Structure of twins

Formation of hillocks and penetration twins on the (100) surface of single crystal diamond was studied by Tsuno et al. [101]. The misorientation of the (100) surface was less than 3° from the exact (100) lattice plane. A NIRIM-type 

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Fig. 2.69 Reconstruction of twinning structure of (llSjuaci. Projected on (llO)jsiaci (a) Structure of twinning. Note that...

Fig. 5.2.11. Secondary structures of twin ribozymes. The arrows mark the cleavage sites. The substrates are fluorescein-labeled at both ends.

Shmytko, IM, Shekhtman VS, Ossipyan YA, Afonikova NS (1989) Twin structure and structure of twin boundaries in 1-2-3-07.,. Ferroelectrics 97 151-170... 

Figure 13 Structures of twin-coronet porphyrins. Reproduced from J. Am. Chem. Soc., 113, 6865 (1991) by permission of the American Chemical Society...

Catti M, Gazzoni G, Ivaldi G (1983) Structures of twinned P-Sr2Si04 and of a-Sri.9Bao.iSi04. Acta Cryst C 39 29-34... 

Nitrosyl Complexes.—A ledetmnination of the structure of twinned crystals of nitrosylpenta-anuninccobalt dichloride establishes that the Co-N-O angle is 119.0(9)° rather than close to 180° as was found in previous studies. The Co-N(NO) distance of 1.871(6) A is considered long. The... 

Keefer C., Mighell A., Mauer E, Swanson H., and Block S., "The Crystal Structure of Twinned Low-Temperature Lithium Phosphate," Inorg. Chem., 6, 119—25 (1967). 

DOia/TigAl. These materials possess a lamellar structure consisting of layers of twin-related TtAl and layers of TigAl in single crystalline form they are called "polysynthetically twinned (PST) crystals" (for a recent review see Yamaguchi, et al. 1995). 

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. 

Atomic structure of the ordered twin with APB tsrpe displacement... 

Fig. 5. Relaxed structure of the ordered twin with APB type displacement, (a) Flnnls-Slnclalr type potentials, (b) Full-potential LMTO method.

Similarly, in studies of lamellar interfaces the calculations using the central-force potentials predict correctly the order of energies for different interfaces but their ratios cannot be determined since the energy of the ordered twin is unphysically low, similarly as that of the SISF. Notwithstcinding, the situation is more complex in the case of interfaces. It has been demonstrated that the atomic structure of an ordered twin with APB type displacement is not predicted correctly in the framework of central-forces and that it is the formation of strong Ti-Ti covalent bonds across the interface which dominates the structure. This character of bonding in TiAl is likely to be even more important in more complex interfaces and it cannot be excluded that it affects directly dislocation cores. 

Similarly, the (111) GaAs substrate could be used to achieve epitaxial growth of zinc blende CdSe by electrodeposition from the standard acidic aqueous solution [7]. It was shown that the large lattice mismatch between CdSe and GaAs (7.4%) is accommodated mainly by interfacial dislocations and results in the formation of a high density of twins or stacking faults in the CdSe structure. Epitaxy declined rapidly on increasing the layer thickness or when the experimental parameters were not optimal. 

Figure 4. HREM of gold nanoparticles. A is a particle showing no defects, while B shows structural defects (twinnings). TEM micrograph taken with Jeol JEM3010, Department of Physical Chemistry University of Venice (Italy).

The occurrence of twinned crystals is a widespread phenomenon. They may consist of individuals that can be depicted macroscopically as in the case of the dovetail twins of gypsum, where the two components are mirror-inverted (Fig. 18.8). There may also be numerous alternating components which sometimes cause a streaky appearance of the crystals (polysynthetic twin). One of the twin components is converted to the other by some symmetry operation (twinning operation), for example by a reflection in the case of the dovetail twins. Another example is the Dauphine twins of quartz which are intercon-verted by a twofold rotation axis (Fig. 18.8). Threefold or fourfold axes can also occur as symmetry elements between the components the domains then have three or four orientations. The twinning operation is not a symmetry operation of the space group of the structure, but it must be compatible with the given structural facts. 

The structure of the metal particles dispersed on a silica powder support ( Aerosil 380, 70 A average silica particle diameter) has been studied by Avery and Sanders (47) using electron microscopy in both bright and dark field, to determine the extent to which the metal particles were multiply twinned or of ideal structure. Platinum, palladium, and gold were examined. These catalysts were prepared by impregnation using an aqueous solution of metal halide derivatives, were dried at 100°-150°C, and were hydrogen... 

New Zeolitic Structures. Multiply twinned faujasitic zeolites (typically zeolite-Y) have recently been shown (30, 31) to be capable, by recurrent twinning on 111 planes, to generate a new, hexagonal zeolite in which tunnels replace the interconnected cages of the parent cubic structure. 

Figure 3.25 Mechanical twinning in vanadium pentoxide, V205 (a) the formation of needlelike twins at the tip of reduced oxide and (b) the idealized structure of the twin. Arrows in (a) represent the direction of shear forces.

As in the case of twin planes, the antiphase relationship may affect only one part of the structure, for example, the cation substructure, while leaving the anion substructure unchanged. This is particularly common when the anion array can be considered to consist of a close-packed array of ions, which remains unchanged by the antiphase boundary (Fig. 3.28). 

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