Molecular crystals plastic

All the plastic phases listed in the table possess FCC structure crystal-plastic crystal transition temperature AS, entropy change at T, AS , entropy change at T , activation energy for molecular reorientation obtained from NMR spectroscopy. 

The picture is less clear for molecular crystals when the molecules deviate strongly from a globular form. NMR data and tracer diffusion data are then often in disagreement. Diffusion profiles (In c, vs. distance) are found to be curved, which is usually attributed to additional heterogeneous and fast diffusion pathways. For plastic crystals, this could indicate that many of them possess a highly defective structure. Even for the aromatic ring molecule benzene, which forms a non-plastic crystal, one finds a D (NMR)/D (tracer) ratio on the order of 103. This cannot be understood unless one invokes other than bulk lattice mechanisms of diffusion. 

M.F. Horstemeyer et al A multiscale analysis of fixed-end simple shear using molecular dynamics, crystal plasticity, and a macroscopic internal state variable theory. Modell. Sim. Mat. Sci. Eng. 11, 265-286 (2003)... 

We have been particularly interested in the study of the plastic states of organic compounds,41 which are characterized by high values of AS of formation from the crystalline state, the AS of fusion (plastic-liquid transition) being much smaller. We find that the AH as well as the AS of the crystal-plastic transition generally decrease as the temperature range of stability of the plastic phase increases the AH and the AS of the plastic-liquid transition, on the other hand, increase as the temperature range of stability of the plastic phase increases.42 Neutron scattering, NMR spectroscopy, and several other techniques have been employed to study molecular reorientation in the plastic state.41 We... 

A scheme as described here is indispensable for a quantum dynamical treatment of strongly delocalized systems, such as solid hydrogen (van Kranendonk, 1983) or the plastic phases of other molecular crystals. We have shown, however (Jansen et al., 1984), that it is also very suitable to treat the anharmonic librations in ordered phases. Moreover, the RPA method yields the exact result in the limit of a harmonic crystal Hamiltonian, which makes it appropriate to describe the weakly anharmonic translational vibrations, too. We have extended the theory (Briels et al., 1984) in order to include these translational motions, as well as the coupled rotational-translational lattice vibrations. In this section, we outline the general theory and present the relevant formulas for the coupled... 

The method of molecular dynamics (MD) provides a remarkable opportunity for the observation of various mechanisms of processes taking place on a micro-(nano-) level, and for the evaluation of the probability of such processes by repeating experiments dozens of times. Figure IX-37 shows the MD simulation of the deformation and fracture of a two-dimensional crystal. Plastic deformation and formation of a dislocation (AB) at elevated temperature (upper part) and the formation of a brittle crack at low temperature (lower part) are shown in Fig. IX-37, a, while simultaneous processes of crack nucleation influenced by the presence of foreign atoms, and their propagation to the tip of the crack, taking place at elevated temperature, are illustrated in both lower and upper portions of Fig. IX-37, b [40,41]. 

A number of two-dimensional NMR experiments have been introduced in high-resolution solid-state NMR studies, designed, for example, to investigate chemical exchange processes [64], to retrieve chemical shift anisotropies [65] and dipolar couplings [66], and to probe spin-diffusion proces.ses 67j. Opella has proposed an intemuclear distance-determined, spin-diffusion mechanism in molecular crystals [68], and Benn and coworkers have demonstrated C/ C connectivities using the INADEQUATE sequence for the plastic crystal camphor and have used the COSY sequence for Si/ Si connectivities in the reference molecule Q Ms [691. 

SELF-DIFFUSION IN THE MOLECULAR CRYSTAL ADAMANTANE, COMPARISON OF NMR AND PLASTIC-FLOW METHODS. 

A loss of the orientational order of a molecular crystal due to free rotation of molecules around x, y and z-axes with the positional order remained results in plastic crystals. The point group symmetry increases to that characteristic of crystals with spherical atoms. However, such crystals are much softer. An example is sohd methane CH4 at low temperature. 

In most molecular crystals, the molecules are in a fixed orientation relative to their neighbors and rotation is restricted however, in some crystals, called plastic crystals, the molecules can rotate freely and are orientationally disordered. Examples of plastic crystals are the beta phase of solid molecular nitrogen, which is stable between 35 and 64 K, and the high-temperature solid phase of succinonitrile (N=C—C2H4—C=N), which is stable between 238 K and its melting point at 335 K. Typically, plastic crystals... 

The internal molecular arrangement of a solid in general can be projected as a continuum between well-ordered crystalline state and completely disordered amorphous state (Fig. 1.2). A crystalline material is depicted as having three-dimensional long-range symmetry operators over a domain of at least 1000 individual molecules. A mesophase material (liquid crystals, plastic crystals) is depicted as having intermediate symmetry operators, and an amorphous state has no symmetry operators (Klug and Alexander 1974). 

In the harmonic method and its extensions it is always assumed that the amplitudes of the molecular vibrations about their equilibrium positions and orientations remain small. It will be illustrated by several examples in Sect. 2.3 that this is often not realistic for molecular crystals. In the plastic phases there is not even a well-defined equilibrium orientation of the molecules, but also in the ordered phases the librational amplitudes may become substantial. The motions in such systems have been studied by classical methods, in particular the Molecular Dynamics method and the Monte Carlo method [73]. The advantage of these methods is that they can also be applied to study liquids, and the melting of solids, and other systems (glasses, solutions, mixed crystals) which have lost translational periodicity. Large amplitude motions in molecular crystals can also be studied quantum mechanically, however, by the methods described below. 

Low-energy (LE) surfaces, for which the chemical binding energy is of the order of kT, which are generally hardly wettable. They include molecular crystals and plastics. In this case, we have... 

All liquids spread on glass, metals, and ionic crystals. By contrast, wetting may be total or partial on plastics and molecular crystals, depending on which specific liquid is used. The empirical criterion worked out by Zis-man allows us to classify solids. Each solid substrate has a critical surface... 

Plastics and molecular crystals generally have a low 7c and, therefore, are poorly wettable by water. One technique to increase their wettability is to coat them with gold. However, it would be a mistake to believe that gold-coated plastic behaves like bulk gold. The liquid does interact with gold, but that does not mean that interactions with the plastic substrate are entirely... 

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