Twinning domains, crystal

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. 

Sn should form twinned crystals with domains in three orientations. 

Figure 7. a) Image of a crystal exhibiting twinning domains. The twin boundaries (T.8) are white arrowed, b) electron diffraction pattern characterized by a splitting of the spots hhO. 

In a merohedral twin, the twin law is a symmetry operator of the crystal system, hut not of the point group of the crystal. This means that the reciprocal lattices of the different twin domains superimpose exactly and the twinning is not directly detectable from the reflection pattern. Two types are possible. 

Rg. 42. High temperature hexagonal (A) structure of a Sm203 crystal, with domains of monoclinic (B) structure exhibiting twins (T). 

Figure 10 Projection of twinning domains in a crystal. The change in the orientation of the Cu04 groups is observed in one perovskite triple layer.

Because of the optical anisotropy in lower-symmetry crystals, twinning can frequently be detected immediately in the polarizing microscope. The effects of superposed twinned regions may give rise to very complex and often beautiful optical patterns. Ferroelectric domains are visible in polarized light when accompanied by reorientation of the optical indicatrix (Figure 2). The motion of domain walls under applied electric fields can be studied with the polarizing microscope. 

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