Clusters isotropic phase

The isotropic phases occurring above the square honeycomb phases and the nematic phase of 89/12 can be considered as cybotactic isotropic phases with disrupted square honeycomb strucmre, whereas those occurring above Colbex phases (compounds 89/7-89/11) are predominately composed of triangular honeycomb clusters. The phase sequence SmA -Colhex-Colsqu/p4g/n/Ncyb-Colsqu/ p4mm, shown in Fig. 5.29, requires a certain length of the rodlike core. For molecules with shorter cores, e.g., compounds 88/ [181], the sequence occurs without the Co sqJp4gm phase and the nematic phase. 

Non-standard EC has also been observed in bent-core nematics, where < 0, Ca < 0 is also realized and where flexoelectricity is strong. There is still no detailed theoretical description, which would have to include the strong frequency dependence of as well as the unusual viscosity and elastic properties these might indicate smectic cluster formations not only in the nematic but even in the isotropic phase. 

A second effect of the same sample shown in Fig. 3 is also interesting. By cooling the isotropic liquid deviations of the static dielectric constant q from the straight line by approaching the is/N transition were measured. Such behavior was at first detected by Bradshaw and Raynes [24], confirmed by Thoen and Menu [25] and explained as an antiparallel correlation of the strong dipoles in the nematic-like clusters formed in the isotropic phase. For less polar molecules the driving force for the antiparallel orientation can also be produced by a combination of the steric and dipolar forces as shown by Kresse and Kremer [26] and demonstrated in Fig. 3. 

Incoherent Clusters. As described in Section B.l, for incoherent interfaces all of the lattice registry characteristic of the reference structure (usually taken as the crystal structure of the matrix in the case of phase transformations) is absent and the interface s core structure consists of all bad material. It is generally assumed that any shear stresses applied across such an interface can then be quickly relaxed by interface sliding (see Section 16.2) and that such an interface can therefore sustain only normal stresses. Material inside an enclosed, truly incoherent inclusion therefore behaves like a fluid under hydrostatic pressure. Nabarro used isotropic elasticity to find the elastic strain energy of an incoherent inclusion as a function of its shape [8]. The transformation strain was taken to be purely, dilational, the particle was assumed incompressible, and the shape was generalized to that of an... 

A further consequence of intermediate-range interactions adding up are very high almost isotropic repulsive barriers around compact clusters. This has consequences for the density scaling [32] and favors small islands with more narrow distributions of sizes and spacings than the ones obtained without interactions [29]. We finally note that atomic superlattices with smaller lattice constant may be stabilized by dipolar interactions of relatively short range. The most prominent examples for such interactions are alkali metals on metal surfaces. A phase transition from a dilute liquid into a well-ordered solid has been reported for Cs/Ag/Si(lll)-( /3 x %/3) [33]. 

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