Grain boundary defect twist

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],... 

Fig. 6. However, these two structures are incompatible with one another and cannot co-exist and the molecules still fill space uniformly without forming defects. The matter is resolved by the formation of a periodic ordering of screw dislocations which enables a quasi-helical structure to co-exist with a layered structure. This is achieved by having small blocks/sheets of molecules, which have a local smectic structure, being rotated with respect to one another by a set of screw dislocations, thereby forming a helical structure [15]. As the macroscopic helix is formed with the aid of screw dislocations, the dislocations themselves must be periodic. It is predicted that rows of screw dislocations in the lattice will form grain boundaries in the phase, see Fig. 7, and hence this structurally frustrated phase, which was theoretically predicted by Renn and Lubensky [15], was called the twist grain boundary (TGB).

A helical director field also occurs in the chiral smectic-C phase and those smectic phases where the director is tilted with respect to the layer normal (Figure 1.13(c)). In these cases, the pitch axis is parallel to the layer normal and the director inclined with respect to the pitch axis. Very complicated defect structures can occur in the temperature range between the cholesteric (or isotropic) phase and a smectic phase. The incompatibility between a cholesteric-like helical director field (with the director perpendicular to the pitch axis) and a smectic layer structure (with the layer normal parallel or almost parallel to the director) leads to the appearance of grain boundaries which in turn consist of a regular lattice of screw dislocations. The resulting structures of twist grain boundary phases are currently extensively studied. The state of the art in this topical field is summarized in Chapter 10. 

Although liquid crystals have been known for more than 100 years, discovering the structures of their many thermodynamic phases is an activity that persists to this day [1]. The sheer variety has been impressive, including such novelties as uniaxial, biaxial, and ferroelectric fluids phases with hexatic order and chiral phases such as the blue phase (BP) and twist grain boundary (TGB) phase, which are stabilized by a lattice of defects. Many of these phases are unique in condensed matter physics their presence never fails to challenge our notions of how matter can arrange itself in the aggregate. 

Another class of frustrated phase results from the frustration between bend or twist deformations in smectic phases (Section 5.6) and the tendency to form a layered structure. Twisted grain boundary phases are frustrated smectic phases and both SmA and SmC versions have been observed. The phases are denoted TGBA and TGBC respectively and are formed by chiral mesogens. The phases are macroscopically chiral and result from arrays of screw dislocations (i.e. defects in lattice order) which lead to a twist in the director between grains of layers, i.e. to a helical rotation of layers. 

Very large GB systems (over 80,000 molecules) have also been recoitly studied to investigate some of the most distinctive features of liquid crystals topological defects [27,28], until now simulated only with lattice models [29]. In particular, the twist grain boundary phase in smectics [27] and the formation of a variety of defects in nematics by rapid quenching [28] have been examined. 

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