Twin walls/boundaries

Several models for the structure of the twin walls have been proposed (30, and ref. therein). The simplest of these is illustrated in Figure 4. The twin wall is formed by three consecutive diagonals. The copper atoms located on the central one have two-fold coordination with corresponding oxygen stoichiometry of x = 6.0. Those located on the two neighboring rows on each side of the central diagonal have three-fold coordination and oxygen stoichiometry x = 6.5. Within each domain, away from the boundary, the composition is x = 7.0 and copper has the usual four-fold planar coordination of... 

Fig. 18. Example of the precipitation microstructure found in bulk-hardened 2-17 magnets of the high-Wc type Sm(Co0 67Fe0 23Cufl fl8Zrc,02)R.M fully heat treated to MHC = 23 kOe. Left section containing the c-axis note the rhomboid cells of the twinned 2-17 matrix, the 1-5 boundary phase surrounding the cells, and the thin bands of z-phase in the base plane, crossing many cells and walls. Right basal...

Twin domains and their boundaries can form complex microstructural patterns in ferroelastic materials. It has been shown theoretically and experimentally that diffusion of impurity ions or vacancies can be enhanced within twin domain walls [1-4]. Furthermore, impurities [5] or oxygen vacancies [6-8] may be pinned in the wall or in its close vicinity. The fact that walls and bulk material show different dielectric transport and diffusion rates could explain the high ionic conductivity in doped heavy twinned lanthanum gallate, which is of interest... 

It should be noted in passing that a [ 1121] twin is nothing but a special case of a KB, where a basal plane dislocation is nucleated every c-lattice parameter [136]. The fundamental difference between a KB and a twin is in the shear angle for the latter, it is crystallographic, but for the former it is not. What determines the angle of a kink boundary is the number of mobile dislocation walls that end up in that boundary. 

The density fluctuations arising near the boundary from the implementation of the conservative force at the waU is a cause of concern when implementing the no-sUp boundary condition. To address this, a no-sUp scheme based on the equivalent force between the waU and fluid particles may he introduced. In this method, layers of frozen particles are chosen as in the frozen-layer scheme, but a key difference is that the coefficient for the conservative force is adjusted in a manner so as to incorporate wall and fluid particle interactions. This leads to a correct no-slip implementation without fluctuations in density. Another proposal to reduce density fluctuations near the walls represents the boundary with two layers of frozen particles. The particles that penetrate the boundary are made to bounce back into the computational domain. No-sUp at the wall is produced by using a twin image of the system being simulated. The second image has the same configurations and dynamics as the first, but operates with a different... 

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