and high-pressure density experiments. Phase transitions as a function of temperature and pressure between the different phases have been characterized at normal pressure as well as at high pressures (up to 300 MPa). From the p-v-T diagram (and the derived p-T diagram) the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. Special attention is given to the lattice symmetry of the orientational disordered phase II, characterized as rhombohedral (a = 14.639(8) angstrom, alpha = 89.44(1)degrees at 240.2 K). The existence of a glass transition from the monoclinic low-temperature stable ordered phase (III) to its nonergodic state (associated with the freezing of exchange positions between Cl and Br atoms) is analyzed in terms of the asymmetry of the intermolecular interactions, and a new "fingerprint" for the glass transition is proposed on the basis of the aspherism index. Lattice parameters as a function of temperature were determined in order to build up the thermal expansion tensor.

Crystals grown by evaporating C60 solutions in n-nonane were found to be crystals of the C60·2/3 n-nonane solvate, after they were stored for eight years in the dark at room temperature. The lattice is an I-centered orthorhombic lattice (a=10.10(1) Å, b=10.19(1) Å, c=48.71(5) Å), that forms with a negative excess volume of about −3.1%, and loses its solvent from about 399 K with an enthalpy change of about 49 J per gram of solvate. Thermodynamic and crystallographic data for available C60–n-alcane solvates are discussed and a van der Waals volume of ≈603 Å3 per C60 isolated molecule is proposed.

The stable solid polymorphism of cyclooctanol (C8H16O, for short C-8-OH) is revealed to be a complex problem and only two stable solid phases, denoted on cooling from the liquid as phases I and II, are found using static (thermodynamic and x-ray diffraction) as well as dynamic (dielectric spectroscopy) experimental techniques. Both solid phases are known to exhibit glass transitions if they are cooled down fast enough to prevent transition to ordered crystalline states. Although glass transitions corresponding to both phases had been well documented by means of specific heat measurements, x-ray measurements constitute, as far as we know, the first evidence from the structural point of view. In addition, a great amount of dielectric works devoted to phase I and its glass transition, were published in the past but next to nothing relating to the dielectric properties of phase II and its glass transition. The nature of the disorder of phase II will be discussed.

Orientationally disordered (OD) stable and metastable mixed crystals of the two-component system 2,2-dichloropropane ((CH(3))(2)CCl(2)) + methylchloroform ((CH(3))CCl(3)) have been characterized from crystallographic and thermodynamic points of view. Continuous series of mixed crystals in the stable rhombohedral (R) phase points out an isomorphism relationship. The monotropic behavior of the non-isomorphous OD metastable phases in the pure components (simple cubic, SC, and face-centered cubic, FCC. respectively) with respect to the R stable phase is partially retained by the mixed crystals. In particular, SC mixed crystals were found persistent enough as metastable crystals in the whole composition range with the exception of the pure compound (CH(3))CCl(3), for which only the FCC metastable phase was experimentally available. On the contrary, samples with compositions greater than X = 0.70 in (CH(3))CCl(3) crystallized into either the FCC or SC, both being metastable with respect to the R phase. Thus, three kinds of OD mixed crystals appear which give rise to three independent OD + Liquid equilibria. The thermodynamic analysis of such non-interfering equilibria, [R + L], [SC + L] and [FCC + L], enables us to infer the stability relationships between the whole of OD phases.

The two-component system cycloheptanol (C7) + cyclooctanol (C8) has been studied by means of thermal analysis, X-ray powder diffraction, and dielectric spectroscopy. In a first step, the polymorphism of pure C7 and C8 has been revised, and new unpublished crystallographic data of the stable phases have been reported. Evidence for the isomorphism relationship between the simple cubic (SC) phases of both of the pure components has been seen through the continuous evolution of the lattice parameters, the continuous evolution of the dielectric strength, and the existence of a two-phase equilibrium [SC + L]. Despite the solid polymorphism of the pure components the SC state has been observed to be the sole mixed solid state between, 170 K and [SC + L] equilibrium. To perform a complete thermodynamic analysis, measurements of the excess enthalpy in the liquid state were undertaken. These measurements together with the experimental melting-phase diagram allow us to determine the excess thermodynamic properties of the mixed SC state via a compensation law. Dielectric data have been used to interpret the excess properties, and the existence of short-range orientatonal order for the mixed SC state has been inferred. In addition, this OD mixed state has been classified as extraordinary in light of the log T-C-log T-m plot.

The polymorphism of cyclopentanol (C5H10O) has been further investigated by X-ray powder diffraction experiments and it has been found to agree with the most recent thermodynamic study [J. Chem. Thermodyn. 1995, 27, 953]. In addition to the previously reported orientationally disordered hexagonal phases I and II, the lattice symmetry of the low-temperature ordered phases III and IV have been determined by means of pattern-matching analyses both as monoclinic with Z = 24 (Cc and C2/c, respectively). The patterns revealed no significant differences between phases III and IV and the transition between both phases was found through careful determination of the variation of the lattice parameters as a function of the temperature. The strength of intermolecular interactions as well as the anisotropy in the solid phases has been analyzed by the study of the isobaric thermal-expansion tensor and the results are discussed in the light of previously reported literature covering dynamics disorder.

