Polymorphism plastic phases

As mentioned above, polymorphs may also be related by order-disorder transitions, e.g. the onset of free rotation of a group of atoms, or local tumbling in semi-plastic or plastic phases. This may be due to random orientation of the molecules or ions, but is also diagnostic of the onset of a reorientational motion. Roughly spherical molecules and ions are likely to show order-disorder phase transitions to a plastic state. In the cases of co-crystals or of crystalline salts this process may affect only one of the components, leading to semi-plastic crystals (an example will be discussed below). Order-disorder phase transitions have often... 

Investigations into the behaviour of ionic solids are important in the search for new battery and fuel cell electrolytes. Defects, plastic phases, disorder and polymorphism in crystalline solids (and their association with ionic conductivity) and associated solid-solid equilibria are thus of considerable interest. The basis of our understanding of these phenomena and their relationship to the characteristics of the con5)onent ions includes studies of materials that are ionic liquids. 

Australia, is the doyen of materials scientists who study the elastic and plastic properties of minerals under hydrostatic pressure and also phase stability under large shear stresses (Paterson 1973). J.-P. Poirier, in Paris, a professor of geophysics, was trained as a metallurgist one of his special skills is the use of analogue materials to help understand the behaviour of inaccessible high-pressure polymorphs, e.g., CaTi03 perovskite to stand in for (Mg, FelSiOi in the earth s mantle (Poirier 1988, Besson el al. 1996). 

In some cases there is evidence of multiple solid-solid transitions, either crystal-crystal polymorphism (seen for Cl salts [20]) or, more often, formation of plastic crystal phases - indicated by solid-solid transitions that consume a large fraction of the enthalpy of melting [21], which also results in low-energy melting transitions. The overall enthalpy of the salt can be dispersed into a large number of fluxional modes (vibration and rotation) of the organic cation, rather than into enthalpy of fusion. Thus, energetically, crystallization is often not overly favored. 

Although polymorphism in plastic crystals is less frequent than in liquid crystals, it does exist. Tetrakis(methylmercapto)methane, C(SCH3)4, for example, has four crystal modifications of which the three high temperature forms have a high degree of plasticity 100). Also, it has been observed that plastic crystals are frequently mutually soluble 16b), a consequence of the less restrictive crystal structures. Phase separation of these solutions occurs often on transition to the fully ordered crystal, giving rise to quite complicated phase diagrams102). 

We note that the phase diagrams for PLC-containing blends are more complicated than those of ordinary engineering plastics. For PLCs additional phases have to be taken into account since the PLCs themselves may exhibit complex polymorphism. 

The first thing that we found was that defects had to be a minimum size to have any effect whatsoever. A vacancy, or even a divacancy, can be passed over by a shock wave without so much as a hiccup. However, beyond a certain size, a void (or vacancy cluster) can produce not only a warp in the shock front, but serious heating upon its collapse. Such an overheat can subsequently lead to initiation of chemical reaction originating at the void, even for a shock wave that would not have been strong enough to produce initiation in a perfect crystal (see Fig. 12). For materials that don t react chemically, such hotspots can still be nucleation sites for other shock-induced phenomena, such as plastic flow or polymorphic phase transformations. 

The samples show a complex polymorphism during tensile deformation. The relationships between the different mechanical behavior and the stress-induced phase transitions are discussed in terms of a general view, outlining the concept that stress-induced phase transitions during plastic deformation of... 

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