L. Carpentier

Solid-State Vitrification of Crystalline Griseofulvin by Mechanical Milling

The thermodynamic, dynamic, and structural changes of crystalline griseofulvin upon high-energy ball milling at room temperature have been studied. The investigations have been performed by differential scanning calorimetry (DSC), dielectric relaxation spectroscopy, and powder X-ray diffraction. The results indicate that this compound undergoes a direct crystal-to-glass transformation upon milling, whereas no glass transition can be clearly detected upon heating because of the exceptional sub-glass transition temperature (T(g) ) recrystallization of the milled sample. This intrinsic difficulty for characterizing the glassy state has been overcome using three independent strategies: (i) comparison of the evolutions upon milling of both the crystalline powder and the quenched liquid, (ii) use of fast DSC to delay the recrystallization event, and (iii) search for dielectric β relaxations typical of glasses in the milled compound.

Novel Tailormade Bi4MO4(PO4)2 Structural Type (M = Mg, Zn)

In the Bi(2)O(3)-MO-P(2)O(5) ternary system, the commonly observed sizable 1D ribbon-like units have been extended to their 2D infinite end member, leading to the novel tailormade Bi(4)MO(4)(PO(4))(2) compounds. It contains planar [Bi(2)O(2)](2+) derivatives, separated by two slabs of PO(4), which create channels hosting the M(2+) cations (M = Mg, Zn). For both compounds, supercell orderings occur comparatively to the predicted ideal crystal structure (V(Mg) = 2V(ideal) and V(Zn) = 8V(ideal)). In the Mg case a transition into the ideal lattice occurs above 450 °C. In spite of the conceptual assembly of 2D motifs, the final architecture is three-dimensional due to strong interbonds. Thus, our work gives new insights on the possibility for versatile organization of original secondary building units (SBUs) able to self-assemble into predicted structural edifices. Single-crystal and powder XRD versus temperature, high-temperature (31)P NMR, as well as transmission electron microscopy were used for structural characterization. Preliminary electric characterization is also reported.

Nuclear magnetic resonance and dielectric investigations of molecular motions in a glassy crystal: the mixed compound (CN-adm)0.75(Cl-adm)0.25.

The dynamic properties of plastic crystalline mixed adamantanes derivatives namely cyanoadamantane (75%) and chloroadamantane (25%) were investigated by dielectric and nuclear magnetic resonance (NMR) spectroscopy, covering a spectral range of 12 decades in the temperature range 110-420 K. Phase transformations were studied and dynamical parameters of the plastic (I), glassy (Ig), and ordered (III) phases were determined and compared with those of pure compounds. The dynamics of the supercooled plastic phase is characterized by an alpha-process exhibiting an Arrhenius behavior which classified the mixed compound as a strong glass former. In the plastic phase, NMR relaxation times were interpreted by using a Frenkel model, which takes into account structural equilibrium positions. This model explains adequately the experimental results by considering two molecular motions. In both the glassy state and plastic phase the motional parameters agree with those of 1-cyanoadamantane. On the contrary, in the ordered phase, the motional parameters related to the uniaxial rotation of chloroadamantane molecules indicate an accelerated motion.

Conversion of the glacial state into the crystal in triphenyl phosphite

The conversion of the glacial state of triphenyl phosphite into the crystal on heating was analyzed from Raman and dielectric investigations. Both spectroscopic probes reveal the appearance of liquid during the heating process, interpreted as a partial melting. Assuming the description of the glacial state in terms of nanocrystals of the stable crystalline phase, embedded in the matrix of non-transformed supercooled liquid, the smallest nanocrystals can be considered merely as nuclei of the crystal with dimensions in the range of the critical radius of nucleation. Consequently, these nanocrystals become unstable on heating and melt. No consideration of structural frustration is needed to explain a partial melting of the glacial state, and no trace of topological defects was detected during the conversion of the glacial state into the crystal.

