Twin domain

The lattice parameters a and b become different during the transition from the tetragonal to the orthorhombic structure. Either a > b or a < b results, depending on the direction of the rotation of the octahedra strands. During the phase transition both directions of rotation occur at random domains appear, in which either a > b or a < b. The obtained crystal is a twinned crystal or twin, consisting of intergrown domains. The twin domains are... 

Definition An intergrowth of two or more macroscopic individuals of the same crystal species is a twin, if the orientation relations between the individuals conform to crystallographic laws. The individuals are called twin partners, twin components or twin domains. 

Figure 18 YBa2Cug07. (0.37

Besides all these localized phenomena, which take place in a more or less random way, a systematic contrast modulation appears in the crystals, the direction of which varies with the a-parameter, from one twinning domain to the other, as shown in Figure 18. In these images, a row of bright dots, parallel to the b-axis, alternates with a darker one, leading to a mean 2 x a periodicity. 

The toroidal structure of PCNA has distinct front and back faces that might provide a variety of sites for interaction with other proteins. The loop region between the twin domains of each monomer is a highly immunogenic exposed site that is important for interaction with other proteins. Such functional protein partners are thought to be crucial in regulating the role of PCNA in replication and repair. The aforementioned symmetry could define the directionality of PCNA movement as it slides along DNA. 

Carlo simulations [16] or free energy minimizations [18], in u, yield twin domain walls oriented along a diagonal. 

For example, Wakayama and coworkers discovered the formation of two oppositely orientated domains of 5,10,15,20- lclrakis(3,5-di-ferf-bulylphcnyl)-porphine deposited onto a Cu surface using low-temperature scanning tunneling microscopy [124]. It was demonstrated that the twin domains were produced by a pair of chiral conformations induced by a combination of symmetrical tilting and twisting rotation of the phenyl rings with respect to... 

Lines represent contact Q/Qo- There are three hnes in the middle of the twin domain wall that are not labelled they represent the Q/Qo valnes of 0.40, 0.00, and -0.40 respectively. Notice the steepness of the gradient of Q/Qo through the twin domain wall. The two stmctures represent sheared twin atomic confignrations in the bnlk (far from the domain wall and surfaces). 

The relation between W and Ws, the domain wall widths in the bulk and at the surface, can be seen in Figure 8. The effect of the surface relaxation is clearly visible as the order parameter at the surface Qs never reaches the bulk value Qo- The distribution of the square of the order parameter at the surface shows the structure that some of the related experimental works have been reported (Tsunekawa et al. 1995, Tung Hsu and Cowley 1994), namely a groove centred at the twin domain wall with two ridges, one on each side. 

In addition, the square of the surface order parameter is proportional to the chemical reactivity profile of the twin domain wall interface at the surface (Locherer et al. 1996, Houchmanzadeh et al.(1992). Intuitively, one would expect the chemical reactivity of the surface to be largest at the centre of the twin domain wall, falling off as the distance from the centre of the wall increases. Contrary to the expected behaviour, a more complex behaviour is found. The reactivity, a monotonic function of Q, is expected to fall off as the distance from the centre of the wall increases, but only after if has reached a maximum of a distance of - 3 IF from the centre of the domain wall. If such a structure is expected to show particle adsorption (e.g. in the MBE growth of thin films on twinned substrates) we expect the sticking coefficient to vary spatially. In one scenario, adsorption may be enhanced on either side of the wall while being reduced at the centre. The real space topography of the surface is determined by both sources of relaxation-twin domain wall and the surface. These are distinct and, when considered separately, the wall... 

Figure 8. (a) Q/Qo (solid line, proportional to the strain at the snrfaee) and Q/Qo (dashed line, proportional to the strain in the bnlk). The widths of the twin domain wall in the bnlk, IF, and at the snrfaee, IF, are the same, (b) The sqnare of the order parameter at the snrfaee, q], related to the ehemical potential, and indieative of the areas of maximnm deeoration, where the distortion of the lathee is at a maximnm. 

In an attempt to predict the possible experimental results of AFM investigations of the surface structure of the twin domain wall, the effect that the tip at the end of an AFM... 

The results can be used as a guide for the future experimental work. In order to determine the twin domain wall width W in the bulk, one only needs to determine the characteristic width IFs of the surface structure of the domain wall. Previously, these features of the twinning materials were investigated using mainly X-ray techniques. In fact, the theoretical work leads to the conclusion that the only necessary information for the determination of the twin domain wall width W are the real space positions of the particles in the surface layer. 

Blackburn J, Salje EKH (1999a) Formation of needle shaped twin domains in cordierite A computer simulation study. J Appl Phys 85 2414-2422... 

Blackburn J, Salje EKH (1999b) Time evolution of twin domains in cordierite a computer simulation study. Phys Chem Minerals 26 275-296... 

Tsatskis I, Salje EKH (1996) Time evolution of pericline twin domains in alkali feldspars A computer-simulation study. Am Mineral 81 800-810... 

Nord, G.L., Lawsort, C.A. (1989) Order-disorder transition-induced twin domains and magnetic properties in ihnenite-hematite. Am Mineral 74 160-176... 

Tweed twinning. When impurity concentrations are sufficiently large, twin walls are stabilized and thus twin wall densities can be extremely high. As a result, mean twin domain sizes can decrease dramatically for relatively small concentrations of solute. In some instances, the twin walls adopt a characteristic checkerboard-like microstructure known as a tweed pattern with orthogonal modulations of -100-200 A. The formation and energetics of tweed twins are discussed in Chapter 3 of this volume by Salje and also in Putnis and Salje (1994) and Salje (1999). Examples of compositionally induced tweed textures include the doped superconductor YBa2(Cui-jcM f)307.8 for M = Co and Fe (Van Tendeloo et al 1987, Schmahl et al 1989, Xu et al. 1989) as well as As-doped lead phosphate (Bismayer et al. 1995) and Sr-doped anorthite (Tribaudino et al. 1995). Diffraction patterns of twin tweeds exhibit characteristic cross-shaped intensities superimposed on the primary spots. 

Although these NMR data clearly support a dynamical model for disorder in P-cristobalite, they are not sensitive to whether the motions of adjacent oxygens are correlated (as required for a model of re-orienting twin domains), or, whether the motion is continuous or a hopping between discrete positions they indicate only that the path of each oxygen traces a pattern with 3-fold or higher symmetry over times of the order 4.7-10 s. Thus, these results cannot discriminate between models based on RUMs or dynamical twin domains, and place only a lower limit on the timescale of the motions. A tighter restriction... 

Figure 12. Polyhedral representation of a fragment of the quartz stracture showing the loeal relationship of the two DauphinD twin orientations of the a-phase (left and right) to the average structure of p-quaitz (center). The orientation of the Si[0]4 tetrahedra in a-quartz results from a rotation about the a-axes from the p-quartz stracture (0 = 16.5° at 25°C). DauphinD twin domains are related by rotations of opposite sense. [Modified after Heaney (1994), Fig. 3, p. 8 and Fig. 5, p. 11.]...

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