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An order-disorder phase transition has been studied in 2-Chloronaphthalene by both calorimetry and X-Ray diffraction. The transformation is reversible and does not destroy the single crystal. It extends over a large temperature range 253-309 K and can be considered as a "high order" transition: the high temperature form corresponds to a totally disordered phase and the low temperature form to a semi-ordered one.

At T ∼ 24K, the quasi one dimensional organic superconductor (TMTSF)2CϱO4 shows a structural transition leading to a doubling (a × 2b × c) of the unit cell. The neutron average structure (i.e. superlattice reflections not included) at T = 7K has already been published. In the present paper, we report the results of a rigid body refinement of the low temperature (T ∼ 10K) structure from both the earlier set of neutron main reflection intensities and an extra set of X-ray superlattice intensities.This improved structural description clearly shows that the anion orientations alternate in the b direction only and that there is no significant TMTSF displacement from the average position.We compare these results with recent structural data obtained on (TMTSF)2ReO4 which exhibits below T ∼ 180 K an insulating ground state instead of the metallic one for the perchlorate salt.

We have performed structural investigation of the (TMTSF)2PF6 salt at low temperature (neutron diffraction) and under pressure (X. Ray diffraction). Evolutions of the main structural parameters are described. Their descriptions allow the first structural approach of pressure effects on the Bech-gaard salts and give comparative informations for a same compound.

The 2-Chloronaphthalene and 2-Bromonaphthale-ne present two types of disordered structures (form II and form I). The passage II ⇆ I corresponds to a second order transition, the premonitory effects of which extend over a large temperature range (50 K). The disorders is induced by molecular motions (reorientation or/and diffusion).

The phase relations in the x 2-fluoronaphthalene-(1 - x) 2-naphthol system have been determined by X-ray diffraction, DTA and heating optical microscopy. Three stable solid phases occur: β which is a solid solution of 2-fluoronaphthalene in the low temperature form of 2-naphthol (OHII), α which is a solid solution of 2-fluoronaphthalene in the high temperature form of 2-naphthol (OHII) and α' which is a solid solution of 2-naphthol in 2-fluoronaphthalene (FI). The structure of the β phase is derived from the ordered structure OHII (monoclinic, Ia, Z = 8) whereas the structure of the α and α' phases are respectively derived from the disordered structures OHI and FI (monoclinic, P21/a, Z = 2).The main features of the phase diagram are a transition zone (II-I) for solid solutions rich in 2-naphthol, the limits of which at 293 K are x = 0.20 and x = 0.45 and a peritectic at Tp= 338 K with a miscibility gap from x = 0.60 to x = 0.87; the limits of demixion at 293 K are x = 0.53 and x = 0.95.This example points out that volume relationships are not decisive in organic solid-state solubilities.Le diagramme de phases du systeme x fluoro-2 naphthalene-(I - x) naphtol-2 a été déterminé par méthodes radiocristallographique, thermique et optique. Il est de type Roozeboom IV avec un péritectique situé à x = 0.87 et à Tp= 338 K. Il est caractérisé également par un phénomene de polymorphisme Il-I affectant des solutions solides riches en naphtol-2 (forme II: monoclinique, Ia, Z = 8 - forme I: monoclinique, P21/a, Z = 2, structure désordonnée).Cet exemple met en lumière l'insuffisance des seules considerations d'homéomorphisme moléculaire pour décrire les conditions de formation d'un cristal mixte organique.

The crystal structure at room temperature of the complexe salt 4-aminocinnamic acid cadmium chloride (HOOC-CH=CH-(C6H4)-NH3)2 CdCl4 has been solved by X-ray diffraction method. The structure exhibits bidimensional arrangement with alternated inorganic-organic layers, in a monoclinic symmetry P21/m: a = 0.5389(1) nm; b = 3.0165(7) nm; c = 0.6999(3) nm; β = 104.09(3)°. Structural considerations allow to explain the photoinactivity (polymerization) of the compound at room temperature upon irradiation with UV light in contrast to some similar butadiene derivatives which undergo a stereospecific 1,4-addition reaction. Thermal investigations by powder X-ray diffraction and calorimetric methods show during heating process, an exothermic irreversible transformation in the solid state at high temperature concurrently with the disappearance of the tridimensional organization in the structure. A complete interpretation of this phenomenon needs further complementary investigation.

L'étude du polymorphisme cristallin du 2-fluoronaphatalène et du 2-chloronaphtalène est suivie de la détermination du diagramme d'équilibre entre ces composés. On note, en particulier, l'existence d'un domaine de syncristallisation totale des formes hautes températures (Formes I) dont la structure cristalline est affectée d'un désordre par réorientations moléculaires.II est montre que ces composes peuvent interdiffuser a I'état solide. Les coefficients d'interdiffusion D ont pu étre calcults par la méthode de Boltzmann-Matano: á 317°K, ils sont de l'ordre de cmz 10-10 cm2 s-1. Ces valeurs soulignent I'importance du phtnomkne observé: les mobilités de ces molecules organiques sont comparables-"à température normalisbe" égale - à celles des atomes metalliques dans certains alliages.The crystalline phase transitions of pure 2-fluoronaphthalene and Zchloronaphthalene have been studied by X-ray diffraction analysis. The high temperature form (form I) of each compound is affected by an orientational molecular disorder: the non symmetrical molecule acquires the pseudosymmetry of naphthalene in the disordered phase.The phase diagram of the 2-fluoronaphthalene 2-chloronaphthalene system was determined: it is a Rooseboom type III with unlimited solubility between forms I above 308°K. Below 308°K, an heterogeneous region of solid solutions I + II is observed for concentrations near of pure 2-chloronaphthalene.Chemical diffusion studies have been carried out in the whole range of solid miscibility (T4 = 313°K and Td = 317°K). Experiments were performed on polycrystalline materials. The interdiffusion results in the formation of continuous solid solutions. Interpenetration was followed by using classical sectionning technique and determining the concentration of each layer by X-ray analysis. The studied diffusion obeys Boltzmann's law i.e. that isothermal diffusion is only concentration-dependent. Diffusivities were computed by Matano method. The interdiffusion coefficients values are about 10-10 cm2 s-1. This result proves that some organic molecules are able to interdiffuse with relative high mobilities in the solid state.