The complete stable phase diagram of the two-component system (heptyloxy)cyanobiphenyl (7OCB) + (octyloxy)cyanobiphenyl (8OCB) was determined by means of modulated differential scanning calorimetry (MDSC), optical microscopy, and X-ray diffraction measurements. It was experimentally established that the 7OCB + 8OCB two-component system exhibits a monotropic reentrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk's equal G analysis, was performed, including the calculation of the monotropic reentrant behavior and a discussion of the stable melting phase diagram. Moreover, the specific-heat critical exponents (alpha), through second-order SmA to N transition, was obtained. If these alpha-values, together with those corresponding to other systems sharing cyanobiphenyl (nCB and nOCB) compounds, are plotted against the normalized nematic ranges, a common and uniform crossover trend is found. This behavior allows one to predict, in a simple way, the order of the SmA to N transition.

The two-component system octylcyanobiphenyl(8CB)+decylcyanobiphenil(10CB) has been studied by means of Modulated Differential Scanning Calorimetry, optical microscopy and X-ray diffraction. Although the general trends of the phase diagram are similar to that proposed by Marynissen et al., additional evidences for the existence of TCP have been reported, the composition being about 0.2, which is slightly lower than that proposed in the aforementioned work. Additionally, the smectic mesophase of the 8CB+10CB mixtures has been characterised through X-ray diffraction and optical measurements as smectic A for the whole composition range. Such a result is against the recent work of Oweimreen and Hwang.

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 X-9OCB = 0.63 for the system 8OCB + 9OCB and X-9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (alpha) 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 alpha-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 (T-AN/T-NI)(TCP) of 0.99 and also a value of (T-AN/T-NI)(3D-XY) of about 0.96.

Triclinic C-60.2ferrocene, which melts peritectically at 495 K (Delta(pi)H = +50 J g(-1)), can form by direct union of its components with a negative excess volume of -68 Angstrom(3) per formula unit, although the enthalpy of deferrocenation into fcc C-60 and ferrocene vapor is virtually the same as for the sublimation of pure ferrocene. C-60 solubility in molten ferrocene is about 0.5 mol % at 495 K. The strong anisotropy of the thermal expansion tensor illustrates the anisotropy of intermolecular interactions inferred from the crystal structure in the 90-300 K range. The existence of nonnegligible interactions is corroborated by the persistence of the crystalline local order in the amorphous phase obtained by grinding, which reverts to the initial crystalline phase upon heating. Thus, from the thermodynamic and structural points of view, C-60.2ferrocene behaves as the C-60.2S(8) solvate.

The polymorphism of 2,2-dichloropropane ((CH3)(2)CCl2) has been further investigated by both thermal and X-ray powder diffraction experiments. From the former the phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 200 MPa). From the p-T slopes of the two-phase coexistence lines, the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. The existence of two low-temperature stable ordered phases (III and II), one high-temperature orientationally disordered phase (rhombohedral, Ib) and one additional monotropic orientationally disordered phase la, has been confirmed. The structure of the low-temperature ordered phase II has been determined by X-ray powder diffraction and Rietveld profile refinement as monoclinic C2/c, with lattice parameters a = 10.6402(3) Angstrom, b = 5.4074(2) Angstrom, c = 10.7295(3) Angstrom, and beta = 116.274(3)degrees at 175.2 K. The strength of intermolecular interactions as well as the anisotropy have been analyzed by means of the thermal-expansion tensor.

The trimorphism of (CH3)(3)CCl, a member of the (CH3)(4-n)CCln series, is reexamined as a function of the temperature and pressure. Its topological p-T diagram is constructed resorting to the available thermodynamic and crystallographic data for the four observed crystalline solids and the data extrapolated from binary phase diagrams sharing compounds of this series. The p,T coordinates of the five stable triple points are calculated together with the slopes of the two-phase equilibrium curves. Through these results, the uncharacterized phase IV (a high-pressure orientationally disordered stable phase) is shown to coincide with the rhombohedral phase (R) which is the orientationally disordered stable phase for the members with n = 2, 3, and 4 at ordinary pressure. In addition, the existence of a isomorphous high-pressure R phase is postulated for the first member [neopentane, (CH3)(4)C] of the series.

The experimental determination by means of DSC together with a thermodynamic analysis of the two-component phase diagram of octylcyanobiphenyl (8CB) + octyloxycyanobiphenyl (8OCB) has been performed. The thermodynamic consistency of the two-component system has been proved and similar excess Gibbs energies for mixtures in the isotropic, nematic and smectic phases have been obtained. In order to study the order of the nematic to smectic A phase transition, specific heat measurements obtained using modulated DSC have been made on three mixtures of 8CB + 8OCB as well as on the pure components. An evolution of the alpha-critical exponent has been found when the composition of the mixture changes between pure 8CB and pure 8OCB showing crossover behaviour between 3D-XY universal class and the tricritical point. As the 8OCB content in the mixture is further increased, the alpha-value decreases with the decrease in the McMillan ratio (T-NA/T-NI) giving rise to a value of about 0.96 at which the 3D-XY mode would be hypothetically reached. The tricritical point would be hypothetically reached at a McMillan ratio of about 0.99 and the extrapolated tricritical amplitude ratio A(-)/A(+) is estimated to be 1.6, a value very different from that obtained for the He-3-He-4 tricritical transition, but comparable with other experimental results reported in the literature.

The preparation and characterization of the unstable-in-air cubic solvate C-60 . 12BrCCl(3) (a = 27.591(3) Angstrom) and. the air-stable hexagonal solvate C-60 . 2BrCCl(3) (alpha(H) = 10,16(:3) Angstrom, c(H) = 10.89(7) Angstrom) are described. Results are compared with those for parent C-60 solvates and discussed in terms of potential superconducting properties.