Microtwins

The core structure of the 1/2 [112] dislocation is shown in Fig. 4. This core is spread into two adjacent (111) plames amd the superlattice extrinsic stacking fault (SESF) is formed within the core. Such faults have, indeed, been observed earlier by electron microscopy (Hug, et al. 1986) and the recent HREM observation by Inkson amd Humphreys (1995) can be interpreted as the dissociation shown in Fig. 4. This fault represents a microtwin, two atomic layers wide, amd it may serve as a nucleus for twinning. Application of the corresponding external shear stress, indeed, led at high enough stresses to the growth of the twin in the [111] direction. 

Similar to PbSe, the controlled growth of lead telluride, PbTe, on (111) InP was demonstrated from aqueous, acidic solutions of Pb(II) and Cd(II) nitrate salts and tellurite, at room temperature [13]. The poor epitaxy observed, due to the presence of polycrystalline material, was attributed to the existence of a large lattice mismatch between PbTe and InP (9%) compared to the PbSe/InP system (4.4%). The characterization techniques revealed the absence of planar defects in the PbTe structure, like stacking faults or microtwins, in contrast to II-VI chalcogenides like CdSe. This was related to electronic and structural anomalies. 

The well known microtwinning phenomena that are inherent to the structure transition of orthorhombic YBa2Cu307 to tetragonal... 

Figure 31 BiPbSr2YCusOfi Several types of satellites are observed in different areas of the same crystals ([001]) (a) satellites set up along a direction roughly parallel to [120] (q-6.25) (b) bidimens-ional system of satellites resulting from the existence of misori-ented areas and double diffraction phenomena, the angle between both systems is close to 100 (c) satellites along [110] with q 3.65 (d) previous satellites have disappeared but streaks remain along (II0 (e) multisplitting of the spots and first satellites they are indications of distortions and microtwins.

Figure 5.8. Schematic diagrams of an inclined microtwin in a thin crystal, (a) The thickness of the microtwin is such that it produces a displacement of half a planar spacing across it. (b) A wider microtwin produces no relative displacement.

The investigation by Fitz Gerald (1980) of microtwins in plagioclase feldspars is probably the most detailed yet undertaken of this type of defect in important rock-forming minerals and provides an excellent example of the theoretical and experimental procedures involved in characterizing these defects from their TEM contrast. 

Correspondingly, for a microtwin that is n unit cells thick,... 

Three microtwins (A, B, and C) in Cazadero albite (Ano) imaged in DF with sbc different diffraction vectors g are shown in Figure 8.13. From... 

Rgure 8.14. DF a = ir type fringes due to microtwins in Cazadero albite. (a)g=002, beam axis near [2l0] (b) g = 111, beam axis near [onj ... 

Figure 8.15. DF images of microtwins in Wyangala plagioclase (Anjo). (a) g = 20T, beam axis near [To2]. Microtwins are viewed edge-on. (b) Same region as (a), again with g = 201, but the beam axis is near [Il2], From Fitz Gerald 1980.)...

Crystals deformed at a constant strain-rate (e = 10 s ) with a confining pressure of 300 MPa and 400 C in an orientation expected to activate the (100) [010] slip system, developed numerous microtwins in (100) and some dislocations that were not fully characterized. However, interesting dislocations and associated faults were observed in specimens scratched on a (110) surface. Figure 9.32 is typical of the dislocation microstructures observed in these specimens and shows segments of dislocation loops bounding planar defects on (100). 

Figure 9.34. Diagram showing a (100) microtwin one unit cell wide in a-spodumene projected onto (010).

If the faults observed by van Duysen and Doukhan (1984) in scratched a-spodumene are microtwins with R = j[001], as suggested, then they would be out-of-contrast for all g = hkA. However, since images with these reflections are not given, the microtwin model caimot be adequately tested... 

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