Grain boundary, twist

The structures of phases such as the chiral nematic, the blue phases and the twist grain boundary phases are known to result from the presence of chiral interactions between the constituent molecules [3]. It should be possible, therefore, to explore the properties of such phases with computer simulations by introducing chirality into the pair potential and this can be achieved in two quite different ways. In one a point chiral interaction is added to the Gay-Berne potential in essentially the same manner as electrostatic interactions have been included (see Sect. 7). In the other, quite different approach a chiral molecule is created by linking together two or more Gay-Berne particles as in the formation of biaxial molecules (see Sect. 10). Here we shall consider the phases formed by chiral Gay-Berne systems produced using both strategies. 

Bayon, R., Coco, S. and Espinet, P. (2002) Twist-Grain Boundary Phase and Blue Phases in Isocyanide Gold(I) Complexes. Chemistry of Materials, 14, 3515-3518. 

Twist grain boundary (TGB) liquid crystal phase, 15 95... 

A high-angle tilt grain boundary results when 6 > 15°. For 6 < 10°, a low-angle tilt grain boundary results, and Eq. (1.37) can be simplified to tmO 6 = b/h. In a similar manner, a twist grain boundary is a set of screw dislocations. 

A, B, and C in vicinal (001) twist grain boundary in Au. Static array of screw dislocations in background accommodates the twist deviation of the vicinal boundary shown from the crystal misorientation of the nearby singular twist boundary to which it is vicinal. Excess selfinterstitial defects were produced m the specimen by fast-ion irradiation and were destroyed at the grain-boundary dislocations by climb, causing the boundary to act as a defect sink, (a) Prior to irradiation, (b) Same area as in (a) after irradiation, (c) Diagram showing the extent of the climb. From Komer et al. [24],... 

Lubensky TC, Renn SR (1990) Twist-grain-boundary phases near the nematic smectic-A smectic-C point in liquid crystals. Phys Rev A 41 4392-4401... 

The twist grain boundary (TGB) phases predicted by Renn and Lubensky have been intensively studied in the few last years. The general structure of the TGB phase is shown schematically in Figure 5.4. Because the symmetry of the Sm A phase does not allow continuous helical twisting, the chiral superstructure is realized in a stepwise manner Small smectic grains rotate around a helical axis, while screw dislocations build the... 

Navaliles, L., Barois, P., Nguyen, H. T. X-ray measurement of the twist grain boundary angle in the liquid crystal analog of the Abrikosov phase. P%s. Rev. Lett. 71, 545 (1993). 

Kitzerow, H.-G. Twist grain boundary phases. In C. Bahr, H. Kitzerow Eds. Chirality in liquid crystals. New York Springer Vet ag, 297 2000. ... 

It is likely that the other more complex hyperbolic mesophases are also to be found in chiral systems twisted grain boundary and blue phases (whose twist numbers are less than and greater than unity respectively, cf. section 4.14), are also prevalent in proteinaceous matter. (Note that a blue phase of DNA in water has recently been reported [11]). 

The HREM image in Fig. 10.8(d) shows a curved boundary between the grain on the top and the grain on the bottom. The boundary has a 90°/[100] misorientation and, like the 90° [100] tilt boundaries, this boundary is structurally coherent and free of any second phase. A grain boundary along a horizontal plane in this image would be a pure twist grain boundary. 

The effect of the two types of boundary on the measurements of resistivity vs. temperature are shown in Fig. 12.21. None of the leads, the c-axis material and the a-axis material measured in the [010] and [001] directions, shows ideal behavior. This is, at least in part because of the necessity of some compromise in the deposition temperature between ideal a-axis and c-axis conditions. However, the results for the twist grain boundary are consistent with that found previously for (103) films measured across many such grain boundaries there... 

Fig. 12.20. Fabrication of single grain boundaries. The lines indicate the CuOg planes of the aligned a-axis region on a PBCO buffer layer and the shaded areas show the c-axis region on the bare substrate. The black pattern is the photolithographic mask used for defining the four measurement lines the BPF and the twist grain boundaries, and the [010] and [001] leads in the a-axis film.

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