Ph. Negrier

Miscibility study in stable and metastable orientational disordered phases in a two-component system (CH3)CCl3+CCl4

The orientationally disordered stable and metastable mixed crystals of the two-component system methylchloroform ((CH3)CCl3)+carbon tetrachloride (CCl4) have been characterised from a crystallographic and thermodynamic point of view. The monotropic behaviour of the metastable phase in the pure components is maintained for the whole range of composition. The lattice symmetry of the stable orientationally disordered phase of methylchloroform has been found to be isostructural with that of the carbon tetrachloride compound. Continuous series of both stable and metastable mixed crystals give rise to a double isomorphism relationship, one for the stable state and another for the metastable stable of the pure components.

Miscibility and molecular interactions in plastic phases: Binary system pentaglycerin/ tris(hydroxymethyl)aminomethane

Abstract The phase diagram of the plastic crystals pentaglycerin (PG) and tris(hydroxymethyl)-aminomethane has been established by means of thermal analysis and X-ray powder diffraction from room temperature to the liquid state. The binary system is characterized by two eutectoid invariants corresponding to solid to plastic phase transitions and by one eutectic invariant corresponding to the melting process. The phase diagram exhibits a demixing region in the plastic state as a consequence of the difference between the disordered high-temperature forms of the pure compounds. The determination of the solubility boundaries in the plastic state shows a narrow two phase domain (about 0.13 in molar fraction). The evolution of the volume occupied by a molecule in the molecular alloys in the plastic phases can be interpreted taking into account the different intermolecular interactions in these phases and by geometrical effects.

Coefficients of molecular homeomorphism and crystalline isomorphism in the series of 2-R-naphthalene (R=H, F, Cl, CH3, SH, Br)

Crystallographic data on the stable and metastable phases of the high-temperature forms of the substances of the series of substituted naphthalenes are given. The data are used to calculate for each pair of substances the coefficient ∊K of molecular homeomorphism as well as the coefficient ∊m of crystalline isomorphism, which allow a comparison of the (stable and/or metastable) crystal framework.

High-pressure properties inferred from normal-pressure properties

From the stable and metastable normal-pressure phase equilibria involved in two-component systems sharing compounds of the series CCl4−nBrn, n = 0,...,4, several thermodynamic properties concerning non-experimentally available phase transitions have been determined. To do so, the well-established concept of crossed isodimorphism has been considered to involve the isomorphism relationships between the low-temperature monoclinic phases as well as, for both rhombohedral and face-centred cubic, orientationally disordered phases appearing in the compounds of the series. On the basis of such relations, the thermodynamic properties of the two-phase equilibria are extrapolated as a function of mole fraction to the pure compounds for which the involved transitions do not exist at normal pressure. The obtained thermodynamic properties are used to build up the topological pressure–temperature phase diagrams of the compounds of the series. The results are compared with the experimental pressure–temperature phase diagrams obtained by means of density measurements as a function of pressure and temperature.

Miscibility in plastic phases: binary system NPG (neopentylglycol)/AMP (2-amino,2-methyl-1,3-propanediol)

Abstract The phase diagram of the plastic crystals neopentylglycol (NPG) and 2-amino, 2-methyl-1,3-propanediol (AMP) has been determined. The phase diagram shows an extensive diversity of two-phase regions, due to the polymorphism of the pure substances. The binary system is characterized by two eutectoid invariants corresponding to the solid to plastic transitions and by one eutectic invariant corresponding to the melting process. The solubility boundaries, determined by means of X-ray powder diffraction, strongly depend on which phases are involved. The demixing region in the plastic state shows a very narrow two-phase domain (approximately 0.05 in mole fraction). The study of the mole-fraction dependence of the cell dimensions of the molecular alloys in the plastic phase is explained by means of the existence of different intermolecular interactions and geometrical effects.

X-ray diffraction investigation of a spin crossover hysteresis loop

The nature and the mechanism of the magnetic hysteresis for the thermal spin crossover exhibited by an iron (II) compound is investigated by means of variable-temperature powder and single-crystal x-ray diffraction. The unit cell temperature dependence clearly evidences the amplitude of the strong structural rearrangement that accompanies the spin crossover—corresponding to a variation of 8.6% for one of the unit cell parameters—as well as the structural hysteresis width. In this regard, the present x-ray study reveals significant differences in the spin crossover features according to the nature of the sample—powder or single crystal—that should be taken into account in the analysis of physical properties. Concerning the interplay between structural and magnetic transitions, quenching effects show that the structural transition and the spin crossover are indissociable. Furthermore, investigations of the mechanism itself of the thermal spin crossover confirm the presence of spin-like domains in the conversion region, either in the cooling or in the warming loops. The non-dependence with temperature of these domains inside the hysteresis loop demonstrates the stability of the microscopic and macroscopic structures in the corresponding thermodynamic conditions. This result is of interest in the context of the potential use of hysteresis loops to obtain high-temperature photo-conversion.

Solid state equilibrium in the n-alkanols family: the stability of binary mixed samples.

The stability of binary mixed samples in the normal alkanols family is studied here via the C18H37OH–C20H41OH and C19H39OH–C20H41OH systems. The stability of mixed samples depends on the method used for their preparation. In the samples obtained by the dissolution–evaporation (D + E) method (with diethyl ether), the phases in the solid–solid and solid–liquid equilibria are stable after preparation. However samples obtained by the melting–quenching (M + Q) method (quenching in liquid nitrogen) are only stable for phases in the solid–liquid equilibria. The phases observed in the solid–solid equilibria evolve over time, even after two years’ storage at low temperature (279 K).

Stable and metastable orientationally disordered mixed crystals of the two-component system (CH3)2CCl2+CCl4

Orientationally disordered stable and metastable mixed crystals of the two-component system 2,2-dichloropropane ((CH3)2CCl2)+carbon tetrachloride (CCl4) have been characterised from crystallographic and thermodynamic points of view. The monotropic behaviour of the metastable phases in the pure components is retained by the mixed crystals. Continuous series of mixed crystals in the stable rhombohedral phase gives rise to an isomorphism relationship. The lattice symmetries of the metastable mixed crystals are found to be simple cubic and face centred cubic, the latter corresponding to a large concentration domain. A strong correlation between thermodynamic functions and optical properties is proposed.

Measurement and Analysis of the Naphthalene — Octafluoronaphthalene Phase Diagram: Complex Formation in Solid and Liquid

Abstract Naphthalene–octafluoronaphthalene room pressure solid–liquid phase diagram has been assessed by means of differential scanning calorimetry and powder X-ray diffraction analyses. Thermodynamic properties are analysed through the associated liquid model. The solid states consists of six crystalline phases with well defined composition: one for naphthalene, two for octafluoronaphthalene and three for an equimolecular intermolecular compound. This complex melts congruently at (406.3±0.3) K, i.e., about fifty degrees higher than its components and the resulting liquid is moderately associated. The solid–solid phase transitions of this complex are strongly energetic. The corresponding overall change of entropy stands for two third times the entropy of fusion. The consecutive increasing of crystalline symmetry and positional disorder is clearly visible on diffraction patterns.

A system with a less than 2 degree melting window in the range within −31 °C and −45 °C: chlorobenzene-bromobenzene

Abstract Chlorobenzene and bromobenzene, liquid substances at room temperature, crystallize with very similar structures, without any known polymorphism at ordinary pressures. They are isomorphous, showing complete solid state solubility, with little deviation from ideality. Thus, as it is confirmed by thermal cycling experiments, their alloys would be suitable as phase change materials for cold storage applications if one needs precise temperature control at a temperature between −31 and −45 °C.