Time-resolved dielectric investigation of relaxation kinetics in metastable caffeine

Using time-resolved dielectric relaxation spectroscopy, we have studied the kinetics of the first-order phase transformation of caffeine from its high temperature rotational solid form to the room temperature phase. The results indicate that the kinetics can be fitted by a highly stretched exponential law. The kinetics, thus, differ from that of a ‘typical’ nucleation and growth process. This unusual non-sigmoïdal evolution can be understood as being dominated by slow nucleation in grains with a dispersion of their size ranging below the characteristic dimension of the nucleation and growth process. The relaxation times of polarization are characterized by an activation energy closely correlated with that of the nucleation process.

Relaxation modes in glass forming meta-toluidine

The dynamics in supercooled meta-toluidine was studied using dielectric relaxation, modulated differential scanning calorimetry, proton spin-lattice relaxation times, and viscosity measurements. The combination of these different techniques has clearly shown a large decoupling of the relaxation modes whose origin is attributed to the formation of clusters via the NH2 bonding. This decoupling starts at a temperature also corresponding to a change of the dynamical behavior from a high temperature Arrhenius evolution to a Vogel-Fulcher-Tamman low temperature evolution.

NMR in molecular crystals: An example: fluoroadamantane C10H15F

The 1H, 19F and 13C relaxation times of fluoroadamantane C10H15F are measured over a wide temperature range. These relaxation times are analysed with two dynamical descriptions: an isotropic rotational diffusion and a Frenkel jump model. In this jump model, the structural equilibrium positions are taken into account and therefore two molecular motions are able to explain adequately the experimental results obtained in the plastic phase: a three-fold uniaxial rotation around the dipolar C-F axis and a tumbling reorientation of this axis between cubic axes. The refinements are first carried out using 1H and 19F NMR results in conjunction with the residence time deduced from the dielectric relaxation. Finally, by introducing the 13C NMR results obtained in the plastic phase a precise determination of the two residence time can be made from the NMR results alone.

Influence of phase transformation on the low temperature dielectric measurements in 1-cyanoadamantane

Dielectric measurements have been performed on the supercooled plastic crystal phase of cyanoadamantane under isothermal and isochronal conditions in the temperature range [170-250 K], each experiment with a new sample. The measurements performed in the course of isotherms allowed us to emphasize the effects of the transformations close to the glass transition temperature leading to a change in the dynamic behavior of the compound. The temperature dependence of the dielectric relaxation times exhibits an Arrhenius evolution on the whole metastable domain and gives at the calorimetric glass transition temperature a value of 1 s, i.e., two orders of magnitude lower than the usual value [tau(T(g)) congruent with 100 s]. These results indicate that the Vogel-Fulcher-Tammann behavior observed with other techniques does not appear in the Brillouin zone center (q=0) and that the glass transition for this glass former is not associated with the freezing out of the tumbling motion of individual molecular dipoles but to the freezing out of fluctuations of an antiferroelectric local ordering.

Dielectric Studies of the Mobility in Pentitols

Broad-band dielectric relaxation measurements were performed for the four pentitols isomers, xylitol, adonitol, L-arabitol and D-arabitol. The comparison of the dynamical properties of these compounds shows similarities between the secondary relaxation processes but also important differences for the temperature dependence of the primary process characterized by the steepness index. These differences enable us to distinguish two groups of compounds that correspond to two kinds of molecular conformation. We show that the formation of more or less extended networks of hydrogen bonds, which reflects the more or less non-Arrhenius variation of the primary relaxation, can be related to the differences of conformation of the studied isomers.

Structure determination of L-arabinitol by powder X-ray diffraction.

Powder X-ray diffraction patterns of the commercial phase of L-arabinitol were recorded with a laboratory diffractometer. The starting structural model was found by a Monte-Carlo simulated annealing method. The final structure was obtained through Rietveld refinements with soft restraints on the interatomic bond lengths and bond angles. H atoms of hydroxyl groups were localized by minimization of the crystalline energy. The cell is triclinic with the space group P1 and contains two molecules. The crystalline cohesion is achieved by an important network of O-H···O hydrogen bonds.